New Horizons
February 1930: Pluto Discovered

See more Featured Images
View additional LORRI images
Learn more about the Science Ops Center
Read more about Jupiter science

This site requires that JavaScripts be enabled in your browser.

 Hubble Discovers a Fifth Moon Orbiting Pluto

New Horizons: NASA's Pluto-Kuiper Belt Mission
HomeOverviewScienceMissionSpacecraftEducationNews CenterGalleryLinks
New Horizons in the News
Media Contacts
75 Years of Pluto


Patsy and Clyde
Clyde and Patsy Tombaugh, circa 1935. (Image: New Mexico State University Archives

Read about
Patsy Tombaugh’s
“Life with Clyde”

New Horizons Team Remembers Patsy Tombaugh
January 16, 2012

It was January 2006, just days before the New Horizons spacecraft lifted off from Cape Canaveral Air Force Station, Florida. A reporter asked Patsy Tombaugh, widow of Pluto’s discoverer, Clyde Tombaugh, what her husband might have thought about the first mission to the planet he found in 1930.

“He’d be very happy about it,” she said, “because he’d really want to know what they were finding out about Pluto.”

Today, the team fulfilling that wish mourns Patsy Tombaugh, who died Jan. 12 in Las Cruces, N.M. She was 99.

"I will never forget Patsy's enthusiasm in New Horizons and her pride in what we're doing,” said Alan Stern, New Horizons principal investigator, from the Southwest Research Institute. "I'll also never forget her smile and winning way. On behalf of the entire New Horizons team, I want to express our condolences to Patsy's family and friends, and to say that when we explore Pluto in three years, she will be as much on our minds and in our hearts, as she is today."

Toast to New Horizons
Halfway there: In late 2010, New Horizons Principal Investigator Alan Stern leads the Tombaugh family in a toast to the mission with specially made “Halfway to Pluto” coffee mugs. From left: Annette Tombaugh Sitze, Patsy Tombaugh, Stern, and Wilbur Sitze. (Image: Alan Stern)

Patricia “Patsy” Edson met Clyde Tombaugh in the spring of 1933; her older brother James and Clyde were astronomy majors at the University of Kansas, and good friends. They married in June 1934; daughter Annette was born in 1940 and son Alden in 1945. But, as Patsy wrote in a 2005 New Horizons web story, their “family” seemed much larger.
“Living with Clyde Tombaugh was like having the celestial universe in the next room, but I found it a very good neighbor,” she wrote.

“Of course, I always had to share Clyde not only with Mars, Jupiter, the Moon, and stars, but also with the public. As the Space Age grew so did Clyde's fan mail. From all over this world came letters from all age groups asking for information or autographs. He once said that he had received at least 30,000 letters. He tried to answer each one.”

Patsy at Launch
Awaiting the launch of the New Horizons spacecraft from Kennedy Space Center/Cape Canaveral Air Force Station, Fla., in January 2006, Patsy Tombaugh points toward Pluto. (Image: Michael Soluri)

Clyde died in 1997. In addition to her son and daughter, Patsy is survived by five grandchildren, nine great-grandchildren and one great-great-grandchild. A memorial service is planned for Feb. 12 in Las Cruces.

Read other tributes to Patsy Tombaugh

NASA - National Aeronautics and Space Administration The Johns Hopkins University Applied Physics Laboratory Department of Energy Southwest Research Institute JPL KinetX
Editor: JHU/APL Webmaster
JHU/APL Official: Ms. K. Beisser
Last verified: 1/17/12
+ Contact JHU/APL
Back to New Horizons Main Page

Southwest Research Institute (SwRI) 2011 News Release -
 SwRI Researchers Discover New Evidence for Complex Molecules on Pluto's Surface


Image showing the "Grand Tack Scenario", which  demonstrates that the gas giant Jupiter may have briefly migrated into the inner solar system and influenced the formation of Mars (right), stripping away materials that resulted in its relatively small size in comparison to Venus (left) and the Earth.
Image courtesy NASA/STScI
The Cosmic Origins Spectrograph aboard NASA's Hubble Space Telescope recently discovered a strong ultraviolet-wavelength absorber on Pluto's surface.
Boulder, Colo. — Dec. 20, 2011 —The new and highly sensitive Cosmic Origins Spectrograph aboard the Hubble Space Telescope has discovered a strong ultraviolet-wavelength absorber on Pluto's surface, providing new evidence that points to the possibility of complex hydrocarbon and/or nitrile molecules lying on the surface, according to a paper recently published in the Astronomical Journal by researchers from Southwest Research Institute and Nebraska Wesleyan University.
Such chemical species can be produced by the interaction of sunlight or cosmic rays with Pluto's known surface ices, including methane, carbon monoxide and nitrogen.
The project, led by SwRI's Dr. Alan Stern, also included SwRI researchers Dr. John Spencer and Adam Shinn, and Nebraska Wesleyan University researchers Dr. Nathaniel Cunningham and student Mitch Hain.
"This is an exciting finding because complex Plutonian hydrocarbons and other molecules that could be responsible for the ultraviolet spectral features we found with Hubble may, among other things, be responsible for giving Pluto its ruddy color," said Stern.
The team also discovered evidence of changes in Pluto's ultraviolet spectrum compared to Hubble measurements from the 1990s. The changes may be related to differing terrains seen now versus in the 1990s, or to other effects, such as changes in the surface related to a steep increase in the pressure of Pluto's atmosphere during that same time span.
"The discovery we made with Hubble reminds us that even more exciting discoveries about Pluto's composition and surface evolution are likely to be in store when NASA's New Horizons spacecraft arrives at Pluto in 2015," Stern added.
This research was supported by a grant from the Space Telescope Science Institute.
Editors: A copy of the science paper by Stern et al. is available at For more information about NASA's New Horizons mission to Pluto, go to
Images to accompany this story:
For more information, contact Maria Martinez, (210) 522-3305, Communications Department, Southwest Research Institute, PO Drawer 28510, San Antonio, TX 78228-0510.


The PI's Perspective
Is the Pluto System Dangerous?
November 7, 2011

Pluto's newest found moon, P4, orbits between Nix and Hydra, both of which orbit beyond Charon. Could there be still more moons of Pluto? Perhaps, and the New Horizons team plans to look harder to ensure that we don't run into something that could damage or destroy New Horizons.

New Horizons remains healthy and on course, now almost twenty two times as far from the Sun as the Earth is, and approaching six years into its 9.5-year journey to the Pluto system.

We’ve taken the spacecraft out of hibernation to perform maintenance activities, and to re-point our radio antenna to compensate for Earth’s movement around its orbit. This “hibernation wakeup” started November 5 and will last until November 15. Then New Horizons will hibernate again until early January, when we’ll perform a more extensive, almost month-long wakeup.

I’ll provide an update soon about how the November hibernation wakeup went and what’s in store for the January wake up and our cruise to Active Checkout 6, which begins next May. But in this PI’s log, I want to concentrate on a question that has recently come to the fore on the mission: “Is the Pluto system dangerous to New Horizons?”

If you’re wondering what I mean, I’m referring to the fact that the more moons that pop up in the system, the more we have to worry that there are still more undiscovered moons that are too small and faint to detect. When we discovered P4 this summer, along with possible evidence of a couple of still-fainter moons (something we need more study to confirm or reject), we began to worry about just how many tiny moons Pluto might have and whether we might have to dodge them.

Even more worrisome than the possibility of many small moons themselves is the concern that these moons will generate debris rings, or even 3-D debris clouds around Pluto that could pose an impact hazard to New Horizons as it flies through the system at high speed. After all, at our 14-kilometer-per-second flyby speed, even particles less than a milligram can penetrate our micrometeoroid blankets and do a lot of damage to electronics, fuel lines and sensors.

So to assess that hazard, we brought together about 20 of the world’s experts in ring systems, orbital dynamics and state-of-the-art astronomical observing techniques to search for small satellites and rings at distant Pluto. This group convened November 3-4 at the Southwest Research Institute’s offices in Boulder, Colo., where the New Horizons science team is centered. During this two-day workshop, a series of technical talks and discussions sections examined every aspect of the hazards that debris and small moons orbiting in the Pluto system might pose.

The presenters and attendees of the New Horizons Pluto Encounter Hazards Workshop on November 4, 2011.

We found a plausible chance that New Horizons might face real danger of a killer impact; and that to mitigate that hazard, we need to undertake two broad classes of work.

First, we need to look harder at the Pluto system for still undiscovered satellites and rings. The best tools for this are going to be the Hubble Space Telescope, some very large ground-based telescopes, telescopes that can make stellar occultation observations of the space between Pluto and Charon where New Horizons is currently targeted, and thermal observations of the system by the ALMA radio telescope array just now being commissioned.

Then, we need to plan for an alternate, safer route through the Pluto system in case those observations reveal strong evidence that our current trajectory is too hazardous. Studies presented at the Encounter Hazards Workshop indicate that a good “safe haven bailout trajectory” (or SHBOT) could be designed to target a closest-approach aim point about 10,000 kilometers farther than our nominal mission trajectory. More specifically, a good candidate SHBOT aim point would be near Charon’s orbit, but about 180 degrees away from Charon on closest-approach day. Why this location? Because Charon’s gravity clears out the region close to it of debris, creating a safe zone.

Making this situation still more complex is the fact that debris created in the Pluto system may not lie in a plane, as in  other ring systems, but might instead be contained in a fat torus (donut-shaped) or even a nearly-spherical 3-D cloud if the debris coming off small satellites has high velocity (such debris is created by impactors from the Kuiper Belt, which hit at pretty high speeds of 1-2 kilometers per second.)

The question of whether the Pluto system could be hazardous to New Horizons remains open –but one we’ll be studying hard over the next year, with everything from computer models to big ground-based telescopes to the Hubble.

I’ll report on results as we obtain them, but it is not lost on us that there is a certain irony that the very object of our long-held scientific interest and affection may, after so many years of work to reach her, turn out to be less hospitable than other planets have been. We’ll see.

Ms. Patsy Tombaugh, at Kennedy Space Center for the New Horizons launch in January 2006. (Photo by Michael Soluri)

Happy Birthday Patsy

In closing, I want to add a timely note and wish Patsy Tombaugh, the widow of Pluto discoverer Clyde Tombaugh, a very happy 99th birthday today (November 7). As Patsy begins her 100th year, we on New Horizons marvel at her longevity, her energy, and her wonderful sense of humor. Happy birthday Patsy!

Well, that’s my update for now. Thanks again for following our journey across the deep ocean of space to a truly new frontier.  And I hope you’ll keep on exploring – just as we do!
- Alan Stern

View The PI's Perspective Archive >


The PI’s Perspective: Visiting Four Moons, in Just Four Years, for All Mankind

August 16, 2011

In June and July, members of the New Horizons science team, using the Hubble Space Telescope, discovered and confirmed that Pluto has a fourth moon! The new satellite, provisionally called P4, is fainter and therefore likely much smaller, than either Nix or Hydra or Charon – Pluto’s other three known moons. For comparison, while Charon is about as wide as the U.S. state of Colorado, Nix and Hydra are closer to the width of Vermont, and P4 is likely to be no wider across than Boulder County, Colorado.

New Horizons remains healthy and on course, now approximately 21 times as far from the Sun as the Earth is – well on its way, between the orbits of Uranus and Neptune.

On 2011 July 1, we completed our 2011 checkout of the spacecraft and its payload, which went very well. During that checkout, which spanned May and June, we conducted the first of two tests of our REX radio occultation experiment, using the moon to cut off a radio signal transmitted to New Horizons from Deep Space Network (DSN) antennas on Earth. This test involved many organizations, including the REX team, our mission operations team at the Johns Hopkins Applied Physics Laboratory, and the DSN itself. And it worked perfectly!

In June, we also accomplished some cruise science for our space plasma instruments – SWAP and PEPPSI – studying the charged-particle populations of the solar system. And over May and June we completed some much-needed spacecraft tracking; from every indication, we’re so close to a perfect course toward Pluto that will not need to conduct a course correction maneuver until at least 2013.

With this checkout behind us, we’ve returned our attention to planning the Pluto encounter. More specifically, the project team is working on the four-day command loads just before and just after the core nine-day load for Pluto closest approach. These “bookend” loads are the most critical portions of our Approach Phase 3 (AP3) and Departure Phase 1 (DP1) observations. Work is also well under way on the farther-out portions of AP3 and DP1. In addition, we’re analyzing and prioritizing responses to some 280 potential contingency scenarios for the 2015 encounter – just in case.

Pluto’s newest found moon (P4) orbits between Nix and Hydra, both of which orbit well beyond Charon. Could Pluto have additional moons? Perhaps, and the New Horizons team has proposed to the Hubble Space Telescope to look even harder – so that we can best plan our encounter observations, and ensure we don’t run into something we could have avoided.

New Horizons PI Alan Stern presented a mission overview talk last month at the U.S. Space and Rocket Center’s auditorium in Huntsville, Alabama.

And if all that isn’t keeping our little mission team busy enough, we’re also planning our first encounter rehearsal on New Horizons for summer 2012.

Designed as a stress test for the spacecraft, the “24-hour rehearsal” will include an intense one-day portion of the encounter near closest approach. We’ve rehearsed this portion of the timeline (and more) on our New Horizons ground simulator at APL in Maryland, and passed with flying colors. But there is no substitute for actually going through these paces on our spacecraft, verbatim. That kind of real-world test will prove the spacecraft is up to the task, and it’ll also let us check out some aspects of the timeline that we can only do in space. A good example: New Horizons will actually perform all the timelined attitude maneuvers and turns, which our ground simulator can only pretend to do.

The 24-hour rehearsal in 2012 is a precursor to the full, nine-day-long core encounter rehearsal that we will run on New Horizons during summer 2013.

What else is coming for New Horizons? We have our usual pair of “precession wakeups” from hibernation this November and in January 2012. In these annual activities we re-point our spacecraft and its communications antenna to account for Earth’s motion around the sun, and perform some spacecraft maintenance. Also in January, we will perform our second (and final) REX radio science lunar occultation test.

On tap in the coming months will be preparations for the new fault protection/autonomy and command and data handling software we’ll send up to the spacecraft next summer, as well as the planning for a jam-packed active checkout for 2012. The “C & DH” software change will feature a key bug fix, tracked down by project engineer Steve Williams of APL, which should reduce the probability of any computer resets during the 2015 encounter.

And also in the coming months, we’ll be kicking off our extensive space plasma hibernation cruise science observations with a test in October, and if all goes well, operational data collects beginning in early 2012.

While our spacecraft and operations teams are performing all those activities, our science team is leading a search for Kuiper Belt flyby targets, conducting studies of Pluto and its moons using ground-based telescopes, and planning a major scientific conference on the Pluto system for the summer of 2013.

That conference will allow the world’s astronomers and planetary scientists to review the state of knowledge about the Pluto system before our encounter and to begin detailed planning of ground-based and space-based campaigns to observe the planet and its moons in conjunction with the New Horizons flyby. The conference will also give researchers a chance to develop educated predictions about what New Horizons may find. We are discussing a similar kind of meeting for educators in 2014.

The New Horizons project expects to propose a stamp to the U.S. Postal Service to commemorate the historic flyby of Pluto and its moons that we will accomplish in 2015. But at least one country has already issued New Horizons stamps! Can you tell which country?

Finally, I’ll close by giving a big shout out of congratulations to NASA’s Dawn mission, which is now making the first reconnaissance of Vesta – the fourth-largest asteroid – which is about 500 kilometers across. Dawn’s exploration of Vesta is just beginning, but is already yielding spectacular results and showing us a much more complex world than many scientists imagined Vesta to be. You can read more about Dawn’s exploration of Vesta here.

From those of us looking forward to exploring a world almost 2,500 kilometers in diameter to those exploring fascinating Vesta, New Horizons salutes you. First-time exploration of new worlds simply rocks!

Well, that’s my update for now. Thanks again for following our journey across the deep ocean of space to a truly new frontier. I hope you’ll keep on exploring – just as we do!

- Alan Stern

View The PI's Perspective Archive >



See Explanation.  Clicking on the picture will download the highest resolution version available.
Pluto's Newly Discovered Moon P4 in the Pluto System
Credit: NASA, ESA, Mark Showalter (SETI Institute)
Explanation: Nix and Hydra were first introduced to human eyes in Hubble Space Telescope images from May 2005, as Pluto's second and third known moons. Now Hubble images have revealed a fourth satellite for the icy, dwarf planet. Provisionally designated P4, it completes an orbit of Pluto in about 31 days. Presently Pluto's smallest and dimmest known moon, P4 is estimated to be 13 to 34 kilometers across dependent upon its albedo. The newly discovered satellite was first spotted in Hubble observations from June 28, and later confirmed in a follow-up on July 3 and July 18. These two panels are composites of both the short and long exposures that include brighter Pluto itself along with Pluto's largest moon Charon. Camera noise and image artifacts also show up in the long exposure segments. The Hubble observations were made while searching for faint rings around the distant world in support of NASA's New Horizons mission, set to fly by the Pluto system in July 2015. 

Fourth Moon Adds to Pluto's Appeal
July 20, 2011

The tiny moon: With an estimated diameter of 8 to 21 miles (13 to 34 kilometers), P4 is the smallest moon discovered around Pluto. By comparison, Charon, Pluto's largest moon, is 648 miles (1,024 kilometers) across, and the other moons, Nix and Hydra, are in the range of 20 to 70 miles in diameter (32 to 113 kilometers).

On the anniversary of the first landing of men on our moon, New Horizons mission team scientists have announced the discovery of a fourth moon around Pluto – adding to the scientific treasure trove that awaits NASA’s Pluto-bound New Horizons when it arrives in 2015.

“Could this planet get any more interesting?” says New Horizons Principal Investigator Alan Stern, of the Southwest Research Institute, Boulder Colo. “We already know that when New Horizons provides the first close-up look at Pluto in July 2015, we’ll see planetary wonders we never could have expected. Yet this discovery gives us another hint of what awaits us in the Pluto system, and we’re already thinking about how we want to study this new moon with New Horizons. What a bonus for planetary science and for New Horizons!”

A Hubble Space Telescope observing team led by Mark Showalter of the SETI Institute, Mountain View, Calif., and Douglas Hamilton of the University of Maryland, College Park, detected the new moon in five sets of Hubble Space Telescope images taken over the past two months. Astronomers are still trying to better peg orbital details on the object, designated “S/2011 P1” or “P4” until it receives a permanent name. They’ve put its diameter at between 8 and 21 miles (13 to 34 kilometers) and estimate that it travels on a circular, equatorial orbit nearly 37,000 miles (about 59,000 kilometers) from Pluto – placing the new moon between the orbits of the moons Nix and Hydra.

Read the NASA/Hubble Space Telescope release

These two images, taken about a week apart by NASA's Hubble Space Telescope, show four moons orbiting the distant, icy dwarf planet Pluto. The green circle in both snapshots marks the newly discovered moon, temporarily dubbed P4, found by Hubble in June. The new moon lies between the orbits of Nix and Hydra, two satellites discovered by Hubble in 2005. It completes an orbit around Pluto roughly every 31 days. The moon was first seen in a photo taken with Hubble's Wide Field Camera 3 on June 28, 2011. The sighting was confirmed in follow-up Hubble observations taken July 3 and July 18. P4, Nix, and Hydra are so small and so faint that scientists combined short and long exposures to create this image of Pluto and its entire moon system. The speckled background is camera "noise" produced during the long exposures. The linear features are imaging artifacts. The tiny satellite was uncovered in a Hubble survey to search for rings around the frigid dwarf planet.

Signs of the new satellite in the June-July 2011 images also sent the observing team – which includes Stern, Hal Weaver of the Johns Hopkins University Applied Physics Laboratory (APL) in Laurel, Md., and Andrew Steffl and Leslie Young of Southwest Research Institute – combing though older Hubble snapshots. Knowing where to look, Weaver found strong evidence for P4 in images from June 2010. Steffl saw the same in a set taken in February 2006. Together these “archival” records add to the available suite of data needed for scientists to accurately determine the moon’s orbit.

The discovery of a fourth moon adds to Pluto’s complex and fascinating story. Discovered in 1930 by American astronomer Clyde Tombaugh, Pluto was first considered an icy oddball with a strange orbit – yet it harkened the existence of thousands of similar frozen mini-worlds in a planetary outpost known as the Kuiper Belt. Pluto’s moon Charon was discovered in 1978; it’s the largest moon in the solar system relative to the planet it orbits. (In fact, the pair orbits a common center of gravity that sits outside each body – earning it the rare status of a “double planet.”)

Charon got company in 2005, when a team led by Weaver and Stern found two smaller moons – subsequently named Nix and Hydra – again using Hubble.

“Pluto continues to amaze us,” says Weaver, the New Horizons project scientist. “Who would have thought a dwarf planet could support such a complex satellite system? The hunt will now be on for similarly complex systems around other Kuiper Belt objects.”

Aside from giving the New Horizons team something to plan for and look forward to, the discovery offers a peek into Pluto’s violent past. “The discovery of this moon reinforces the idea that the Pluto system was formed during a massive collision 4.6 billion years ago,” says Weaver. “The smaller satellites, including this one, probably came together in the resulting debris disk.”

This discovery also suggests other small bodies may lurk in the Pluto system – and, if so, New Horizons should root them out when it flies by. But more “targets” might not be the best thing for the mission.“The discovery of P4 is exciting, but it also raises the possibility that our New Horizons spacecraft may enter a more hostile environment than we previously imagined,” says Glen Fountain, New Horizons project manager from APL.

“We don’t want our spacecraft running into any debris that’s still hanging around from the massive collision that spawned the formation of Pluto’s smaller satellites,” adds Stern. “For this reason we’re going to look with New Horizons, and even before we get there, we’ll be looking with Hubble and other tools, we hope.”

New Horizons launched on Jan. 19, 2006, picking up a gravity boost from (and conducting scientific investigations of) Jupiter in February 2007. The spacecraft is now more than 1.8 billion miles (2.9 billion kilometers) from Earth, on course to fly past Pluto on July 14, 2015, then on to one or more other Kuiper Belt objects in a possible extended mission. The Johns Hopkins Applied Physics Laboratory built and operates the New Horizons spacecraft and manages the mission for NASA; Stern leads the mission as principal investigator.


































Double Occultations This Week Will Reveal More Details About Pluto

by Nancy Atkinson on June 23, 2011 from
Several teams of astronomers are taking advantage of a rare double event this week to learn more about the atmosphere and makeup of Pluto and its moons. The dwarf planet will occult, or pass in front of two different stars this week. One of the best viewing sites for these two events is in Hawaii, and eclipse-chaser Dr. Jay Pasachoff is there to record both events. “To see those occultations, we have to be in a particular set of places on Earth, those over which the shadow of the object in starlight passes,” Pasachoff wrote in a guest post on the Planet Hunters blog. “Since the stars are so far away, their light is essentially parallel and the shadows of the objects on Earth are the same as the sizes of the objects.”
If all goes well, we will know a lot more about the Pluto system, Pasachoff said.

Map of where the occultation would be visible on June 22-23, 2011.

Last night, June 22/23, both Pluto and its moon Charon occulted a magnitude 14.4 star, with each occultation lasting a minute or so and separated from each other by 12 minutes. “The event is particularly exciting because if we capture both Pluto and Charon nearly simultaneously, we can find out about the system’s internal orbits with higher precision than before, perhaps allowing a refinement of the center of mass and thus the masses and densities of each object,” Pasachoff said.
Also, the first deployment for an occultation of the NASA/German SOFIA observatory took place last night to view the Pluto occultation, flying at an altitude of 43,000 feet off the west coast of Central America.
“The scientific goal is to catch the ‘central flash,’ which conveys vital information about conditions in Pluto’s global atmosphere,” wrote American Astronomical Society press officer Rick Fienberg on Twitter. Fienberg was part of the press corps that was accompanying the flight.
On Sunday/Monday night, June 26/27, Pluto will occult a different star, and over a much narrower path, its small moon Hydra might also occult another star.
Pasachoff said that the most recent predictions for last night’s occulations shifted the prediction south, so that Hawaii is slightly off the main predicted path, to its north. But other teams are in Cairns, Australia, to see if it goes that far south.
For the June 26/27 event (June 27 UT but June 26 in Hawaii), the star is magnitude 13.6. “That is a couple of magnitudes brighter than most of the stars we have observed being occulted,” Pasachoff said, “so the data would be particularly low-noise. In addition to the occultation of Pluto itself, whose southern limit is predicted to pass through the Hawaiian islands, the tiny Pluto moon Hydra is to be occulted, though that narrow path’s prediction now passes north of the Hawaiian islands. We have arranged for telescopes in Yunnan, China, in Japan, Taiwan, and Thailand to observe with us, and MIT’s Matt Lockhart is en route to Yunnan with one of our POETS (Portable Occultation, Eclipse, and Transit System) cameras. We have Australian sites still observing as well, just in case the actual path is hundreds of kilometers south of the predictions.”
Earlier occultations by Pluto studied by Pasachoff and his colleagues showed that Pluto’s atmosphere was warming and that the atmosphere would probably remain warm enough by 2015 for the New Horizons spacecraft to detect and study it with its on-board instruments, and was part of the incentive for the mission to launch when it did.
To learn more about the occultations and the research, check out this main stellar occultation website from Williams College, where Pasachoff is located, which has links to the work of other researchers as well.
Maps and details of the predictions can be found here, and more details about Pluto occultations websites can be found here.
We’ll try to provide an update of the events when details become available.
You can follow Universe Today senior editor Nancy Atkinson on Twitter: @Nancy_A. Follow Universe Today for the latest space and astronomy news on Twitter @universetoday and on Facebook.
Tagged as: Charon, Hydra, Jay Pasachoff, Nix, Pluto
This image obtained from NASA on Thursday shows the newest images of Pluto taken by the Hubble Space Telescope. Pluto, the dwarf planet on the outer edge of our solar system, has a dramatically ruddier hue than it did just a few years ago, NASA scientists said Thursday after examining the photos.
AFP/Getty Images
A new examination of photographs of Pluto from the Hubble Space Telescope shows the dwarf planet changing colors and at one point getting redder in what may indicate distinct changes of seasons, National Geographic News says.
 The photographs, taken from 2002 to 2003, were compiled in a four-year project that involved 20 computers running simultaneously, lead investigator Marc Buie, of the Southwest Research Institute in Boulder, Colo., tells reporters.
Buie says the melding of 384 Hubble pictures of Pluto created ""my best guess at a true-color appearance" of the dwarf planet.
"If you were sitting in a spacecraft puttering around Pluto and looking out the window, this is what you'd see, but in higher resolution," he says.
Astronomers noted that the Southern Hemisphere became significantly darker and redder from 2000 to 2002, while the Northern Hemisphere got brighter, NGN says.
This likely is because of ice melting and refreezing during the change of seasons, astronomer Mike Brown, of the California Institute of Technology in Pasadena, said at the briefing.


Pluto, which was downgraded from full planet status to dwarf  planet four years ago, orbits the sun every 248 years, AFP notes
.The latest pictures should offer astronomers ideas on what to focus on when the New Horizons spacecraft reaches Pluto in 2015, NGN says.



Wanted: Kuiper Belt Targets
New Horizons team launches search for post-Pluto flyby prospects
April 20, 2011
The New Horizons team, working with astronomers using some of the largest telescopes on Earth, will begin searching this month for distant Kuiper Belt objects that the New Horizons spacecraft hopes to reconnoiter after completing its observations of the Pluto system in mid-2015. 
No spacecraft has ever visited the Kuiper Belt, a distant, donut-shaped region of the solar system filled with small planets and comets that formed early in the solar system’s history.

Diagram of the Kuiper Belt, beyond the orbit of Neptune, more than 3 billion miles from the Sun. Pluto's orbit in the Kuiper Belt is shown in yellow; the trajectory of the New Horizons spacecraft is shown in red.
While the main target for NASA’s New Horizons mission is Pluto and its three moons, New Horizons was built with post-Pluto Kuiper Belt object (KBO) flybys in mind.
"We have enough fuel on New Horizons, and there are enough Kuiper Belt objects out there, that we have a good chance of visiting at least one of them, probably one that's at least 50 kilometers [about 30 miles] across,” says New Horizons co-investigator John Spencer, of Southwest Research Institute, who is coordinating the search effort. “But first, we have to find them.”
Spencer cites two reasons why suitable target KBOs aren’t already known. First, they are likely to be more than 10,000 times fainter than Pluto — near the very limit of what large telescopes can detect. Second, by a twist of fate, the current location of objects that New Horizons can reach is superimposed on the dense star fields of the Milky Way’s center, in the constellation Sagittarius, which is the hardest region of the Kuiper Belt to search for faint KBOs. 

Enlisted for the New Horizons KBO-target search: The 8-meter Subaru and 3.6-meter Canada-France-Hawaii (inset) telescopes at Mauna Kea Observatory in Hawaii. (Credit: Subaru Telescope; CFHT)
“As a result, we have to conduct a special, dedicated search to find our target KBOs,” says New Horizons Principal Investigator Alan Stern, also of the Southwest Research Institute.  “And, because it will take two to three years to net a range of potential targets and refine their orbits and physical characteristics well enough to select the best one or two for New Horizons, we have long planned to begin this work in 2011, so we can have our targets selected and propose this extended mission to NASA before we get to Pluto.”
To conduct the KBO search, the New Horizons project has recruited an international team of astronomers from nine institutions in the U.S., Canada, France and Chile, which has secured 140 total hours of observing time between April-July 2011 on some of the world's premier telescopes, including the 8-meter Subaru and 3.6-meter Canada-France-Hawaii telescopes at Mauna Kea Observatory in Hawaii, and the twin 6.5-meter Magellan telescopes at Las Campanas Observatory in Chile.

In this artist's concept created by Dan Durda of SwRI, New Horizons flies by a Kuiper Belt object.
During the search, which begins later this month, these telescopes will take thousands of wide-field images, containing millions of stars. The search team will then sort through this mass of data to find a few moving points of light, with orbits in the Kuiper Belt. After follow-up observations next year to refine the orbits of these flyby candidates, one or two of them may become a New Horizons target.
“Other than New Horizons, no existing or planned spacecraft has the chance to explore KBOs, which are ancient and highly scientifically valuable relicts of the era of outer solar system formation,” Stern says. “We’re very proud to carry that banner for NASA and the scientific community.”





See Explanation.  Clicking on the picture will download  the highest resolution version available.
Dim World, Dark Nebula
Credit & Copyright: Ray Gralak
Explanation: Dim, distant, dwarf planet Pluto can be hard to spot, especially in recent months as it wanders through the crowded starfields of Sagittarius and the central Milky Way. But fortunately for backyard Pluto hunters, it crossed in front of a dark nebula in early July. The diminutive world is marked with two short lines near the center of this skyscape recorded from New Mexico Skies on July 5. Pluto stands out only because obscuring dark nebula Barnard 92 (B92) blocks the background of the Milky Way's congeries of faint, innumerable stars. Another of astronomer E. E. Barnard's cataloged dark markings on the sky, B93, is easy to pick out just left of B92. Prominent at the lower left is open star cluster NGC 6603. In fact, Pluto, dark nebulae, and star cluster all lie within a portion of M24, also known as the Sagittarius Star Cloud, filling most of the frame.


More Surprises From Pluto - Is It a Comet ????

by Jason Major on April 24, 2011 From

Artist's illustration of Pluto's surface. Credit: NASA

Ah, Pluto. Seems every time we think we’ve got it figured out, it has a new surprise to throw at us.

First spotted in 1930 by a young Clyde Tombaugh, for 76 years it enjoyed a comfortable position as the solar system’s most distant planet. Then a controversial decision in 2006 by the International Astronomical Union, spurred by suggestions from astronomer (and self-confessed “planet-killer”) Mike Brown*, relegated Pluto to a new class of worlds called “dwarf planets”. Not quite planets and not quite asteroids, dwarf planets cannot entirely clear their orbital path with their own gravitational force and thus miss out on full planetary status. Besides immediately making a lot of science textbooks obsolete and rendering the handy mnemonic “My Very Eager Mother Just Served Us Nine Pies” irrelevant (or at least confusing), the decision angered many people around the world, both in and out of the scientific community. Pluto is a planet, they said, it always has been and always will be! Save Pluto! the schoolkids wrote in crayon to planetarium directors. The world all of a sudden realized how much people liked having Pluto as the “last” planet, and didn’t want to see it demoted by decision, especially a highly contested one.

Yet as it turns out, Pluto really may not be a planet after all.

It may be a comet.

But…that’s getting ahead of ourselves. First things first.

Discovery data showing carbon monoxide spectrum. Credit: J.S. Greaves / Joint Astronomy Centre.

Recent discoveries by a UK team of astronomers points to the presence of carbon monoxide in Pluto’s atmosphere. Yes, Pluto has an atmosphere; astronomers have known about it since 1988. At first assumed to be about 100km thick, it was later estimated to extend out about 1500km and be composed of methane gas and nitrogen. This gas would expand from the planet’s – er, dwarf planet’s – surface as it came closer to the Sun during the course of its eccentric 248-year orbit and then freeze back onto the surface as it moved further away. The new findings from the University of St Andrews team, made by observations with the James Clerk Maxwell telescope in Hawaii, identify an even thicker atmosphere containing carbon monoxide that extends over 3000 km, reaching nearly halfway to Pluto’s largest moon, Charon.

It’s possible that this carbon monoxide atmosphere may have expanded outwards from Pluto, especially in the years since 1989 when it made the closest approach to the Sun in its orbit. Surface heating (and the term “heating” is used scientifically here…remember, at around -240ºC (-400ºF) Pluto would seem anything but balmy to us!) by the Sun’s radiation would have warmed the surface and expelled these gases outwards. This also coincides with observations made by the Hubble Space Telescope over the course of four years, which revealed varying patterns of dark and light areas on Pluto’s surface – possibly caused by the thawing of frozen areas that shift and reveal lighter surface material below.

“Seeing such an example of extra-terrestrial climate-change is fascinating. This cold simple atmosphere that is strongly driven by the heat from the Sun could give us important clues to how some of the basic physics works, and act as a contrasting test-bed to help us better understand the Earth’s atmosphere.”

–  Dr. Jane Greaves, Team Leader

In fact, carbon monoxide may be the key to why Pluto even still has an atmosphere. Unlike methane, which is a greenhouse gas, carbon monoxide acts as a coolant; it may be keeping Pluto’s fragile atmosphere from heating up too much and escaping into space entirely! Over the decades and centuries that it takes for Pluto to complete a single year, the balance between these two gases must be extremely precise.

Read more about this discovery on the Royal Astronomical Society’s site.

Pluto's elliptical orbit

So here we have Pluto exhibiting an expanding atmosphere of thawing expelled gas as it gets closer to the Sun in an elliptical, eccentric orbit. (Sound familiar?) And now there’s another unusual, un-planet-like feature that’s being put on the table: Pluto may have a tail.

Actually this is an elaboration of the research results coming from the same team at the University of St Andrews. The additional element here is a tiny redshift detected in the carbon monoxide signature, indicating that it is moving away from us in an unusual way. It’s possible that this could be caused by the top layers of Pluto’s atmosphere – where the carbon monoxide resides – being blown back by the solar wind into, literally, a tail.

That sounds an awful lot, to this particular astronomy reporter anyway, like a comet.

Just saying.

Anyway, regardless of what Pluto is or isn’t, will be called or used to be called, there’s no denying that it is a fascinating little world that deserves our attention. (And it will be getting plenty of that come July 2015 when the New Horizons spacecraft swings by for a visit!) I’m sure there’s no one here who would argue that fact.

New Horizons’ upcoming visit will surely answer many questions about Pluto – whatever it is – and most likely raise even more.


Artist's impression of Pluto's huge atmosphere of carbon monoxide.Credit:P.A.S. Cruickshank.

The new discovery was presented by team leader Dr. Jane Greaves on Wednesday, April 20 at the National Astronomy Meeting in Wales.

Article reference: Discovery Of Carbon Monoxide In The Upper Atmosphere Of Pluto


*No disrespect to Mr. Brown intended…he was just performing science as he saw fit!



Tagged as: atmosphere, carbon monoxide, comet, IAU, mike brown, New Horizons, planet, Pluto, Solar System, University of St Andrews

{ 40 comments… read them below or add one }

Lawrence B. Crowell April 24, 2011 at 3:20 am

If we were to be scientifically accurate we would drop the whole concept of planet as it has been used. Planet came from wandering star, which we know they are not. Further, if we are to peel these dwarf planets off, and there are a couple of them about as big as Pluto such as Sedna, based on physical characteristics, then should terrestrial planets be consdiered the same type of object as a gas giant or jovian? The definition appears to hinge upon the idea a planet needs to gravitationally clear its orbital region. This definition is a bit uncertaint to me. We had an article recently about the horseshoe orbiting asteroid. Clearly Earth has not completely cleared its interplanetary space. Jupiter has similar objects called Trojans.

There are three types of objects, terrestrials, jovians and K-dwarfs (Kuiper belt dwarf planets — like Pluto). They all three are physically quite distinct from each other. They have different compositions, mass ranges and so forth. Maybe be we should call these objects cometoids, for comets and these dwarf planets are from the same region, likely have a similar chemistry and so forth.


Log in to Reply

iantresman April 24, 2011 at 4:54 pm

LC wrote: “Planet came from wandering star, which we know they are not.”

Doesn’t that depend on your frame of reference. See “Wandering Stars: The Movements and Visibility Cycles of the Naked Eye Planets

Log in to Reply

Lawrence B. Crowell April 24, 2011 at 5:40 pm

I meant the word planet came from greek for wandering star.


Log in to Reply

Torbjorn Larsson OM April 24, 2011 at 10:48 pm

FWIW, the criteria isn’t “completely cleared” but cleared, i.e. the planet dominates orbital mass. Pluto, Ceres et cetera fails that.

Log in to Reply

DrNothing April 25, 2011 at 3:50 pm

If Earth were out that far, it too would ail (per modeling) to clear it’s orbit either.**

Log in to Reply

DrNothing April 25, 2011 at 3:58 pm

LOL! I just read this and realize I seem to have had a case of reportersiezurites… I MEANT to say that if Earth were out that far, it would fail, as modeling suggests, to clear it’s orbit as well.

Log in to Reply

DrNothing April 25, 2011 at 3:55 pm

Truly this new (and arguably first) non-vague definition of the word clearly needs work, as many involved admit. And you bring up a great point that I hadn’t heard of or considered before per ‘rockies’ being considered in the same class as ‘gassies’ making comparably little sense. I say that we ape the Greeks’ treatment of the word, “Love”. Different words for different types of them, but still use the overlaying term, “Planet” to speak in generalities on the topic.

Log in to Reply

cosmic avenger April 24, 2011 at 3:41 am

I’ve been teaching high school science for 15 years, initially I discussed Pluto and its status as a planet as a discussion of definitions and characteristics for similar and dissimilar ideas. I have always had students come up with the idea that Pluto seemed more like a comet than a planet.

Log in to Reply

VIGNESHRAJU April 24, 2011 at 4:32 am

It is very useful to us.These discovery would also helps us to contact with extraterrestrial life in our universe.

Log in to Reply

Nexus April 24, 2011 at 6:13 am
johnnymorales April 24, 2011 at 6:21 am

All this interest in Pluto would be better redirected at Triton which research indicates is probably a near twin of Pluto.

If we did so, not only would we learn all we need to know about Plutinos, we’d also learn a lot more about Neptune.

A 2 for 1 that somehow is completely overlooked due to the fascination with Pluto.

Log in to Reply

Sili April 24, 2011 at 10:34 pm

I think it’s more a matter of a mission to Triton having to be an orbiter. Which is far more expensive than the fly-by of Pluto, something that’s being done on a shoestring budget.

Log in to Reply

Jon April 25, 2011 at 5:38 pm

I think a Cassini like orbiter around Neptune and Uranus would be awesome in general. Let’s get landers on that thing. =D

Also, don’t planets and well, everything with an atmosphere (even a highly variable one that is seasonal) have tails of some kind? Why isn’t everything a comet?

Log in to Reply

Feenixx April 24, 2011 at 8:29 am

Neil deGrasse Tyson has been saying all along that he reckons “Pluto is a comet” – and some people I know say that he is just “being controversial”. It seems that he does have a valid point.

So, what with both, Pluto and Charon, orbiting the centre of their combined gravity, which lies outward from the surface of either of the two: can we call it a “double comet”?

Log in to Reply

Feenixx April 24, 2011 at 8:31 am

… or a “binary comet” – as in “binary star system”?

Log in to Reply

vagueofgodalming April 24, 2011 at 10:16 am

It strikes me that Jane Greaves isn’t very politically aware, since her quote there plays into both the ‘it’s all natural’ and ‘climate scientists admit they don’t know any basic physics’ memes regarding climate change – nether of which I would think she believes.

Log in to Reply

David April 24, 2011 at 1:41 pm

With an atmosphere that extensive, maybe Pluto is a small gas giant with a rocky core and transparent atmosphere.

Log in to Reply

Jorge April 24, 2011 at 1:56 pm

Newsflash: Venus loses low mass gaseous molecules to space due to the effect of the solar wind. That was, in fact, how it lost most of its water. The molecules get blown away as a thin gaseous (you guessed it) tail.

Hey, look, Venus is a comet!

Dumb, huh?

Want to know what’s even dumber? Earth is a comet, for it does the exact same thing. Only our tail is thinner than venus’, because the solar wind here is not as strong and because we’re heavier. But it’s there, so the Earth is a comet. Shabam! Take that, you fools that thought the Earth was a planet! Silly, silly people…

There’s only one planet in the whole Solar System, and that’s Jupiter. The rest? Comets, comet cores, things with tails in general.

Actually, and seriously now, Jupiter itself has a tail, a huge magnetic tail filled with charged matter, measured recently by New Horizons, but I’m not sure if it also includes matter coming from the outer layers of its own atmosphere or not. Probably not: its magnetic field may be too strong for that. But all of the smaller planets lose atmosphere to space. It’s a common process, known for many years, and I find truly silly that the discovery of a similar process ocurring in Pluto would generate articles like this one. Even Mercury, which has almost no atmosphere, loses matter to space. Planets customarily show thin comet-like tails, and they don’t even have to be small. Look at HD 209458 b, aka Osiris, both larger and more massive than Jupiter, with a tail detectable from Earth!

Let’s try not to silly it up whenever the topic is Pluto, OK?

Log in to Reply

solarx2 April 24, 2011 at 2:28 pm

ok, jorg good points but the planets you speak of are not made of ice and have almost perfectly circular orbits. a comet by definition is made of something that is frozen and has an eccentric orbit that causes it to periodically suffer bouts of intense melt. also, several of the planets you mentioned have strong magnetic fields, which would disqualify them from the comet designation outright. pluto, could technically fit the definition, seems a little far-fetched to me though. we really need a good classification system that everyone agrees on.

Log in to Reply

Lawrence B. Crowell April 24, 2011 at 3:06 pm

The Earth will lose all its water in 2 billion years or less. In fact with the increased heating of the sun in about 1 to 1.5 billion years Earth will start to look more like Venus. Planets and bodies in general outgas into space.

The Jovian and Saturnian moons, and certainly those of Uranus and Neptune appear to be plutoid-like, and these may in fact be left overs from the early solar system evolution where the Kuiper belt may have extended all the way into the “frost radius.”


Log in to Reply

Jorge April 24, 2011 at 4:48 pm

Well, FWIW, the more I learn about these things, the more I think the division between comets and non-comets makes no sense at all.

Because a rock is nothing but an ice of silicate gas. If you rise the temperature enough, what you get is silicate gas although, as commonly happens with the more volatile organic molecules, they tend to quickly subdivide in simpler chemical species. But the truth remains that an ice is in its essence a mineral, just like any other. If you lower the temperature enough, almost anything becomes an ice; if you rise it enough, everything turns into gas.

And all bodies are combinations of different substances that evaporate or sublimate at different temperatures. Of course, those that form at lower temperatures and/or are subject to lower temperatures during their existence, will incorporate a larger percentage of substances that turn into gas at lower temperatures than those that don’t. But there is no clear division. Even the smallest comets that drop by and sprout magnificent tails include “rocky materials” (i.e., silicates), and even Mercury has a tiny percentage of what’s commonly called volatiles. It’s all a vast continuum, as has been proven recently by those asteriods showing cometary activity and by the inert comet nuclei that have been discovered. In a very real sense, a comet is just an asteroid that wanders too close to its star. Or a planet. Or an asteroid or planet, confortably sitting in their low-e orbit, subject to the action of a star that expanded a bit too close to them.

Speaking of eccentricity, Pluto’s is only a bit larger than Mercury’s, and there are other dwarf planets with much more circular orbits. And if you go to the Extrasolar Planets Encyclopedia’s interactive catalog and order the whole shebang by eccentricity, you’ll find that about one quarter of all extrasolar planets discovered so far are more eccentric than Pluto. Not much of a divisor there.

Log in to Reply

laurele April 25, 2011 at 10:17 pm

Many giant exoplanets have orbits far more eccentric than that of Pluto. One has a very comet-like orbit around its star. Yet it makes no sense to classify them as comets. Clearly, planets can have elliptical orbits too. Pluto’s orbit is stable, unlike that of comets, which lose mass every time they come close to the Sun, eventually disintegrating altogether. Pluto never comes into the inner solar system, and it is much bigger than the largest known comets. Also, Pluto is not “made of ice,” as it is 70-75 percent rock and geologically differentiated into core, mantle, and crust just like Earth–and unlike any comet.

Log in to Reply

Jason Major April 24, 2011 at 4:25 pm

I realize that the “Comet Pluto” idea is a bit imaginative, but obviously whenever classifications are made there’s going to be a hazy zone between them. Perhaps actual planets – dwarf or not – exhibit some comet-like characteristics as well but obviously we’re not going to rethink what to call Earth, Venus, Saturn, et al….at least not anytime soon! Maybe comets would thus also be classified by a smaller, irregular size and shape, whereby Pluto would then be out of that group. But the expanding and receding atmosphere, affected as it is by solar heating and blown into a tail, as well as its origins out near the Kuiper Belt, admittedly does make one wonder what type of sub-categories “dwarf planet” status might require. Still, Mercury behaves nothing like Jupiter or Saturn yet we comfortably call each a planet, so this may end up being more an exercise in semantics than anything else. Regardless, Pluto is fascinating! While we all quibble over what’s-called-what New Horizons is quietly zipping through the blackness of space, preparing for its all-too-brief rendezvous with an enigma. We should save our assumptions until then, when undoubtedly everyone will be greeted with unexpected surprises!

Log in to Reply

Moonshine April 24, 2011 at 6:43 pm

Thankyou Jason for your input. I agree on this totally !

I hope that in a 100 years if we don’t kill ourself off first, that this same topic will be settled or continue to inspire a more intellegent refinement of the classifcation system.

Log in to Reply

RUF April 24, 2011 at 9:55 pm

K-dwarfs? Does that mean “Kuiper”? Hadn’t heard that one. I like the Jovian, Terrestial, dwarf scheme.

The exact origin of a word may have a meaning that no longer applies — but it still works. Some still say “dial 911″ though dial phones are gone, and we “tape” shows on a DVR. Just throwing that out there. I like the word planet. Doesn’t really matter what it used to mean.

One last thing — Could all icy moons like Enceladus and Europa be HUGE captured comets? Doesn’t sound to far-fetched to me (except maybe their size).

Log in to Reply

Jason Major April 24, 2011 at 11:00 pm

How about J-Planet, T-Planet, D-Planet for terms? (Sounds like nicknames from Jersey Shore…)

Log in to Reply

Torbjorn Larsson OM April 24, 2011 at 11:11 pm

Also possibly excepting their angular momentum. Captures would have roughly equal chances of being retrogrades in orbit (and possibly spin), right?

As opposed to scattered and coalesced impacts as seems predicting our and Mars moons (and even a putative recent moon impact on the later – Mars may have had at least 3 moons at the same time). And whatever process seems to have formed most of the giants moons.

But there _are_ captures too. (Say, Triton.)

So how did the giant’s moons form? Can it be used to tell between non-retrograded captures and the rest?

Anyway, IIRC composition is gradual over the planetary system (due to where ices froze out). Maybe that can be used.

Log in to Reply

Torbjorn Larsson OM April 24, 2011 at 11:13 pm

I like the word planet.

It has a certain gravitas.

[/ducks out before IVAN3MAN starts to throw planets and other pun stuff on me.]

Log in to Reply

Torbjorn Larsson OM April 24, 2011 at 11:00 pm

I was going to say something along the lines of Jorge and Jason, there is a continuum with clutter. Especially the tail characteristic in itself strikes me as silly. Mercury’s tail is 6-16 planet radii IIRC.

It is together that these things (solar heated CO atmosphere, tail; like many comets) makes a tentative impression.

Also, it does shore up support for the value of the planet criteria which is (more or less) based on orbital properties alone, regardless of the individual pathway and (most of) the internal physical characteristics of the population.

Log in to Reply

Nyx April 25, 2011 at 5:21 am

More hints we get, more Pluto become fascinating. In 2015 Pluto will be 26 years after its perihelion (Sept. 1989, Wikipedia). The next one will be in 2237. While 2015 it’s not very late, I feel like we missed some opportunities and maybe discoveries. I still remember astronomers urging a mission, in the previous decades. But I’m happy and curious about New Horizons. I can’t wait to see what New Horizons will find there.

Log in to Reply

Andrew James April 25, 2011 at 6:45 am

Your name is closely associated with Pluto, so I’d expect you should know all about this (or at least you muse would!).

Log in to Reply

the_alien_in_me April 25, 2011 at 10:13 am

Ah, humans, silly as ever… of course there is a continuum in nature, be it in shapes or sizes, colours or compositions. we just can’t seem to live without putting everything into little boxes, each one with a very distinct name. ok, i won’t dispute the usefulness of such a scheme in many instances, but we should be aware that in many other occasions it just doesn’t reflect the strangeness of things in the real universe…
as for the word planet, we should keep in mind that earth also was not considered a planet, until copernicus. so, if pluto was “demoted”, can we say the earth was “promoted”? probably not, i mean, once the centre of the universe… there’s only one way, and it’s down in the scale of importance…. ;-)

Log in to Reply

Torbjorn Larsson OM April 25, 2011 at 2:57 pm

As long as we are philosophizing, well, yes, but the other side of the coin is that both graduations and classifications are useful as you note.

Also I like to ponder how we can use or impose non-linearities to make qualitative as opposed to quantitative distinctions.

For example, transistors are continuous analog devices, yet by design we can make them operate as digital devices in the discrete regime. (Maybe a bad example since the device non-linearity isn’t strictly necessary for the circuit non-linearity.) Never underestimate the power of a little bit.

Log in to Reply

Dark Gnat April 25, 2011 at 2:57 pm

The idea of classifying planets and other bodies into two or three catagories is rather silly.

Maybe we should extend the OBAFGKMLT should extend down to planets. It’s partly there, as L and T represent brown dwarfs.

Base them on mass, size and composition. Allow for a seperate catagory for rogue/ejected bodies.

Log in to Reply

Andrew James April 25, 2011 at 3:54 pm

Stars emit radiation from their surfaces or atmospheres, planets absorb more radiation than they receive emit radiation.

You also missed some of the classes. I.e. Y (<600K) [None observed so far, though exoplanet Wasp 12b could be one!] Also below M are the 'C' and 'S' classes, and above O is the W class or Wolf Rayets (as either WC or WN subclasses) P and Q are planetary nebulae and Q for novae.

If you wished to add a letter for planets, sadly all twenty-six are assigned to some spectral aspect or another I.e. E= emission lines, H=Non-polarised magnetic white dwarf, V=variable, X, Y and Z are stellar metallicities, and R to Z can be confused for variable stars.

Perhaps we can use the "Star Trek" classification of planets. I.e Memory Alpha's (The earth, for instance, is a M type planet!)

Log in to Reply

Andrew James April 25, 2011 at 3:58 pm

Err…. Easteritis:
Correction: “Stars emit more radiation from their surfaces or atmospheres than they receive, planets absorb more radiation from their surfaces or atmospheres than they emit.”
Now I feel better…

Log in to Reply

Jorge April 25, 2011 at 4:22 pm

Well, I don’t think it’s all that silly. In biology, earlier taxonomy (before all the genetic stuff was discovered) was based on dichotomical keys. With the vast difference that biology is inherently a historical process, wereas astronomy is much more phisics-driven (i.e., in biology what the ancestors of a given living species are play a fundamental role in determining what that species is, wereas in astronomy what a given body is, wilst also influenced by its history, is much more influenced by the phisical constraints on what it can be, given its mass and proximity to other masses and energy sources), that idea of dichotomical keys can also be used as a starting point for astronomical classification. You just have to find the criteria that minimize the inevitable grey areas.

For instance: upwards to a certain mass range, all objects will undergo fusion of deuterium in their cores; downwards they won’t; upwards we have brown dwarfs (which I much rather call “planetars” as in “intermediate objects between planets and stars”, because they are neither brown nor particularly dwarfish), downwards we have planets.

Upwards of a certain mass range, all objects will round up due to their own gravity, reaching states close to hidrostatic equilibrium; downwards, they won’t and their shape will be determined by chemical bounds, not gravity; upwards we have planets, downwards we have minor objects.

(note that these two complementary dichotomies are based on the same thing: mass. As all complementary dichotomies should be.)

And so on. We can and should use the same dichotomical approach when subcategorizing, sub-subcategorizing, etc. If we choose the basic criteria well, we can go as deep as we’d like without having to touch the upper levels. We can dichotomize the planets by the presence or absence of a superficial discontinuity, thus separating gas giants from the rest; we can dichotomize the planets by how dominant they are in their local orbital neighborhoods, thus separating belt planets from main planets; we can dichotomize those that have a superficial discontinuity by it being a liquid or a solid interface; we can dichotomize those that have a solid interface by it being made of silicates or what is commonly referred to as ices, and so on, and so forth.

Log in to Reply

Dark Gnat April 25, 2011 at 2:58 pm

Sorry for the typo.

Log in to Reply

Aqua April 25, 2011 at 5:52 pm

Now THAT would be a BIG comet! Is there a size limit for the largest comet possible?

Log in to Reply

Sowff April 25, 2011 at 7:08 pm

Pluto is a planet and Charon is one of its three moons. If someone also wants to call Charon a binary and secondary planet, that is fine, but it is still also a moon of Pluto. Mercury also has a tail. No comet has an atmosphere and 3 moons orbiting it, or even two moons and a binary object orbiting it. The whole comet talk thing is silly.



Eris Gets Dwarfed (Is Pluto Bigger?)

Something happened on 2010 November 6th that could upend the bragging rights among the kingpins of trans-Neptunian space. If the early results hold up, this time it's the dwarf planet Eris's turn to be demoted, and Pluto might have just regained its status as the largest object in the Kuiper Belt.

Path of Eris's occultation
The final prediction for an occultation by Eris yielded a track across northern Chile, home to many professional observatories.
Marcelo Assafin & others
The scene for this drama is high in the Chilean Andes, where yesterday three teams of astronomers caught Eris in the act of passing directly in front of a 17th-magnitude star in north-central Cetus. Astronomers had known about this occultation well in advance, but the predicted path had shifted up and down along South America, leaving astronomers unsure who — if anyone — might see the event.

Well, the news from Chile is that three observing teams, all using relatively modest telescopes, saw the star blink out.

According to Emmanuël Jehin (University of Liège, Belgium), who tracked the event with the 24-inch (60-cm) TRAPPIST telescope at La Silla Observatory, "I got seven frames where the light of the star is gone." Since the images were taken 4½ seconds apart, Jehin says the occultation lasted about 27 seconds.

Star occulted by Eris
Images taken every 4½ seconds clearly show the brief disappearance of a star behind the dwarf planet Eris on November 6, 2010 (Universal Time).
Emmanuël Jehin & others / TRAPPIST Observatory
Then word came that the occultation had been witnessed by two more telescopes some 460 miles (740 km) to the north. Sebastian Saravia, Alain Maury, and Caisey Harlingten saw the star disappear for 76 seconds through Harlingten's 20-inch (50-cm) PlaneWave telescope at the San Pedro de Atacama Celestial Explorations Observatory. (Maury's account of the event is here.)

The outage was also recorded by a remotely operated 16-inch (40-cm) telescope in another dome at the SPACE site, under the control of Jose-Luis Ortiz (Institute of Astrophysics of Andalusia, Spain).

Any positive result would be cause enough to celebrate — never before has an occultation involved an object so far away. But successful observations from widely separated sites create two chords across Eris's shadow that yields a unique solution for its diameter (assuming that the object is spherical).

Animation of Eris occultation
This video from a 20-inch telescope at San Pedro de Atacama Celestial Explorations Observatory shows how Eris briefly covered a 17th-magnitude star during an occultation on November 6, 2010. (Video frame rate is faster than real time.)
Alain Maury / SPACE Observatory
That number, according to Bruno Sicardy (Paris Observatory), is hard to pin down exactly because timings derived from the three telescopes' light curves have some uncertainty. Even so, Sicardy notes in an email, "Almost certainly Eris has a radius smaller than 1,170 km" — and that would make it ever-so-slightly smaller than Pluto, whose radius is thought to be 1,172 (±10) km. Don't be surprised if the final value gets pushed another 50 or 60 km lower.

"This is huge," notes occultation guru David Dunham. And you'll get no disagreement from Mike Brown (Caltech), who headed the team that discovered Eris several years ago. "Most of the ways we have of measuring the sizes of objects in the outer solar system are fraught with difficulties," Brown notes on his website. "But precisely timed occultations like these have the potential to provide incredibly precise answers."

We've gotten comfortable thinking that Eris is larger than Pluto, and why not? That's what Wikipedia says, so it must be true! Seriously, images taken in December 2005 by Brown and others with the Hubble Space Telescope indicated a diameter of 1,500 miles (2,400 km), just 5% larger than Pluto's. But the true size remained uncertain because even Hubble's supersharp gaze is only barely able to resolve Eris's disk. (Remember: it's some 9 billion miles from the Sun, three times as far away as Pluto.

Observations made with the Spitzer Space Telescope yielded a diameter nearer 1,600 miles (2,600 km), and another group, using the IRAM radio telescope in Spain, upped the value to something closer to 1,900 miles (3,000 km). However, astronomers now realize that Eris's spin axis is pointing toward the Sun, an aspect that would keep the sunlit hemisphere warmer than average and skew any infrared measurements toward higher values.

SPACE Observatory in Chile
A sunset view of San Pedro de Atacama Celestial Explorations Observatory in Chile. The 20-inch (50-cm) telescope at left and the 16-inch (40-cm) telescope two domes away successfully recorded Eris's stellar occultation on November 6, 2010.
Alain Maury
The relatively brief occultation (it had been predicted to last nearly 2 minutes), combined with negative observations from a pair of telescopes in Argentina, suggested right away that Eris wasn't as large as thought. So the outcome from Chile is actually more in line with the Hubble result from 2005.

Eris's mass, determined from the orbit of its moon Dysnomia, is about 125% that of Pluto — and that's unchanged. So if the occultation results hold up, then the density of Eris must be higher, 2.5 g/cm3 or more, and its albedo (reflectivity) at least 90%.

"A year ago I would have declared that result to be thoroughly crazy," Brown commented today via email, "as it just seems unreasonable that Eris would have a density that high" — though he adds, "The albedo is already so ridiculously high that just a little more ridiculousness is OK." But earlier this year Brown and others found that the largish Kuiper Belt denizen 50000 Quaoar is essentially a dense rock (averaging at least 3 g/cm3). "So I would say that densities, too, are up in the air," Brown comments.

One more thing: occultations are very powerful probes of whether an object has an atmosphere, and for now all the team members I contacted are mum on whether Eris has one. The object's high albedo, combined with its great distance, suggests that the surface must be absorbing too little sunlight to vaporize any ice present there. But astronomers were shocked to find (during an occultation in 1988) that Pluto has a thin atmosphere — so stay tuned!

Posted by Kelly Beatty, November 7, 2010
related content: Solar system news



Halfway to Pluto, New Horizons Wakes Up in 'Exotic Territory'

Play AudioDownload AudioJoin Mailing List

June 18, 2010:  Zipping through space at nearly a million miles per day, NASA's New Horizons probe is halfway to Pluto and just woke up for the first time in months to look around.

New Horizons Update (new horizons, 200px)
An artist's concept of New Horizons.

"Our spacecraft is way out in exotic territory, in the middle of nowhere," says Hal Weaver, New Horizons project scientist at Johns Hopkins University. "And we have a lot to do."

It's the perfect opportunity to test New Horizon's instruments before the probe reaches Pluto in 2015. "We don't want to miss a single breathtaking moment during the Pluto encounter," he says. "So we're checking everything out now to make sure we're ship-shape and ready to go."

The 9 weeks of testing commenced on May 25th. Mission controllers plan a thorough checkout and recalibration of all seven science instruments onboard.

First up is LORRI, the Long-Range Reconnaissance Imager, one of the largest interplanetary telescopes ever flown.

"On July 14, 2015, the date of closest approach, we'll be able to distinguish objects on Pluto as small as a football field," says Weaver. "That's about 300 times better resolution than anything we have now."

LORRI will be working together with "Ralph," a spectrometer designed to probe the surface of Pluto at visible and infrared wavelengths. Ralph will reveal Pluto's temperature, color, and chemical composition.

"During the current tests, we'll point both LORRI and Ralph at something in the sky to make sure they can be operated together with full sensitivity. Since New Horizons is far from any large bodies right now, we'll aim the cameras at a star field to test them."

New Horizons Update (Pluto sundogs, 550px)
Space artist Ron Miller's concept of hypothetical geysers and sundogs on Pluto.

Just after New Horizons passes Pluto in 2015, Pluto will appear as a waning crescent to the spacecraft looking back at it. Weaver speculates that during this phase LORRI might be able to spot hazes high in Pluto's atmosphere or perhaps evidence for cryo-volcanism (i.e., volcanoes that spew icy cold material rather than hot magma) on Pluto's surface.

"During a flyby of Neptune in 1989, Voyager 2 spotted dark streaks on Neptune's moon Triton that appear to have been produced by geysers spewing dirty, frozen nitrogen particles. We could see something similar on Pluto."

New Horizons Update (instruments, 200px)
Click here for information about New Horizon's seven main science instruments.

As New Horizons passes into Pluto's shadow in 2015, a UV imaging spectrometer named "Alice" will look back toward the sun through Pluto's atmosphere. This should reveal how molecules in Pluto's atmosphere absorb sunlight, and thus what the atmosphere is made of.

"We know the sun's extreme brightness can make these activities difficult for our instruments. So we'll set up the same angles during the current testing to determine what we'll really be able to see and what kinds of information can be pulled out."

Cameras and spectrometers won't be the only busy instruments. REX, New Horizon's Radio Science EXperiment, will detect and observe radio signals coming all the way from NASA's Deep Space Network on Earth.

"The way those signals bend as they pass through Pluto's atmosphere will tell us a lot about the atmosphere's pressure and thickness."

The team is counting on test results to be "all systems go." If everything checks out, there's a lot to look forward to.

"We have high hopes for this mission," concludes Weaver. "Pluto awaits. The most exciting thing is that we don't know what we're going to see when we get there."

Stay tuned for 2015.

Author: Dauna Coulter | Editor: Dr. Tony Phillips | Credit: Science@NASA

More Information

New Horizons -- home page

Check it Out: System Tests, Science Observations and a Course Correction -- latest news about New Horizon's ongoing wake up period

Ever Farther Across the Ocean of Space to a Distant and Unknown Shore -- an essay by New Horizons principal investigator Alan Stern


February 25, 2010

The Approach Begins

Another milestone passed! Today Feb 25,  NASA’s New Horizons Spacecraft is 15.96 astronomical units (about 2.39 billion kilometers, or 1.48 billion miles) from the Sun – putting it HALFWAY between Earth’s location on Launch Day in January 2006, and Pluto’s place during New Horizons’ encounter with the planet in July 2015.

“From here on out, we’re on approach to an encounter with the Pluto system,” says New Horizons Principal Investigator Alan Stern, from the Southwest Research Institute. “The second half of the journey begins.”

This is rare territory; New Horizons is just the FIFTH probe, after Pioneers 10 - 11 and Voyagers 1 – 2, to traverse interplanetary space so far from the Sun. SEE THE CHARTS BELOW. And it’s the first to travel so far to reach a new planet for exploration.

Humming along at more than 16 kilometers per second – more than 36,600 miles per hour - the spacecraft will next cross a planetary boundary in March 2011, when it passes the orbit of Uranus. Follow the journey.


The Charts Below show the current positions and other interesting data of the five spacecraft which are leaving the Solar System on escape trajectories - our first emissaries to the stars. The graphics and data table are generated dynamically and so always represent the latest positions.

The New Horizons Spacecraft on its way to Pluto has been added to the table and the charts.

View of Orbit from above Ecliptic Plane

View from 10 degrees above Ecliptic Plane
  Pioneer 10 Pioneer 11 Voyager 2 Voyager 1 New Horizons
Distance from Sun (AU) 100.255 80.386 91.504 112.720 16.034
Speed relative to Sun (km/s) 12.081 11.443 15.486 17.077 16.467
Speed relative to Sun (AU/year) 2.548 2.414 3.267 3.602 3.474
Ecliptic Latitude 3.0° 14.4° -33.0° 34.9° 1.7°
Declination (J2000) 25.83° -8.73° -54.43° 12.08° -21.52°
Right Ascension (J2000) 5.063 hrs 18.738 hrs 19.858 hrs 17.172 hrs 18.616 hrs
Constellation Tau Sct Tel Oph Sgr
Distance from Earth (AU) 100.099 80.901 92.039 112.809 16.507
One-way light time (hours) 13.87 11.21 12.76 15.64 2.29
Magnitude of Sun from spacecraft -16.7 -17.2 -16.9 -16.4 -20.7
Spacecraft still functioning ? No No Yes Yes Yes
Launch date Mar 3, 1972 Apr 6, 1973 Aug 20, 1977 Sep 5, 1977 Jan 19, 2006

January 19, 2010

Four Years and Counting

NASA's New Horizons mission team marks four years of flight  as of
Jan 19 – and their Pluto-bound spacecraft is sleeping right through the celebration.

Operators at the Johns Hopkins University Applied Physics Laboratory in Laurel, Md., commanded New Horizons into hibernation on Friday after 10 days of maintenance, during which they downloaded Student Dust Counter data from the solid-state recorder, uploaded software updates to the Solar Wind Around Pluto (SWAP) instrument, made minor adjustments to the spacecraft’s fault-protection system and collected navigation-tracking data.

The spacecraft will remain in hibernation – with the Student Dust Counter powered on and taking more data – until the team wakes it up for an annual checkout that begins 2010 May 25. “This was a short but successful maintenance operation,” says New Horizons Project Manager Glen Fountain, of APL. “The spacecraft is in great shape and continuing on its way, and we’ll keep preparing for Annual Checkout 4 in the spring.”

At 2 p.m. EST on Jan. 19, 2006, New Horizons roars off the launch pad aboard an Atlas V rocket from Complex 41 on Cape Canaveral Air Force Station, Fla.
New Horizons, which launched from Cape Canaveral Air Force Station, Fla., on Jan. 19, 2006, is more than 1.4 billion miles from the Sun – and closer to Pluto than its home planet.  “We’re nearly 1,500 days into flight, and many space missions don't even last that long,” says New Horizons Principal Investigator Alan Stern, of the Southwest Research Institute. “We already have much to be proud of – the fastest spacecraft launch from Earth, and an incredible Jupiter encounter [in early 2007] that could have been a mission in itself. Amazingly, we still have 2,000 days to go before we reach Pluto."

Launch Reflections

The New Horizons science team gathers for its annual meeting today and tomorrow at APL. Among the status reports and discussions of Kuiper Belt Object search plans and data-collection opportunities during the cruise to Pluto, you can expect some reflecting on the start of New Horizons’ historic voyage.

“It doesn’t seem like it’s been four years since New Horizons was launched because we’ve had so much to do during this time,” says New Horizons Project Scientist Hal Weaver, of APL.

Weaver says that after the science team completes most of its Pluto encounter planning this year, it will turn attention toward the data they expect to get out of New Horizons – and optimizing that by pushing various facilities (such as large ground-based telescopes, new space telescopes and networks of small, mobile telescopes) to learn as much about the Pluto system as possible before the New Horizons flyby in July 2015.

“The years 2014-2015 are going to be a ‘golden age’ of planetary exploration as the Dawn mission has its rendezvous with the asteroid Ceres, the Rosetta mission drops a probe on comet 67P/Churyumov-Gerasimenko, and New Horizons passes through the Pluto system,” Weaver says. “I can hardly wait!”

Denis Bogan (left) and Kurt Lindstrom cut the cake to celebrate New Horizons’ fourth launch anniversary during the New Horizons science team meeting at the Johns Hopkins University Applied Physics Laboratory on Jan. 19.  Bogan was the New Horizons program scientist at NASA Headquarters when the mission launched in January 2006; Lindstrom, now at APL, was the NASA New Horizons program executive.  

New Horizons: NASA's Pluto-Kuiper Belt Mission
HomeOverviewScienceMissionSpacecraftEducationNews CenterGalleryLinks
Mission Timeline
Where is New Horizons Now
 • Current Position
 • Passing The Planets
The Team
Mission FAQs


Where Is New Horizons?

The computer-generated images below are simulated views of New Horizons' location in the solar system. The images were created using the Satellite Tool Kit (STK) software, which was developed by Analytical Graphics, Inc.

Images are Updated Every Hour at the Site BELOW:

Click here to follow New Horizons as it Passes Each Planet's Orbit,
 so far, starting with our own Moon.

Next Planetary Orbital Pass: Uranus

New Horizons Will Pass the Orbit of Uranus on 2011 March 18

    New Horizons Will Pass the Orbit of  Neptune on 2014 August 24
       New Horizons Will Reach the Orbit of Pluto on 2015 July 14

  Position of Date in the Charts

This image shows New Horizons' current position dated. The green segment of the line shows where New Horizons has traveled since launch; the red indicates the spacecraft's path toward Jupiter, Pluto and beyond. Positions of stars with magnitude 12 or brighter are shown from this perspective, which is above the Sun and "north" of Earth's orbit.

What Is an AU? The graphics on these pages note New Horizons' distance from Earth, Jupiter and Pluto in AU, or Astronomical Units. One AU is the average distance between the Sun and Earth, about 93 million miles or 149.6 million kilometers.

Heliocentric Velocity. The current position graphic also notes the spacecraft's heliocentric velocity - its speed with respect to the Sun - in kilometers per second. One kilometer per second is equivalent to 0.62 miles per second, or 2,237 miles per hour.


The image “” cannot be displayed, because it contains errors.

What Is 2002 JF56? On June 13, 2006, New Horizons passed approximately 102,000 kilometers (nearly 63,500 miles) from asteroid 2002 JF56, a small, relatively unknown body less than 5 kilometers (3 miles) in diameter. At that distance, the asteroid was so small that the Ralph imager could just barely resolve it. (The higher-resolution LORRI camera couldn't open its door until late August, to guard against accidental Sun pointing. So while LORRI could have resolved the asteroid, the camera couldn't observe it for safety reasons.)

But the "encounter" with 2002 JF56 was still highly useful to New Horizons. The team successfully tested Ralph's optical navigation and moving-target-tracking capabilities, using this flight test to gain valuable experience in tracking moving targets for the Jupiter and Pluto flybys.

In January 2007 the asteroid was renamed "APL" in honor of The Johns Hopkins University Applied Physics Laboratory's leading role in New Horizons and other NASA missions.

Full Trajectory: Overhead View

This image shows New Horizons' current position along its full planned trajectory. The green segment of the line shows where New Horizons has traveled since launch; the red indicates the spacecraft's future path. Positions of stars with magnitude 12 or brighter are shown from this perspective, which is above the Sun and "north" of Earth's orbit.

The image “” cannot be displayed, because it contains errors.

Full Trajectory - Side View

This image shows New Horizons' current position along its full planned trajectory. The green segment of the line shows where New Horizons has traveled since launch; the red indicates the spacecraft's future path. Positions of stars with magnitude 12 or brighter are shown from this perspective, which is slightly above the orbital plane of the planets.

The image “” cannot be displayed, because it contains errors.

NASA - National Aeronautics and Space Administration The Johns Hopkins University Applied Physics Laboratory Department of Energy Southwest Research Institute JPL KinetX
Editor: JHU/APL Webmaster
JHU/APL Official: Ms. K. Beisser
Last verified: 1/03/10
+ Contact JHU/APL
Back to New Horizons Main Page

Latest NewsLatest News RSS Feed

November 12, 2009
New Horizons Roused for Long-Distance Checkup

Call it a burst of activity between naps: the New Horizons team woke its Pluto-bound spacecraft from hibernation this week for some onboard housekeeping. +Read More

New Horizons bumper sticker

September 8, 2009
New Horizons Hits Halfway Mark Between Saturn, Uranus Orbits

New Horizons sails silently today through another milestone on the way to its historic reconnaissance of the Pluto system, reaching the halfway point between the orbits of Saturn and Uranus. +Read More

New Horizons bumper sticker

September 2, 2009
The PI’s Perspective: Science Never Sleeps

Mission Principal Investigator (PI) Alan Stern reviews the last spacecraft checkout, and offers a brief look at what’s in store for the New Horizons team (and spacecraft) this fall. +Read More

The PI's Perspective

Farewell 2009 by ALAN STERN - Principal Investigator (PI)

December 2, 2009

New Horizons is now more than 1,400 days into its 9.5-year journey and well past 15 AU (astronomical units) from the Sun. We still have about 2,050 days ahead of us before we reach the Pluto system, but on Dec. 29, we’ll reach the first of several midway milestones. As the graph below shows, New Horizons will be closer to Pluto (the red line) than to Earth (the blue curve). This marker puts a nice capstone on 2009, during which we moved another 500 million kilometers closer to our favorite planet, so far against the deep.

And on this long journey, things continue to go well. On Nov. 20 we wrapped up 10 days of hibernation wake-up activities and put the spacecraft back into hibernation until early January. The main goal of the wake-up period was to repoint our communications dish antenna to keep up with the changing position of the Earth around the Sun. (By the way, it’s this same motion that causes the blue “Earth” line in the figure above to wiggle back and forth.)

"It's going to make it!" by 7-year-old Piper Benson of Mexico City, depicting New Horizons, with the Pluto-Charon-Nix-Hydra system in the corner, and a small Sun. Piper was born when New Horizons was being designed and will just about be a teenager when we arrive in 2015.

This chart shows the distance of New Horizons to both Earth (blue curve) and Pluto (red line) versus date.

During last month’s wake-up we also downloaded several months of stored science data from the Venetia Burney Student Dust Counter, corrected a recently discovered bug in our fault protection system software, completed about a dozen tracking passes to help refine our trajectory, and uploaded instructions to run the spacecraft through early January. All of the goals of the November wake-up were successfully completed, and telemetry shows that New Horizons is in very good health and almost exactly on its planned course.

Since I wrote you in early September, our ground team has been a lot busier than our spacecraft has, since they never hibernate. In addition to planning both the just-completed November wake-up and the upcoming 10-day January (2010) wake-up, they’ve also completed all but a few final details of the nine-day, Pluto-closest-approach encounter command load for 2015 and verified this command load on the spacecraft simulators at the Johns Hopkins University Applied Physics Laboratory in Laurel, Md. Our ground team has also been hard at work planning next summer’s Active Checkout (ACO), which will run from late May to early July. 

The 2010 ACO is our fourth of the mission, so it’s called ACO-4. Unlike ACO-3, which was very light on activity (to give our ground team more time to work on Pluto encounter planning), ACO-4 will be chockablock with scheduled activities. Among these will be a complete spacecraft and instrument checkout; instrument calibrations, to look for changes since our last set of calibrations in 2008; a trajectory correction maneuver, our first since 2007; a little cruise science focusing on the interplanetary environment and Uranus and Neptune imaging; more fault-protection software upgrades; some tests associated with activities we’ll be conducting at Pluto; and our first-ever full length encounter mode test on the spacecraft.

And just in case you think the ground team still doesn’t have enough to do, they have also begun the detailed planning of the final few weeks of our approach to Pluto that precedes the nine-day close encounter period they’ve already planned out. All of this, mind you, by a team that is about 10 times smaller than the venerable Voyager team when its Uranus and Neptune flybys were planned in the mid- and late-1980s.

Before I close, I want to mention two other items you might find interesting. First, on Nov. 17, a crew from the PBS television series NOVA visited New Horizons at APL for a special, hosted by Neil deGrasse Tyson, that is scheduled to air in early March. The episode will concern the subject of Neil’s book, “The Pluto Files.” I was interviewed in our control center and also met with Neil to discuss Pluto’s planethood. We’ve posted a few pictures from the shoot here.

Speaking of that always-hot topic, there’s a fascinating new book out on the same called “The Case for Pluto,” written by MSNBC’s science guy, Alan Boyle. I was asked to help critique early drafts, and in my opinion, Boyle has thought harder and more deeply about the topic than any journalist ever has. It’s a fun read too.

Alan Stern and Neil deGrasse Tyson of NOVA SCIENCE NOW in the New Horizons control room at the Johns Hopkins Applied Physics Lab for NOVA’s episode concerning Pluto, its planethood and New Horizons.The NOVA crew came to APL on Nov. 17.

Alan Boyle’s newly released book on Pluto and the debates over what kinds of objects are planets makes entertaining and enlightening reading. (Wiley)

We’re also planning our annual New Horizons science team meeting, which occurs every January near the anniversary of our launch in 2006. Among the topics we’ll discuss in detail are plans to start searching for Kuiper Belt Objects that we hope to fly by and reconnoiter after Pluto. Those searches will begin next summer and continue through 2011 and 2012. Hopefully, they’ll net us four to 10 potential targets.

That’s my update for now. Thanks for following our journey to a new frontier. I’ll be back in January, after the next time we wake up our spacecraft. In the meantime, keep on exploring, just as we do!

- Alan Stern

View The PI's Perspective Archive >

March 2nd, 2009

Pluto's Atmosphere Boasts Methane, Warmer Temps

Written by Anne Minard

Artist’s impression of how the surface of Pluto might look, if patches of pure methane rest on the surface. At the distance of Pluto, the Sun appears about 1,000 times fainter than on Earth. Credit: ESO

Pluto is certainly frigid, but new research has revealed its atmosphere is a bit warmer.

Astronomers using the European Southern Observatory's Very Large Telescope have found unexpectedly large amounts of methane in Pluto's atmosphere, which evidently helps it stay about 40 degrees warmer than the dwarf planet's surface. The atmosphere warms to -180 degrees Celsius (-356 degrees Fahrenheit), compared to a surface that's usually -220 degrees Celsius (-428 degrees Fahrenheit).

With lots of methane in the atmosphere, it becomes clear why Pluto’s atmosphere is so warm,” said Emmanuel Lellouch of the Observatoire de Paris in France. Lellouch is lead author of the paper reporting the results, which is in press at the journal Astronomy and Astrophysics.

Pluto, which is about a fifth the size of Earth, is composed primarily of rock and ice and orbits about 40 times further from the Sun than the Earth.

It has been known since the 1980s that Pluto also has a thin, tenuous atmosphere. Abundant nitrogen, along with traces of methane and probably carbon monoxide, are held to the surface by an atmospheric pressure only about one hundred thousandth of that on Earth, or about 0.015 millibars. As Pluto moves away from the Sun, during its 248 year-long orbit, its atmosphere gradually freezes and falls to the ground. In periods when it is closer to the Sun — as it is now — the temperature of Pluto’s solid surface increases, causing the ice to sublimate into gas.

Until recently, only the upper parts of the atmosphere of Pluto could be studied. By observing stellar occultations, a phenomenon that occurs when a Solar System body blocks the light from a background star, astronomers were able to demonstrate that Pluto’s upper atmosphere was some 50 degrees warmer than the surface. Those observations couldn’t shed any light on the atmospheric temperature and pressure near Pluto’s surface. But unique, new observations made with the CRyogenic InfraRed Echelle Spectrograph (CRIRES), attached to ESO’s Very Large Telescope, have now revealed that the atmosphere as a whole, not just the upper atmosphere, has a mean temperature much less frigid than the surface.

Usually, air near the surface of the Earth is warmer than the air above it, largely because the atmosphere is heated from below as solar radiation warms the Earth’s surface, which, in turn, warms the layer of the atmosphere directly above it. Under certain conditions, this situation is inverted so that the air is colder near the surface of the Earth. Meteorologists call this an inversion layer, and it can cause smog build-up.

Most, if not all, of Pluto’s atmosphere is thus undergoing a temperature inversion: the temperature is higher, the higher in the atmosphere you look. The change is about 3 to 15 degrees per kilometer (about 3.3 feet). On Earth, under normal circumstances, the temperature decreases through the atmosphere by about 6 degrees per kilometer.

The reason why Pluto’s surface is so cold is linked to the existence of Pluto’s atmosphere, and is due to the sublimation of the surface ice; much like sweat cools the body as it evaporates from the surface of the skin, this sublimation has a cooling effect on the surface of Pluto.

The CRIRES observations also indicate that methane is the second most common gas in Pluto’s atmosphere, representing half a percent of the molecules. “We were able to show that these quantities of methane play a crucial role in the heating processes in the atmosphere and can explain the elevated atmospheric temperature,” said Lellouch.

Two different models can explain the properties of Pluto’s atmosphere. In the first, the astronomers assume that Pluto’s surface is covered with a thin layer of methane, which will inhibit the sublimation of the nitrogen frost. The second scenario invokes the existence of pure methane patches on the surface.

“Discriminating between the two will require further study of Pluto as it moves away from the Sun,” says Lellouch. “And of course, NASA’s New Horizons space probe will also provide us with more clues when it reaches the dwarf planet in 2015.”

LEAD IMAGE CAPTION: Artist’s impression of how the surface of Pluto might look, if patches of pure methane rest on the surface. At the distance of Pluto, the Sun appears about 1,000 times fainter than on Earth. Credit: ESO

Source: ESO

Enjoyed this article? Subscribe to the feed.

Filed under: Pluto, Solar System

 The PI's Perspective

The PI’s Perspective: One-Third Down

March 19, 2009 UPDATE

We passed the milestone of being of the distance to Pluto last year, but  on March 19, 2009 — after 38 months and almost 2 billion kilometers of flight, New Horizons has completed precisely one-third of the days in its journey to Pluto. That’s quite a milestone, and we on the mission team celebrate the closing of this chapter of our historic journey across the great expanse of our planetary system, and the opening of mid-cruise, as I described in my January postingne-third o

But you won’t have to wait another three years for our next significant distance and flight-time milestones — they come next year, when we cross the halfway point! But whenever quoting such milestones, I have to be careful about the meaning. So when will our spacecraft be halfway to Pluto? Well, that depends on which halfway you mean. (No, I am not kidding.)
New Horizons (on its Atlas V launch vehicle) leaves Earth on January 19, 2006. (NASA)
New Horizons (on its Atlas V launch vehicle) leaves Earth on January 19, 2006. (NASA)
  • If one asks, when have we traveled half the flight time to reach Pluto? That halfway milestone occurs on October 17, 2010.

  • But if one asks, when will we be half as far from the Sun as Pluto will be at the time of our encounter on July 14, 2015? That occurs somewhat earlier, on April 20, 2010.

  • And if one asks, when will we have traveled half the heliocentric distance to Pluto from our launch at Earth? That milestone occurs even earlier, on February 25, 2010.

So, as you can see, the answer to the halfway question depends on precisely what it means to be halfway. In fact, you could even ask when the halfway day was from New Horizons project inception on December 20, 2000, to Pluto arrival on July 14, 2015 — that was April 4, 2008. Or one could ask about the halfway day from when we began our push for a Pluto mission on May 4, 1989 (when we had our first meeting with NASA officials) until Pluto encounter — that was on June 9, 2002. So in some ways we’re already halfway there, and in other ways, we have well over a year to go to reach the halfway point.

Our goal — closest approach to the Pluto system — will occur on July 14, 2015. This is an enchanting new artist’s conception of Pluto’s frosty surface, with Charon in the background. (ESO/L. Calçada)
Our goal — closest approach to the Pluto system — will occur on July 14, 2015. This is an enchanting new artist’s conception of Pluto’s frosty surface, with Charon in the background. (ESO/L. Calçada)
But regardless of which halfway milestone you like, we still have a great deal of work ahead of us and a long time to steward our craft across the great gulf of space to the Kuiper Belt, and we’re looking forward to that.

Since I wrote in early January, New Horizons has remained in hibernation, reporting on its health once or twice a week. New Horizons continues to do very well in flight. We’re almost 100 days into our current hibernation period, and besides taking care of itself autonomously, New Horizons is collecting data with the Venetia Burney Student Dust Counter. During those 11 weeks, New Horizons traveled out about another 70 million miles (112 million kilometers), and we’re just three weeks from reaching 13 astronomical units from the Sun.

It’s hard to believe how far we are from anything else created by humankind. Except for our own, now-derelict third stage, nothing made by people or from the Earth — nothing — is within more than a billion miles of New Horizons.

Back on the ground, we’re just beginning to plan for this year’s annual wakeup from hibernation, which will begin on July 7 and last several weeks.

This year’s wakeup is different from our previous ones in two significant respects. The first difference is that we’ll be conducting the checkout in the summer rather than the fall. Summer checkouts will be our norm from here out, allowing us to get into the rhythm of spring planning and summer activity that is necessary for the July 2015 encounter. The second and even more significant difference between past wakeups and this one is that we’re going to minimize the activities to save time for our mission planners, who are busy writing Pluto encounter close-approach sequences with a deadline of 2010. The minimal wakeup for 2009 also saves us fuel, since we won’t be de-spinning the spacecraft, conducting complex pointed observations with our scientific instruments, and then spinning up again to prepare for the next hibernation cycle.

Although we plan more active wakeups that involve de-spinning and conducting many pointed observations in 2010, 2011, 2013 and 2014, 2012’s checkout will be another minimal one, again saving us labor and fuel.

That catches you up on where we are in our flight plan, but there’s one more thing I want to write about.

When New Horizons arrives in 2015, we will learn a great deal about Pluto’s atmosphere. But for now, Earth-based studies are our only way of making advances.

Early last month, scientists published an exciting new scientific result about Pluto’s atmosphere. Using spectra obtained during an August 2008 event when Pluto occulted a star in the sky over South America, a European team led by Emmanuel Lellouch learned that Pluto’s atmospheric methane (CH4) abundance is now about 0.5 percent, somewhat less than previous measurements over a decade ago. (Methane gas was discovered in Pluto’s atmosphere in the 1990s by New Horizons Co-investigator Leslie Young and mission collaborator Jim Elliot.) Why the CH4 abundance is decreasing isn’t known, but it might be related to the onset of atmospheric and surface cooling as Pluto draws away from the Sun.

Lellouch and company also found that Pluto’s surface pressure is between about 7 and 24 microbars. A microbar is one one-millionth of an Earth atmospheric pressure, so Pluto’s surface pressure is between 1/300th and 1/1000th as on Mars’s surface. Lellouch and his coworkers also found that Pluto’s diameter must be 2,340 kilometers or larger — so previous estimates of 2,300 kilometers or less have now been ruled out — another cool result. Congratulations to Emmanuel and his research team for these exciting new insights into a fascinating planet out on the frontier of the planetary system.

Well, that’s it for now. I'll be back in touch just before we begin checkout in July. In the meantime, keep on exploring, just as we do!

- Alan Stern - the Principal Investigator

View The PI's Perspective Archive >


Pluto in Hi-Def
Click on image to enlarge.
Pluto in Hi-Def
Release Date: Jan 24, 2008
Keywords: LORRI, Pluto
This image demonstrates the first detection of Pluto using the high-resolution mode on the New Horizons Long-Range Reconnaissance Imager (LORRI). The mode provides a clear separation between Pluto and numerous nearby background stars. When the image was taken on October 6, 2007, Pluto was located in the constellation Serpens, in a region of the sky dense with background stars.

Typically, LORRI’s exposure time in hi-res mode is limited to approximately 0.1 seconds, but by using a special pointing mode that allowed an increase in the exposure time to 0.967 seconds, scientists were able to spot Pluto, which is approximately 15,000 times fainter than human eyes can detect.

New Horizons was still too far from Pluto (3.6 billion kilometers, or 2.2 billion miles) for LORRI to resolve any details on Pluto’s surface – that won’t happen until summer 2014, approximately one year before closest approach. For now the entire Pluto system remains a bright dot to the spacecraft’s telescopic camera, though LORRI is expected to start resolving Charon from Pluto – seeing them as separate objects – in summer 2010.

Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute
« Prev  Next » 


Launch Plus Three Years:
Looking Back, Looking Ahead


On the anniversary of New Horizons’ launch from Cape Canaveral Air Force Station, Fla., on January 19, 2006, mission team members reflect on liftoff, a busy first three years of flight and the ongoing voyage to Pluto and beyond.

Countdown to Liftoff

To Ralph McNutt, of the Johns Hopkins Applied Physics Laboratory (APL), launch wasn’t just a beginning – it was the culmination of a hard-fought, nearly two-decade-long battle in the scientific community to secure a mission to the ninth planet.

“When the announcer hit ‘zero’ and the Atlas V rocket began plowing its way through the wispy skin of this pale blue dot we call home, it was a special moment,” says McNutt, principal investigator of the New Horizons Pluto Energetic Particle Spectrometer Science Investigation, or PEPSSI. “We really were on the way, and no one could stop us from taking that path to new lands.”
Since January 19, 2006...
• New Horizons has traveled more than 1.21 billion miles (2.08 billion kilometers).

• The spacecraft’s primary computer has executed 443,380 commands from Earth.

Science team co-investigator Richard Binzel of the Massachusetts Institute of Technology remembers the run-up to launch, a four-year concert of spacecraft design, build and testing, and mission planning that had to reach its crescendo by January 2006, in time to meet a month-long launch period and take advantage of an opportunity to use Jupiter’s gravity as a slingshot toward deeper space.
“The transition from launch to flight is truly phenomenal,” Binzel says. “Before launch, the clock looms so large.  Everything has to be ready at the launch window, or else!  In cruise phase the pace of hard work continues, but now the responsibility feels different. We know New Horizons will reach Pluto!”

Streaks across the sky: Greg Bolt of Perth, Australia, captured these telescope images of New Horizons and its third-stage rocket after launch. Read about them here.


    Next: Asteroids and Jupiter >



Map of Pluto based on Charon eclipses, approximately true colour and giving the highest resolution possible with current technology
Discovered by Clyde W. Tombaugh
Discovery date February 18, 1930
MPC designation 134340 Pluto
Minor planet
category (MPC)
dwarf planet,
and KBO
Adjective Plutonian
Epoch J2000
Aphelion 7 375 927 931 km
49.305 032 87 AU
Perihelion 4 436 824 613 km
29.658 340 67 AU
Semi-major axis 5 906 376 272 km
39.481 686 77 AU
Eccentricity 0.248 807 66
Orbital period 90 613.305 days
248.09 years
Synodic period 366.73 days
Average orbital speed 4.666 km/s
Inclination 17.141 75°
11.88° to Sun's equator
Longitude of ascending node 110.303 47°
Argument of perihelion 113.763 29°
Satellites 3
Physical characteristics
Mean radius 1,195 km[1]
0.19 Earths
Surface area 1.795×107 km²
0.033 Earths
Volume 7.15×109 km³
0.006 6 Earths
Mass (1.305 ± 0.007)×1022 kg[2]
0.002 1 Earths
0.178 moon
Mean density 2.03 ± 0.06 g/cm³[2]
Equatorial surface gravity 0.58 m/s²
0.059 g
Escape velocity 1.2 km/s
Sidereal rotation
−6.387 230 day
6 d 9 h 17 m 36 s
Equatorial rotation velocity 47.18 km/h
Axial tilt 119.591 ± 0.014° (to orbit)[2][3]
North pole right ascension 133.046 ± 0.014°[2]
North pole declination -6.145 ± 0.014°[2]
Albedo 0.49–0.66 (varies by 35%)[4][1]
Surface temp.
min mean max
33 K 44 K 55 K
Apparent magnitude up to 13.65 (mean is 15.1)[1]
Absolute magnitude (H) −0.7[5]
Angular diameter 0.065" to 0.115"[1][6]
Surface pressure 0.30 Pa (summer maximum)
Composition nitrogen, methane

Pluto (pronounced en-us-Pluto.ogg /ˈpluːtoʊ/ (help·info), from Latin: Plūto, Greek: Πλούτων), formal designation (134340) Pluto, is the second-largest known dwarf planet in the Solar System (after Eris) and the tenth-largest body observed directly orbiting the Sun. Originally classified as a planet, Pluto is now considered the largest member of a distinct population called the Kuiper belt.[7]

Like other members of the Kuiper belt, Pluto is composed primarily of rock and ice and is relatively small: approximately a fifth the mass of the Earth's Moon and a third its volume. It has a highly eccentric and highly inclined orbit.

Pluto's eccentricity takes it from 30 to 49 AU (4.4–7.4 billion km) from the Sun, causing Pluto to occasionally come closer to the Sun than Neptune. Pluto and its largest moon, Charon, are often treated together as a binary system because the barycentre of their orbits does not lie within either body.[8] The International Astronomical Union (IAU) has yet to formalise a definition for binary dwarf planets, and until it passes such a ruling, Charon is classified as a moon of Pluto.[9] Pluto has two known smaller moons, Nix and Hydra, discovered in 2005.[10]

From its discovery in 1930 until 2006, Pluto was considered the Solar System's ninth planet. In the late 20th and early 21st centuries, however, many objects similar to Pluto were discovered in the outer solar system, notably the scattered disc object Eris, which is 27% more massive than Pluto.[11] On August 24, 2006 the IAU defined the term "planet" for the first time. This definition excluded Pluto, which the IAU reclassified as a member of the new category of dwarf planets along with Eris and Ceres.[12] After the reclassification, Pluto was added to the list of minor planets and given the number[13][14] 134340. A number of scientists continue to suggest that Pluto should be reclassified as a planet.[15] 




Clyde W. Tombaugh, the discoverer of Pluto.

In the 1840s, using Newtonian mechanics, Urbain Le Verrier predicted the position of the then-undiscovered planet Neptune after analysing perturbations in the orbit of Uranus.[16] Subsequent observations of Neptune in the late 19th century caused astronomers to speculate that Uranus' orbit was being disturbed by another planet in addition to Neptune. In 1906, Percival Lowell, a wealthy Bostonian who had founded the Lowell Observatory in Flagstaff, Arizona in 1894, started an extensive project in search of a possible ninth planet, which he termed "Planet X".[17] By 1909, Lowell and William H. Pickering had suggested several possible celestial coordinates for such a planet.[18] Lowell and his observatory conducted his search until his death in 1916, but to no avail. Unbeknownst to Lowell, on March 19, 1915, his observatory would capture two faint images of Pluto, but would not recognise them for what they were.[19][18]

Due to a ten-year legal battle with Constance Lowell, Percival's widow, who attempted to wrest the observatory's million-dollar portion of his legacy for herself, the search for Planet X did not resume until 1929,[20] when its director, Vesto Melvin Slipher, summarily handed the job of locating Planet X to Clyde Tombaugh, a 23-year-old Kansas man who had just arrived at the Lowell Observatory after Slipher had been impressed by a sample of his astronomical drawings. [20]

Discovery photographs of Pluto

Tombaugh's task was systematically to image the night sky in pairs of photographs taken two weeks apart, then examine each pair and determine whether any objects had shifted position. Using a machine called a blink comparator, he rapidly shifted back and forth between views of each of the plates, to create the illusion of movement of any objects that had changed position or appearance between photographs. On February 18, 1930, after nearly a year of searching, Tombaugh discovered a possible moving object on photographic plates taken on January 23 and January 29 of that year. A lesser-quality photograph taken on January 21 helped confirm the movement.[21] After the observatory obtained further confirmatory photographs, news of the discovery was telegraphed to the Harvard College Observatory on March 13, 1930.[18]


Venetia Burney, the girl who named Pluto
See also: Pluto (mythology)

The right to name the new object belonged to the Lowell Observatory. Tombaugh urged Slipher to suggest a name for the new object quickly before someone else did.[17] Name suggestions poured in from all over the world. Constance Lowell proposed Zeus, then Lowell, and finally her own first name. These suggestions were disregarded.[22]

The name Pluto was first suggested by Venetia Burney (later Venetia Phair), an eleven-year-old schoolgirl in Oxford, England.[23] Venetia was interested in classical mythology as well as astronomy, and considered the name, one of the alternate names of Hades, the Greek god of the Underworld, appropriate for such a presumably dark and cold world. She suggested it in a conversation with her grandfather Falconer Madan, a former librarian of Oxford University's Bodleian Library. Madan passed the name to Professor Herbert Hall Turner, who then cabled it to colleagues in America.[24]

The object was officially named on March 24, 1930.[25] Each member of the Lowell Observatory was allowed to vote on a short-list of three: "Minerva" (which was already the name for an asteroid), "Cronus" (which had garnered a bad reputation after being suggested by an unpopular astronomer named Thomas Jefferson Jackson See), and Pluto. Pluto received every vote.[26] The name was announced on May 1, 1930.[23] Upon the announcement, Madan gave Venetia five pounds as a reward.[23]

The name Pluto was intended to evoke the initials of the astronomer Percival Lowell, a desire echoed in the P-L monogram that is Pluto's astronomical symbol (♇).[27] Pluto's astrological symbol resembles that of Neptune (), but has a circle in place of the middle prong of the trident ().

In Chinese, Japanese, Korean the name was translated as underworld king star (冥王星),[28][29] as suggested by Houei Nojiri in 1930.[30] Many other non-European languages use a transliteration of "Pluto" as their name for the object; however, some Indian languages may use a form of Yama, the Guardian of Hell in Hindu mythology, such as the Gujarati Yamdev.[28] Vietnamese also uses the Vietnamese name for Yama (Diêm Vương) as the name of the planet.

Naming Pluto (Review)

Written by Ian O'Neill

Naming Pluto Promotional PosterFather Films)

Naming Pluto explores the chain of events that lead to Pluto's naming and in 2007 sees Venetia Phair viewing Pluto for the very first time through a telescope, on her 89th birthday, 77 years after Pluto's discovery. A wonderful, intimate look into the story behind how Pluto got its name. A review of the short film directed and produced by Ginita Jimenez, distributed by Father Films.

In recent years, Pluto has seen its status change from being a planet to what many people view as a planetary underclass. The reasons behind this have been set out by the International Astronomical Union (IAU) to cater for the increasing number of Solar System bodies being discovered; the traditional nine planets have had to make room for a growing minor planet population. Unfortunately, Pluto was at the front line as it inhabits a region of space dominated by the gas giant Neptune, plus thousands of other Kuiper belt objects. Although the mysterious body lost its planetary status (as it does not have the ability to "clear its own orbit"), it has taken the title of "dwarf planet" and now has an entire class of object named in its honour: "Plutoids".

However, the recent tumultuous history of the traditional "9th planet" has not impacted the fascination we have for Pluto. It has, and always will be, viewed with intrigue and wonder.

The key to Pluto's romantic tale begins in the year 1930 when a mysterious heavenly was discovered by Clyde Tombaugh, a 23 year-old astronomer working at the Lowell Observatory in Flagstaff, Arizona. However, the honour of naming Pluto didn't rest on Tombaugh's shoulders. Over 5000 miles away in Oxford (UK) an 11 year old girl was having breakfast with her grandfather, wondering what this newly discovered planet should be called…

The Pluto system seen from the surface of Hydra (NASA)Naming Pluto starts out with some stunning visuals from 2006 of NASA's New Horizons Pluto mission launching from Cape Canaveral. Throughout the opening tour of the Solar System, we can hear the voice of Venetia Burney as she is interviewed by NASA Public Affairs officer Edward Goldstein during the launch.

When Goldstein asks whether she had ever seen Pluto through a telescope, the clear and articulate voice of Venetia replies, "I don't think I have. I've just seen a photograph." And so the journey begins, where Venetia explains her fascination with Pluto and a number of experts (including the enigmatic Sir Patrick Moore) help to explain the facts behind the discovery of Pluto to the scientific endeavour of the search for "Planet X".

One of the key moments is when Venetia is describing when she decided on the name for the heavenly body. At age 11, had an acute interest in ancient mythology, so she chose the name because Pluto is the Roman god of the underworld; a fitting name considering the cold, dark nature of Pluto's 248 year orbit. In a fortuitous chain of events, her grandfather, a former librarian of Oxford University's Bodleian Library, passed the suggestion via letter to Professor Herbert Hall Turner saying that his granddaughter had chosen a "thoroughly suitable name: PLUTO." Hall Turner, thrilled with the candidate name, sent Venetia's idea to colleagues in the USA, at the Lowell Observatory.

Venetia talks about her relationship with Pluto. A model of NASA's New Horizons probe is also in view (© Father Films)

Venetia talks about her relationship with Pluto. A model of NASA's New Horizons probe is also in view (© Father Films)

Pouring a cup of tea, Venetia recounts that historic day in 1930. "It was about 8 o'clock and I was having breakfast with my mother and my grandfather," she says very matter of factually. "My grandfather, as usual, opened the paper, The Times, and in it he read that a new planet had been discovered. He wondered what it should be called. We all wondered. And then I said, "why not call it Pluto?" And the whole thing stemmed from that."

A special delight is when Venetia visits St. Anne's Primary School in Surrey to participate in their class project all about Pluto. It goes to show that even young school children have fallen under Pluto's spell. One 9 year-old pupil, Katie, shares her concerns about Pluto's demotion, "Some people say that Pluto isn't a real planet, so I'm looking forward to Venetia coming because I want to find out if that's true."

Sir Patrick Moore shares his views on Pluto (© Father Films)

Sir Patrick Moore shares his views on Pluto (© Father Films)

Legendary astronomer Sir Patrick Moore enthusiastically gives his views on Pluto too, having co-authored a 1980 book with discoverer Tombaugh called Out of the Darkness: The Planet Pluto, he is the ideal character to defend the demotion from planet to dwarf planet by the IAU saying, "It's not demoted! [...] you can call it whatever you like. It's there!" I have been a huge fan of Sir Patrick's writing, and his regular BBC program The Sky at Night is essential astronomy watching, and has been for the last 50 years!

Other guests on the film uncover the various attributes of Pluto's discovery, delving into the history and future of the planetary lightweight on the outermost reaches of the Solar System.

Venetia meets Patrick at his home for the second time (© Father Films)

Venetia meets Patrick at his home for the second time (© Father Films)

The Naming Pluto adventure culminates in 2006 when Venetia and Sir Patrick meet (for the second time) at his West Sussex home to make an attempt at observing Pluto through the telescope in his garden. Patrick was overjoyed to see Venetia again and chuckles as he introduces her to the camera crew, "The lady who named Pluto!"

"Yes, indeed," the ever gracious Venetia replies, smiling.

Unfortunately, the UK summer weather conspired against the possibility of clear skies, and any chance of Patrick's 15" reflector of spying Pluto was lost. However, there is a fantastic twist in the tale, bringing the whole film to a wonderfully emotional ending.

All in all, Naming Pluto is a fabulous tribute, not only to Venetia, but to the astronomical process. Although Pluto has undergone a change in status these last few years, it remains an important, permanent feature of the Solar System. This well-crafted story gives the viewer an excellent overview of Pluto's discovery, naming and the magic it holds today for the 9 year-olds at St. Anne's to Venetia who named the planet nearly 80 years ago…

For more information about Pluto, check out the Guide to Space: Pluto »

A big thank you goes to writer, director and producer Ginita Jimenez for sharing this magnificent production with me. My copy will have pride of place with my growing collection of space science DVDs, a timeless memento of a historic time for astronomy.

If you want your own copy, or want to buy it as a gift, contact Ginita at:

Venetia Phair

From Wikipedia, the free encyclopedia

Venetia Katherine Douglas Phair
DIED ON 2009 APRIL 30 - AGE 90

Venetia Phair at age 11
Born Venetia Burney
July 11, 1918
Died April 30, 2009 (aged 90)
Known for Naming Pluto
Spouse(s) Edward Maxwell Phair (c1920-2006) (m. 1947–2006) «start: (1947)–end+1: (2007"Marriage: Edward Maxwell Phair (c1920-2006) to Venetia Phair" Location: (linkback:
Children Patrick
Parents Charles Fox Burney
Ethel Wordsworth Madan
Relatives Falconer Madan, grandfather

Venetia Katherine Douglas Phair (née Burney) (11 July 1918 – 30 April 2009) was the first person to suggest the name Pluto for the object[1]Clyde W. Tombaugh in 1930. At the time, she was 11 years old and lived in Oxford, England. discovered by



Burney was the daughter of Rev. Charles Fox Burney, Oriel Professor of the Interpretation of Holy Scripture at Oxford, and his wife Ethel Wordsworth Madan. Venetia was the granddaughter of Falconer Madan (1851–1935), Librarian of the Bodleian Library of the University of Oxford.[2] Falconer's brother Henry Madan (1838–1901), Science Master of Eton, had in 1878 had suggested the names Phobos and Deimos for the moons of Mars.[3]

On 14 March 1930, Falconer Madan read the story of the new planet's discovery in The Times, and mentioned it to his granddaughter Venetia. She suggested the name Pluto — the Roman God of the Underworld who was able to make himself invisible — and Falconer Madan forwarded the suggestion to astronomer Herbert Hall Turner, who cabled his American colleagues at Lowell Observatory. Tombaugh liked the proposal because it started with the initials of Percival Lowell who had predicted the existence of Pluto. On 1 May 1930, the name Pluto was formally adopted for this new celestial body.

Venetia was educated at Downe House School in Berkshire and Newnham College, Cambridge, where she studied mathematics. After graduation she became a chartered accountant. Later she became a teacher of economics and mathematics at girls’ schools in southwest London. In 1947 she married Edward Maxwell Phair. Her husband, a classicist, later became housemaster and head of English at Epsom College. In recent years she lived in Epsom.[4]

Only a few months before the reclassification of Pluto from a planet to a dwarf planet, with the debate going on about the issue, she said in an interview that "At my age, I've been largely indifferent to [the debate]; though I suppose I would prefer it to remain a planet."[5]

She died on 30 April 2009 in Banstead.[4]


The asteroid 6235 Burney was named in her honour. The Student Dust Counter, an instrument on board the New Horizons spacecraft, is also named after her.[6]

 References and notes

  1. ^ Pluto is no longer regarded as a planet but as a dwarf planet; but at the time Burney named it, it was considered to be a planet.
  2. ^ "Venetia Phair". Daily Telegraph. May 5, 2009. Retrieved on 2009-05-11. "Venetia Phair, who has died aged 90, had the distinction of being the only woman in the world to have named a planet; in 1930, as a girl of 11, she suggested the name Pluto for the enigmatic celestial body that had just been discovered, and which became (albeit only temporarily) the ninth planet in our solar system." 
  3. ^ The Observatory, Vol. 53, p. 193–201 (1930)
  4. ^ a b Grimes, William (May 10, 2009). "Venetia Phair Dies at 90; as a Girl, She Named Pluto". New York Times. Retrieved on 2009-05-11. "Venetia Phair, as she became by marriage, died April 30 in her home in Banstead, in the county of Surrey, England. She was 90. The death was confirmed by her son, Patrick." 
  5. ^ The girl who named a planet from BBC News Online
  6. ^ Johns Hopkins University Applied Physics Laboratory press release, June 29, 2006

External links

Phair, Venetia Katherine Douglas
Burney, Venetia
Gave Pluto its name

Banstead, Oxfordshire, United Kingdom

Demise of Planet X

Once found, Pluto's faintness and lack of a resolvable disc cast doubt on the idea that it could be Lowell's Planet X. Throughout the mid-20th century, estimates of Pluto's mass were often revised downward. In 1978, the discovery of Pluto's moon Charon allowed the measurement of Pluto's mass for the first time. Its mass, roughly 0.2 percent that of the Earth, was far too small to account for the discrepancies in Uranus. Subsequent searches for an alternate Planet X, notably by Robert Sutton Harrington,[31] failed. In 1993, Myles Standish used data from Voyager 2's 1989 flyby of Neptune, which had revised the planet's total mass downward by 0.5 percent, to recalculate its gravitational effect on Uranus. With the new figures added in, the discrepancies, and with them the need for a Planet X, vanished.[32] Today the overwhelming consensus among astronomers is that Planet X, as Lowell defined it, does not exist. Lowell had made a prediction of Planet X's position in 1915 that was fairly close to Pluto's actual position at that time; however, Ernest W. Brown concluded almost immediately that this was a coincidence, a view still held today.[33]

Physical characteristics

Images of Pluto generated using data from the Hubble Space Telescope.
Hubble map of Pluto's surface, showing great variations in albedo.
Possible structure of Pluto.
1. Frozen nitrogen
2. Water ice
3. Silicate and water ice

Pluto's distance from Earth makes in-depth investigation difficult. Many details about Pluto will remain unknown until 2015, when the New Horizons[34] spacecraft is expected to arrive there.

Appearance and composition

Pluto's apparent magnitude averages 15.1, brightening to 13.65 at perihelion.[1] To see it, a telescope is required; around 30 cm (12 in) aperture being desirable.[35] It looks indistinct and star-like even in very large telescopes because its angular diameter is only 0.11". It is light brown with a very slight tint of yellow.[36]

Spectroscopic analysis of Pluto's surface reveals it to be composed of more than 98 percent nitrogen ice, with traces of methane and carbon monoxide.[37][38] Distance and current limits on telescope technology make it impossible to directly photograph surface details on Pluto. Images from the Hubble Space Telescope barely show any distinguishable surface definitions or markings.[39]

The best images of Pluto derive from brightness maps created from close observations of eclipses by its largest moon, Charon. Using computer processing, observations are made in brightness factors as Pluto is eclipsed by Charon. For example, eclipsing a bright spot on Pluto makes a bigger total brightness change than eclipsing a dark spot. Using this technique, one can measure the total average brightness of the Pluto-Charon system and track changes in brightness over time.[40] Maps composed by the Hubble Space Telescope reveal that Pluto's surface is remarkably heterogeneous, a fact also evidenced by its lightcurve and by periodic variations in its infrared spectra. The face of Pluto oriented toward Charon contains more methane ice, while the opposite face contains more nitrogen and carbon monoxide ice. This makes Pluto the second most contrastive body in the Solar System after Iapetus.[41][dubious discuss]

The Hubble Space Telescope places Pluto's density at between 1.8 and 2.1 g/cm³, suggesting its internal composition consists of roughly 50–70 percent rock and 30–50 percent ice by mass.[38] Because decay of radioactive minerals would eventually heat the ices enough for them to separate from rock, scientists expect that Pluto's internal structure is differentiated, with the rocky material having settled into a dense core surrounded by a mantle of ice. It is also possible that such heating may continue today, creating a subsurface ocean of liquid water.[42]

Mass and size

Pluto's volume is about 0.66% that of Earth

Astronomers, assuming Pluto to be Lowell's Planet X, initially calculated its mass on the basis of its presumed effect on Neptune and Uranus. In 1955 Pluto was calculated to be roughly the mass of the Earth, with further calculations in 1971 bringing the mass down to roughly that of Mars.[43] However, in 1976, Dale Cruikshank, Carl Pilcher and David Morrison of the University of Hawaii calculated Pluto's albedo for the first time, finding that it matched that for methane ice; this meant Pluto had to be exceptionally luminous for its size and therefore could not be more than 1 percent the mass of the Earth.[43][44]

The discovery of Pluto's satellite Charon in 1978 enabled a determination of the mass of the Pluto–Charon system by application of Newton's formulation of Kepler's third law. Once Charon's gravitational effect on Pluto was measured, estimates of Pluto's mass fell to 1.31×1022 kg—less than 0.24 percent that of the Earth.[45] Observations of Pluto in occultation with Charon were able to fix Pluto's diameter at roughly 2390 km.[46] With the invention of adaptive optics astronomers were able to determine its shape accurately.[47]

GanymedeTitanCallistoIoMoonEuropaTritonPlutoFile:Pluto compared2.jpg
Pluto (bottom right) compared in size to the largest satellites in the solar system (from left to right and top to bottom): Ganymede, Titan, Callisto, Io, the Moon, Europa, and Triton

Among the objects of the Solar System, Pluto is not only smaller and much less massive than any planet, but at less than 0.2 lunar masses it is also less massive than seven moons: Ganymede, Titan, Callisto, Io, Earth's Moon, Europa and Triton. Pluto is more than twice the diameter and a dozen times the mass of the dwarf planet Ceres, the largest object in the asteroid belt. However, it is smaller than the dwarf planet Eris, a trans-Neptunian object discovered in 2005.


Pluto's atmosphere consists of a thin envelope of nitrogen, methane, and carbon monoxide, derived from the ices on its surface.[48] As Pluto moves away from the Sun, its atmosphere gradually freezes and falls to the ground. As it edges closer to the Sun, the temperature of Pluto's solid surface increases, causing the ices to sublimate into gas. This creates an anti-greenhouse effect; much like sweat cools the body as it evaporates from the surface of the skin, this sublimation has a cooling effect on the surface of Pluto. Scientists using the Submillimeter Array have recently discovered that Pluto's temperature is 43 K (−230.15 °C), 10 K colder than expected.[49]

Artist's conception of the New Horizons spacecraft passing over Pluto, showing its tenuous atmosphere

Pluto was found to have an atmosphere from an occultation observation in 1985; the finding was confirmed and significantly strengthened by extensive observations of another occultation in 1988. When an object with no atmosphere occults a star, the star abruptly disappears; in the case of Pluto, the star dimmed out gradually.[50]

From the rate of dimming, the atmospheric pressure was determined to be 0.15 pascal, roughly 1/700 000 that of Earth.[51]

In 2002, another occultation of a star by Pluto was observed and analysed by teams led by Bruno Sicardy of the Paris Observatory,[52] James L. Elliot of MIT,[53] and Jay Pasachoff of Williams College.[54] The atmospheric pressure was estimated to be 0.3 pascal, even though Pluto was farther from the Sun than in 1988 and thus should have been colder and had a more rarefied atmosphere. One explanation for the discrepancy is that in 1987 the south pole of Pluto came out of shadow for the first time in 120 years, causing extra nitrogen to sublimate from the polar cap. It will take decades for the excess nitrogen to condense out of the atmosphere.[55] Another stellar occultation was observed by the MIT-Williams College team of James Elliot, Jay Pasachoff, and a Southwest Research Institute team led by Leslie Young on June 12, 2006 from sites in Australia.[56]

In October 2006, Dale Cruikshank of NASA/Ames Research Center (a New Horizons co-investigator) and his colleagues announced the spectroscopic discovery of ethane on Pluto's surface. This ethane is produced from the photolysis or radiolysis (i.e., the chemical conversion driven by sunlight and charged particles) of frozen methane on Pluto's surface and suspended in its atmosphere.[57]


Orbit of Pluto – ecliptic view. This 'side view' of Pluto's orbit (in red) shows its large inclination to Neptune's orbit (in blue). The ecliptic is horizontal

Pluto's orbit is markedly different from those of the planets. The planets all orbit the Sun close to a flat reference plane called the ecliptic and have nearly circular orbits. In contrast, Pluto's orbit is highly inclined relative to the ecliptic (over 17°) and highly eccentric (elliptical). This high eccentricity leads to a small region of Pluto's orbit lying closer to the Sun than Neptune's. Pluto was last interior to Neptune's orbit between February 7, 1979 and February 11, 1999. Detailed calculations indicate that the previous such occurrence lasted only fourteen years, from July 11, 1735 to September 15, 1749, whereas between April 30, 1483 and July 23, 1503, it had also lasted 20 years.

Although this repeating pattern may suggest a regular structure, in the long term Pluto's orbit is in fact chaotic. While computer simulations can be used to predict its position for several million years (both forward and backward in time), after intervals longer than the Lyapunov time of 10–20 million years, it is impossible to determine exactly where Pluto will be because its position becomes too sensitive to unmeasurably small details of the present state of the solar system.[58][59] For example, at any specific time many millions of years from now, Pluto may be at aphelion or perihelion (or anywhere in between), with no way for us to predict which. This does not mean that the orbit of Pluto itself is unstable, however, only that its position along that orbit is impossible to determine far into the future. In fact, several resonances and other dynamical effects conspire to keep Pluto's orbit stable, safe from planetary collision or scattering.

Neptune-avoiding orbit

Orbit of Pluto — polar view. This 'view from above' shows how Pluto's orbit (in red) is less circular than Neptune's (in blue), and how Pluto is sometimes closer to the Sun than Neptune. The darker halves of both orbits show where they pass below the plane of the ecliptic.

Despite Pluto's orbit apparently crossing that of Neptune when viewed from directly above the ecliptic, the two objects cannot collide. This is because their orbits are aligned so that Pluto and Neptune can never approach closely. Several factors contribute to this.

At the simplest level, one can examine the two orbits and see that they do not intersect. When Pluto is closest to the Sun, and hence closest to Neptune's orbit as viewed in a top-down projection, it is also the farthest above the ecliptic. This means Pluto's orbit actually passes above that of Neptune, preventing a collision.[60] Indeed, the part of Pluto's orbit that lies as close or closer to the Sun than that of Neptune lies about 8 AU above the ecliptic,[61] and so a similar distance above Neptune's orbit.[62] Pluto's ascending node, the point at which the orbit crosses the ecliptic, is currently separated from Neptune's by over 21°;[63] their descending nodes are separated by a similar angular distance (see diagram). Since Neptune's orbit is almost flat with respect to the ecliptic, Pluto is far above it by the time the two orbits cross.

This alone is not enough to protect Pluto; perturbations (e.g., orbital precession) from the planets, particularly Neptune, would adjust Pluto's orbit, so that over millions of years a collision could be possible. Some other mechanism or mechanisms must therefore be at work. The most significant of these is a mean motion resonance with Neptune.

This diagram shows the relative positions of Pluto (red) and Neptune (blue) on selected dates. The size of Neptune and Pluto is depicted as inversely proportional to the distance between them to emphasise the closest approach in 1896.

Pluto lies in the 3:2 mean motion resonance of Neptune: for every three orbits of Neptune around the Sun, Pluto makes two. The two objects then return to their initial positions and the cycle repeats, each cycle lasting about 500 years. This pattern is configured so that, in each 500-year cycle, the first time Pluto is near perihelion Neptune is over 50° behind Pluto. By Pluto's second perihelion, Neptune will have completed a further one and a half of its own orbits, and so will be a similar distance ahead of Pluto. In fact, the minimum separation of Pluto and Neptune is over 17 AU; Pluto actually comes closer (11 AU) to Uranus than it does to Neptune.[62]

The 3:2 resonance between the two bodies is highly stable, and is preserved over millions of years.[64] This prevents their orbits from changing relative to one another — the cycle always repeats in the same way — and so the two bodies can never pass near to each other. Thus, even if Pluto's orbit were not highly inclined the two bodies could never collide.[62]

Other factors governing Pluto's orbit

The image “” cannot be displayed, because it contains errors.

Diagram of the argument of perihelion

Numerical studies have shown that over periods of millions of years, the general nature of the alignment between Pluto's and Neptune's orbits does not change.[60][65] However, there are several other resonances and interactions that govern the details of their relative motion, and enhance Pluto's stability. These arise principally from two additional mechanisms (in addition to the 3:2 mean motion resonance).


Pluto's argument of perihelion, the angle between the point where it crosses the ecliptic and the point where it is closest to the Sun, librates around 90°.[65] This means that when Pluto is nearest the Sun, it is at its farthest above the plane of the solar system, preventing encounters with Neptune. This is a direct consequence of the Kozai mechanism,[60] which relates the eccentricity of an orbit to its inclination, relative to a larger perturbing body — in this case Neptune. Relative to Neptune, the amplitude of libration is 38°, and so the angular separation of Pluto's perihelion to the orbit of Neptune is always greater than 52° (= 90°–38°). The closest such angular separation occurs every 10,000 years.[64]

Second, the longitudes of ascending node of the two bodies — the points where they cross the ecliptic - are in near-resonance with the above libration. When the two longitudes are the same — that is, when one could draw a straight line through both nodes and the Sun — Pluto's perihelion lies exactly at 90°, and it comes closest to the Sun at its peak above Neptune's orbit. In other words, when Pluto most closely intersects the plane of Neptune's orbit, it must be at its farthest beyond it. This is known as the 1:1 superresonance.[60]

To understand the nature of the libration, imagine a polar point of view, looking down on the ecliptic from a distant vantage point where the planets orbit counter-clockwise. After passing the ascending node, Pluto is interior to Neptune's orbit and moving faster, approaching Neptune from behind. The strong gravitational pull between the two causes angular momentum to be transferred to Pluto, at Neptune's expense. This moves Pluto into a slightly larger orbit, where it travels slightly slower, in accordance with Kepler's third law. As its orbit changes, this has the gradual effect of changing the pericentre and longitudes of Pluto (and, to a lesser degree, of Neptune). After many such repetitions, Pluto is sufficiently slowed, and Neptune sufficiently speeded up, that Neptune begins to catch Pluto at the opposite side of its orbit (near the opposing node to where we began). The process is then reversed, and Pluto loses angular momentum to Neptune, until Pluto is sufficiently speeded up that it begins to catch Neptune once again at the original node. The whole process takes about 20,000 years to complete.[62][64]


Main article: Moons of Pluto
Pluto and its three known moons. Pluto and Charon are the bright objects in the center, the two smaller moons are at the right and bottom, farther out.
The Pluto system. The region around Pluto and Charon was reduced in brightness so that all four objects could be shown individually in a single image. Photo by David Tholen.

Pluto has three known natural satellites: Charon, first identified in 1978 by astronomer James Christy; and two smaller moons, Nix and Hydra, both discovered in 2005.[66]

The Plutonian moons are unusually close to Pluto, compared to other observed systems. Moons could potentially orbit Pluto up to 53% (or 69%, if retrograde) of the Hill sphere radius, the stable gravitational zone of Pluto's influence. For example, Psamathe orbits Neptune at 40% of the Hill radius. In the case of Pluto, only the inner 3% of the zone is known to be occupied by satellites. In the discoverers’ terms, the Plutonian system appears to be "highly compact and largely empty."[67]


The Pluto-Charon system is noteworthy for being the largest of the solar system's few binary systems, defined as those whose barycentre lies above the primary's surface (617 Patroclus is a smaller example).[68] This and the large size of Charon relative to Pluto has led some astronomers to call it a dwarf double planet.[69] The system is also unusual among planetary systems in that each is tidally locked to the other: Charon always presents the same face to Pluto, and Pluto always presents the same face to Charon. If one were standing on Pluto's near side, Charon would hover in the sky without moving; if one were to travel to the far side, one would never see Charon at all.[70] In 2007, observations by the Gemini Observatory of patches of ammonia hydrates and water crystals on the surface of Charon suggested the presence of active cryo-geysers.[71]

Pluto and Charon, compared to Earth's Moon[2]

(Pronunciation key)

Diameter (km) Mass (kg) Orbital radius (km)
Orbital period (d)
Pluto /ˈpluːtoʊ/ 2390
(70% Moon)
1.305 (7)×1022
(18% Moon)
2 040 (100)
(0.6% Moon)
(25% Moon)
Charon /ˈʃɛərən/,
(35% Moon)
1.52 (7)×1021
(2% Moon)
17 530 (90)
(5% Moon)

Nix and Hydra

Artist's concept of the surface of Hydra. Pluto with Charon (right) and Nix (bright dot on left).
The image “” cannot be displayed, because it contains errors.
Diagram of the Plutonian system. P 1 is Hydra, and P 2 is Nix.

Orbits of 4 bodies in en:Pluto system about barycenter as seen from Earth. Vectorized by Mysid after a NASA image ([1]). The dates are all in the year 2005.

Original summary by en:User:Jyril:

Although a full en:orbital solution for Pluto's satellites cannot be determined from only two Hubble measurements, it turns out that their paths closely follow those expected for objects orbiting the en:Pluto system's barycenter in a perfect circle in the same plane as Charon's orbit. In this diagram, the en:barycenter is the dot in the center, Pluto's orbit is the smallest en:ellipse, Charon's orbit is the next ellipse (its position on May 15 and May 18 are indicated by the filled and open circles, respectively), an orbit consistent with P2's measured positions is next, followed by an orbit consistent with P1's measured positions. For both of the latter cases, the filled squares are positions on May 15 and open squares are positions on May 18. Note that projection effects cause these circular orbits to look elliptical on the plane of the sky.

Two additional moons of Pluto were imaged by astronomers working with the Hubble Space Telescope on May 15, 2005, and received provisional designations of S/2005 P 1 and S/2005 P 2. The International Astronomical Union officially named Pluto's newest moons Nix (or Pluto II, the inner of the two moons, formerly P 2) and Hydra (Pluto III, the outer moon, formerly P 1), on June 21, 2006.[72]

These small moons orbit Pluto at approximately two and three times the distance of Charon: Nix at 48 700 kilometres and Hydra at 64 800 kilometres from the barycenter of the system. They have nearly circular prograde orbits in the same orbital plane as Charon, and are very close to (but not in) 4:1 and 6:1 mean motion orbital resonances with Charon.[73]

Observations of Nix and Hydra to determine individual characteristics are ongoing. Hydra is sometimes brighter than Nix, suggesting either that it is larger or that different parts of its surface may vary in brightness. Sizes are estimated from albedos. The moons' spectral similarity to Charon suggests a 35% albedo similar to Charon's; this value results in diameter estimates of 46 kilometres for Nix and 61 kilometres for the brighter Hydra. Upper limits on their diameters can be estimated by assuming the 4% albedo of the darkest Kuiper Belt objects; these bounds are 137 ± 11 km and 167 ± 10 km, respectively. At the larger end of this range, the inferred masses are less than 0.3% that of Charon, or 0.03% of Pluto's.[74]

The discovery of the two small moons suggests that Pluto may possess a variable ring system. Small body impacts can create debris that can form into planetary rings. Data from a deep optical survey by the Advanced Camera for Surveys on the Hubble Space Telescope suggest that no ring system is present. If such a system exists, it is either tenuous like the rings of Jupiter or is tightly confined to less than 1000 km in width.[75]

In imaging the Plutonian system, observations from Hubble placed limits on any additional moons. With 90% confidence, no additional moons larger than 12 km (or a maximum of 37 km with an albedo of 0.041) exist beyond the glare of Pluto 5 arcseconds from the dwarf planet. This assumes a Charon-like albedo of 0.38; at a 50% confidence level the limit is 8 kilometres.[76]

Kuiper belt

Main article: Kuiper belt
Plot of all known Kuiper belt objects, set against the four outer planets

Pluto's origin and identity have long puzzled astronomers. In the 1950s it was suggested that Pluto was an escaped moon of Neptune, knocked out of orbit by its largest current moon, Triton. This notion has been heavily criticised because, as explained above, Pluto never actually comes near the planet.[77]

Beginning in 1992, astronomers began to discover a large population of small icy objects beyond Neptune that were similar to Pluto not only in orbit but also in size and composition. This belt, known as the Kuiper belt after one of the astronomers who first speculated on the nature of a trans-Neptunian population, is believed to be the source of many short-period comets. Astronomers now believe Pluto to be the largest[7] of the known Kuiper belt objects (KBOs). Like other KBOs, Pluto shares features with comets; for example, the solar wind is gradually blowing Pluto's surface into space, in the manner of a comet.[78] If Pluto were placed as near to the Sun as Earth, it would develop a tail, as comets do.[79]

Though Pluto is the largest of the Kuiper belt objects discovered so far, Triton, which is slightly larger than Pluto, shares many atmospherical and geological composition similarities with Pluto and is believed to be a captured Kuiper belt object.[80] Eris (see below) is also larger than Pluto but is not strictly considered a member of the Kuiper belt population. Rather, it is considered a member of a linked population called the scattered disc.

A large number of Kuiper belt objects, like Pluto, possess a 3:2 orbital resonance with Neptune. KBOs with this orbital resonance are called "plutinos", after Pluto.[81]

Exploration of Pluto

Main article: New Horizons
New Horizons, launched on January 19, 2006

Pluto presents significant challenges for spacecraft because of its small mass and great distance from Earth. Voyager 1 could have visited Pluto, but controllers opted instead for a close flyby of Saturn's moon Titan, resulting in a trajectory incompatible with a Pluto flyby. Voyager 2 never had a plausible trajectory for reaching Pluto.[82] No serious attempt to explore Pluto via spacecraft occurred until the last decade of the 20th century. In August 1992, JPL scientist Robert Staehle telephoned Pluto's discoverer, Clyde Tombaugh, requesting permission to visit his planet. "I told him he was welcome to it," Tombaugh later remembered, "though he's got to go one long, cold trip."[83] Despite this early momentum, in 2000, NASA cancelled the Pluto Kuiper Express mission, citing increasing costs and launch vehicle delays.[84]

After an intense political battle, a revised mission to Pluto, dubbed New Horizons, was granted funding from the US government in 2003.[85] New Horizons was launched successfully on January 19, 2006. The mission leader, S. Alan Stern, confirmed that some of the ashes of Clyde Tombaugh, who died in 1997, had been placed aboard the spacecraft.[86]

In early 2007 the craft made use of a gravity assist from Jupiter. Its closest approach to Pluto will be on July 14, 2015; scientific observations of Pluto will begin 5 months prior to closest approach and will continue for at least a month after the encounter. New Horizons captured its first (distant) images of Pluto in late September 2006, during a test of the Long Range Reconnaissance Imager (LORRI).[87] The images, taken from a distance of approximately 4.2 billion kilometres, confirm the spacecraft's ability to track distant targets, critical for maneuvering toward Pluto and other Kuiper Belt objects.

First Pluto sighting from New Horizons

New Horizons will use a remote sensing package that includes imaging instruments and a radio science investigation tool, as well as spectroscopic and other experiments, to characterise the global geology and morphology of Pluto and its moon Charon, map their surface composition and analyse Pluto's neutral atmosphere and its escape rate. New Horizons will also photograph the surfaces of Pluto and Charon.

Discovery of moons Nix and Hydra may present unforeseen challenges for the probe. Debris from collisions between Kuiper belt objects and the smaller moons, with their relatively low escape velocities, may produce a tenuous dusty ring. Were New Horizons to fly through such a ring system, there would be an increased potential for micrometeoroid damage that could disable the probe.[75]

Planetary status controversy

See also: Definition of planet

Pluto's official status as a planet has been a subject of controversy since at least 1992, when the first Kuiper Belt Object, (15760) 1992 QB1, was discovered. Since then, further discoveries have intensified the debate.

Commemoration as a planet

Pluto is shown as a planet on the Pioneer plaque, an inscription on the space probes Pioneer 10 and Pioneer 11, launched in the early 1970s. The plaque, intended to give information about the origin of the probes to any alien civilization that might in the future encounter the vehicles, includes a diagram of our solar system, showing nine planets.[88] Similarly, an analog image contained within the Voyager Golden Record included on the probes Voyager 1 and Voyager 2 (also launched in the 1970s) includes data regarding Pluto and again shows it as the ninth planet.[89] The Disney character Pluto, introduced in 1930, was also named in honour of the planet.[90] In 1941, Glenn T. Seaborg named the newly created element plutonium in Pluto's honour, in keeping with the tradition of naming elements after newly discovered planets (uranium after Uranus and neptunium after Neptune, although this tradition is also used for some non-planets: cerium is named after Ceres and palladium after Pallas).[91]

New discoveries ignite debate

The EarthDysnomia(136199) ErisCharon(134340) Pluto(136472) Makemake(136108) Haumea(90377) Sedna(90482) Orcus(50000) Quaoar(20000) VarunaFile:EightTNOs.png
Pluto compared to Eris, Makemake, Haumea, Sedna, Orcus, Quaoar, and Varuna compared to Earth (artist's impressions; no detailed photographs exist).

The discovery of the Kuiper belt and Pluto's relation to it led many to question whether Pluto could be considered separately from others in its population. In 2002, the KBO 50000 Quaoar was discovered, with a diameter of roughly 1280 kilometres, about half that of Pluto.[92] In 2004, the discoverers of 90377 Sedna placed an upper limit of 1800 kilometres on its diameter, near Pluto's diameter of 2320 kilometres.[93] Just as Ceresasteroids, so, it was argued, Pluto should be reclassified as one of the Kuiper belt objects. eventually lost its planet status after the discovery of the other

On July 29, 2005, the discovery of a new Trans-Neptunian object was announced. Named Eris, it is now known to be slightly larger than Pluto.[94] This was the largest object discovered in the solar system since Triton in 1846. Its discoverers and the press initially called it the "tenth planet", although there was no official consensus at the time on whether to call it a planet.[95] Others in the astronomical community considered the discovery the strongest argument for reclassifying Pluto as a minor planet.[96]

The last remaining distinguishing features of Pluto were now its large moon, Charon, and its atmosphere. These characteristics are probably not unique to Pluto: several other Trans-Neptunian objects have satellites, and Eris's spectrum suggests that its surface has a composition similar to Pluto's.[97] It also possesses a moon, Dysnomia, discovered in September 2005.

Museum and planetarium directors occasionally created controversy by omitting Pluto from planetary models of the solar system. Some omissions were intentional; the Hayden Planetarium reopened after renovation in 2000 with a model of only eight planets. The controversy made headlines at the time.[98]

IAU decision to reclassify Pluto

The debate came to a head in 2006 with an IAU resolution that created an official definition for the term "planet". According to this resolution, there are three main conditions for an object to be considered a 'planet':

  1. The object must be in orbit around the Sun.
  2. The object must be massive enough to be a sphere by its own gravitational force. More specifically, its own gravity should pull it into a shape of hydrostatic equilibrium.
  3. It must have cleared the neighbourhood around its orbit.[99][100]

Pluto fails to meet the third condition, since its mass was only 0.07 times that of the mass of the other objects in its orbit (Earth's mass, by contrast, is 1.7 million times the remaining mass in its own orbit).[101][102] The IAU further resolved that Pluto be classified in the simultaneously created dwarf planet category, and that it act as the prototype for the plutoid category of trans-Neptunian objects, in which it would be separately, but concurrently, classified.[103]

On September 13, 2006, the IAU included Pluto, Eris, and the Eridian moon Dysnomia in their Minor Planet Catalogue, giving them the official minor planet designations "(134340) Pluto", "(136199) Eris", and "(136199) Eris I Dysnomia".[104] If Pluto had been given a minor planet name upon its discovery, the number would have been a little over a thousand rather than over 100,000. The first minor planet to be found after Pluto was 1164 Kobolda, a month later.

There has been some resistance within the astronomical community toward the reclassification.[105][106][107] Alan Stern, principal investigator with NASA's New Horizons mission to Pluto, has publicly derided the IAU resolution, stating that "the definition stinks, for technical reasons."[108] Stern's current contention is that by the terms of the new definition Earth, Mars, Jupiter, and Neptune, all of which share their orbits with asteroids, would be excluded.[109] His other claim is that because less than five percent of astronomers voted for it, the decision was not representative of the entire astronomical community.[109] Marc W. Buie of the Lowell observatory has voiced his opinion on the new definition on his website and is one of the petitioners against the definition.[110] Others have supported the IAU. Mike Brown, the astronomer who discovered Eris, said "through this whole crazy circus-like procedure, somehow the right answer was stumbled on. It’s been a long time coming. Science is self-correcting eventually, even when strong emotions are involved."[111]

Children protest the reclassification of Pluto, police keep counter-protesters on a different corner.

Among the general public, reception is mixed. Some have accepted the reclassification; others seek to overturn the decision with online petitions urging the IAU to consider reinstatement. A resolution introduced by some members of the California state assembly light-heartedly denounces the IAU for "scientific heresy," among other crimes.[112] The U.S. state of New Mexico's House of Representatives passed a resolution declaring that, in honour of Tombaugh, a longtime resident of that state, Pluto will always be considered a planet while in New Mexican skies, with March 13 being known as "Pluto Planet Day".[113] Some reject the change for sentimental reasons, citing that they have always known Pluto as a planet and will continue to do so regardless of the IAU decision.[114] Others view this rejection as an attempt to bend the rules in order to keep the only planet discovered by an American classified as such.[115]

The ongoing debate over the status of Pluto continues to be acknowledged by the Jet Propulsion Laboratory which, as recently as January 2008, continued to reference it on JPL Photojournal webpages dedicated to Pluto.[116] Researchers on both sides of the debate will gather in August 2008 at Johns Hopkins University for a conference that includes back-to-back talks on the current IAU definition of a planet.[117]


The verb "to pluto" (preterite and past participle: "plutoed") is a neologism coined in the aftermath of the decision. In January 2007, the American Dialect Society chose "plutoed" as its 2006 Word of the Year, defining "to pluto" as "to demote or devalue someone or something", "as happened to the former planet Pluto when the General Assembly of the International Astronomical Union decided Pluto no longer met its definition of a planet."[118][119]

Society president Cleveland Evans stated the reason for the organization's selection of "plutoed": "Our members believe the great emotional reaction of the public to the demotion of Pluto shows the importance of Pluto as a name. We may no longer believe in the Roman god Pluto, but we still have a sense of connection with the former planet."[120]

See also


  1. ^ a b c d e D. R. Williams (September 7, 2006). "Pluto Fact Sheet". NASA. Retrieved on 2007-03-24.
  2. ^ a b c d e f M. W. Buie, W. M. Grundy, E. F. Young, L. A. Young, S. A. Stern (2006). "Orbits and photometry of Pluto's satellites: Charon, S/2005 P1, and S/2005 P2". Astronomical Journal 132: 290. doi:10.1086/504422. arΧiv:astro-ph/0512491. 
  3. ^ Based on the orientation of Charon's orbit, which is assumed the same as Pluto's spin axis due to the mutual tidal locking.
  4. ^ Calvin J. Hamilton (2006-02-12). "Dwarf Planet Pluto". Views of the Solar System. Retrieved on 2007-01-10.
  5. ^ "JPL Small-Body Database Browser: 134340 Pluto". Retrieved on 2008-06-12.
  6. ^ Based on geometry of minimum and maximum distance from Earth and Pluto radius in the factsheet
  7. ^ a b Although Eris is larger than Pluto, it resides in the scattered disc. Wikipedia convention treats this as a distinct region from the Kuiper belt, so Pluto becomes the largest Kuiper belt object
  8. ^ C.B. Olkin, L.H. Wasserman, O.G. Franz (2003). "The mass ratio of Charon to Pluto from Hubble Space Telescope astrometry with the fine guidance sensors-" (PDF). Lowell Observatory 254–259. doi:10.1016/S0019-1035(03)00136-2. Retrieved on 2007-03-13.
  9. ^ O. Gingerich (2006). "The Path to Defining Planets" (PDF). Harvard-Smithsonian Center for Astrophysics and IAU EC Planet Definition Committee chair. Retrieved on 2007-03-13.
  10. ^ B. Sicardy, W. Beisker et al. (2006). "Observing Two Pluto Stellar Approaches In 2006: Results On Pluto's Atmosphere And Detection Of Hydra". Retrieved on 2007-03-13.
  11. ^ "Astronomers Measure Mass of Largest Dwarf Planet". hubblesite (2007). Retrieved on 2007-11-03.
  12. ^ A. Akwagyiram (2005-08-02). "Farewell Pluto?". BBC News. Retrieved on 2006-03-05.
  13. ^ T. B. Spahr (2006-09-07). "MPEC 2006-R19 : EDITORIAL NOTICE". Minor Planet Center. Retrieved on 2006-09-07.
  14. ^ D. Shiga (2006-09-07). "Pluto added to official "minor planet" list". NewScientist. Retrieved on 2006-09-08.
  15. ^ Richard Gray (2008-08-10). "Pluto should get back planet status, say astronomers". The Telegraph. Retrieved on 2008-08-09.
  16. ^ K. Croswell (1997). Planet Quest: The Epic Discovery of Alien Solar Systems. The Free Press. pp. 43. ISBN 978-0684832524. 
  17. ^ a b J. Rao (March 11, 2005). "Finding Pluto: Tough Task, Even 75 Years Later". Retrieved on 2006-09-08.
  18. ^ a b c W. G. Hoyt (1976). "W. H. Pickering's Planetary Predictions and the Discovery of Pluto". Isis 67 (4): 551–564.. doi:10.1086/351668. Retrieved on 27 June 2007. 
  19. ^ Mark Littman (1990). Planets Beyond: Discovering the Outer Solar System. Wiley. pp. 70. ISBN 047151053X. 
  20. ^ a b Croswell, p. 50
  21. ^ Croswell p. 52
  22. ^ B. Mager. "The Search Continues". Pluto: The Discovery of Planet X. Retrieved on 2007-03-27.
  23. ^ a b c P. Rincon (2006-01-13). "The girl who named a planet". Pluto: The Discovery of Planet X. BBC News. Retrieved on 2007-04-12.
  24. ^ K. M. Claxton. "The Planet 'Pluto'". Parents' Union School Diamond Jubilee Magazine, 1891–1951 (Ambleside: PUS, 1951), p. 30–32. Retrieved on 2007-10-15.
  25. ^ "The Trans-Neptunian Body: Decision to call it Pluto". The Times': 15. May 27, 1930. 
  26. ^ Croswell pp. 54–55
  27. ^ "NASA's Solar System Exploration: Multimedia: Gallery: Pluto's Symbol". NASA. Retrieved on 2007-03-25.
  28. ^ a b "Planetary Linguistics". Retrieved on 2007-06-12.
  29. ^ "Uranus, Neptune, and Pluto in Chinese, Japanese, and Vietnamese". Retrieved on 2008-05-24.
  30. ^ Steve Renshaw and Saori Ihara (2000). "A Tribute to Houei Nojiri". Retrieved on 2007-06-12.
  31. ^ P. K. Seidelmann and R. S. Harrington (1987). "Planet X — The current status". U. S. Naval Observatory. Retrieved on 2007-11-04.
  32. ^ Ken Croswell (1993). "Hopes Fade in hunt for Planet X". Retrieved on 2007-11-04.
  33. ^ "History I: The Lowell Observatory in 20th century Astronomy". The Astronomical Society of the Pacific (1994-06-28). Retrieved on 2006-03-05.
  34. ^ "Space Probe Heads To Pluto - Finally". CBS News (2006-01-19). Retrieved on 2007-04-14.
  35. ^ "This month Pluto's apparent magnitude is m=14.1. Could we see it with an 11" reflector of focal length 3400 mm?". Singapore Science Centre. Retrieved on 2007-03-25.
  36. ^ M. Cuk (September 2002). "What color is each planet?". Curious about Astronomy?. Cornell University. Retrieved on 2007-03-25.
  37. ^ Tobias C. Owen, Ted L. Roush et al. (August 6 1993). "Surface Ices and the Atmospheric Composition of Pluto". Science 261 (5122): 745–748. doi:10.1126/science.261.5122.745. PMID 17757212. Retrieved on 29 March 2007. 
  38. ^ a b "Pluto". SolStation (2006). Retrieved on 2007-03-28.
  39. ^ "Hubble Reveals Surface of Pluto for First Time". Hubblesite (1996). Retrieved on 2007-03-26.
  40. ^ E. F. Young; R. P. Binzel; K. Crane (2000). "A Two-Color Map of Pluto Based on Mutual Event Lightcurves". Bulletin of the American Astronomical Society 32: 1083. Retrieved on 26 March 2007. 
  41. ^ Alan Boyle (1999-02-11). "Pluto regains its place on the fringe", MSNBC. Retrieved on 20 March 2007. 
  42. ^ "The Inside Story". New Horizons (2007). Retrieved on 2007-03-29.
  43. ^ a b Croswell p. 57
  44. ^ Pluto's albedo is 1.3–2.0 times greater than that of Earth. "Pluto Fact Sheet". NASA (September 7, 2006). Retrieved on 2007-03-24.
  45. ^ J. Davies (2001). "Beyond Pluto (extract)" (PDF). Royal Observatory, Edinburgh. Retrieved on 2007-03-26.
  46. ^ D. J. Tholen, M. W. Buie, R. P. Binzel, M. L. Frueh (1987). "Improved Orbital and Physical Parameters for the Pluto-Charon System". Science 237 (4814): 512–514. doi:10.1126/science.237.4814.512. PMID 17730324. Retrieved on 26 March 2007. 
  47. ^ L. M. Close, W. J. Merline, D. J. Tholen, T. C. Owen, F. J. Roddier, C. Dumas, (2000). "Adaptive optics imaging of Pluto-Charon and the discovery of a moon around the Asteroid 45 Eugenia: the potential of adaptive optics in planetary astronomy". Proceedings of the International Society for Optical Engineering 4007: 787–795,. Retrieved on 26 March 2007. 
  48. ^ Ken Croswell (1992). "Nitrogen in Pluto's Atmosphere". Retrieved on 2007-04-27.
  49. ^ T. Ker (2006). "Astronomers: Pluto colder than expected". (via Retrieved on 2006-03-05.
  50. ^ "IAUC 4097" (1985). Retrieved on 2007-03-26.
  51. ^ R. Johnston (2006). "The atmospheres of Pluto and other trans-Neptunian objects". Retrieved on 2007-03-26.
  52. ^ B. Sicardy; T. Widemann, et al. (2003-07-10). "Large changes in Pluto's atmosphere as revealed by recent stellar occultations". Nature (Nature) 424: 168. doi:10.1038/nature01766. Retrieved on 5 March 2006. 
  53. ^ "Pluto is undergoing global warming, researchers find". Massachusetts Institute of Technology (2002-10-09). Retrieved on 2007-03-20.
  54. ^ "Williams Scientists Contribute to New Finding About Pluto". Williams College (2003-07-09). Retrieved on 2007-03-20.
  55. ^ R. R. Britt (2003). "Puzzling Seasons and Signs of Wind Found on Pluto". Retrieved on 2007-03-26.
  56. ^ J. L. Elliot, M. J. Person, A. A. S. Gulbis, E. R. Adams, E. A. Kramer, C. A. Zuluaga, R. E. Pike, J. M. Pasachoff, S. P. Souza, B. A. Babcock, J. W. Gangestad, A. E. Jaskot, P. J. Francis, R. Lucas, A. S. Bosh (2006). "The Size of Pluto's Atmosphere As Revealed by the 2006 June 12 Occultation". E Pasadena Division of Planetary Sciences. Retrieved on 2007-04-12.
  57. ^ A. Stern (November 1, 2006). "Making Old Horizons New". The PI's Perspective. Johns Hopkins University Applied Physics Laboratory. Retrieved on 2007-02-12.
  58. ^ Gerald Jay Sussman; Jack Wisdom (1988). "Numerical evidence that the motion of Pluto is chaotic". Science 241: 433–437. doi:10.1126/science.241.4864.433. PMID 17792606. 
  59. ^ Jack Wisdom; Matthew Holman (1991). "Symplectic maps for the n-body problem". Astronomical Journal 102: 1528–1538. doi:10.1086/115978. 
  60. ^ a b c d X.-S. Wan, T.-Y. Huang, and K. A. Innanen (2001). "The 1 : 1 Superresonance in Pluto's Motion". The Astronomical Journal 121 (2): 1155–1162. doi:10.1086/318733. Retrieved on 6 September 2008. 
  61. ^ Maxwell W. Hunter II (2004). "Unmanned scientific exploration throughout the solar system". NASA Programs, Lockheed Missiles & Space Company. Retrieved on 2007-03-28.
  62. ^ a b c d Renu Malhotra (1997). "Pluto's Orbit". Retrieved on 2007-03-26.
  63. ^ David R. Williams. "Planetary Fact Sheet". NASA. Retrieved on 2007-03-31.
  64. ^ a b c Hannes Alfvén and Gustaf Arrhenius (1976). "SP-345 Evolution of the Solar System". Retrieved on 2007-03-28.
  65. ^ a b J. G. Williams; G. S. Benson (1971). "Resonances in the Neptune-Pluto System". Astronomical Journal 76: 167. doi:10.1086/111100. Retrieved on 17 April 2007. 
  66. ^ Guy Gugliotta. "Possible New Moons for Pluto." Washington Post. November 1, 2005. Retrieved on October 10, 2006.
  67. ^ S.A. Stern, H.A. Weaver, A.J. Steffl, M.J. Mutchler, W.J. Merline, M.W. Buie, E.F. Young, L.A. Young, J.R. Spencer (2006). "Characteristics and Origin of the Quadruple System at Pluto" (subscription required). Nature 439: 946–948. doi:10.1038/nature04548. 
  68. ^ Derek C. Richardson and ­ Kevin J. Walsh (2005). "Binary Minor Planets". Department of Astronomy, University of Maryland. Retrieved on 2007-03-26.
  69. ^ B. Sicardy et al. (2006). "Charon's size and an upper limit on its atmosphere from a stellar occultation". Retrieved on 2007-03-26.
  70. ^ Leslie Young (1997). "The Once and Future Pluto". Southwest Research Institute, Boulder, Colorado. Retrieved on 2007-03-26.
  71. ^ "Charon: An ice machine in the ultimate deep freeze". Gemini Observatory (2007). Retrieved on 2007-07-18.
  72. ^ International Astronomical Union (2006-06-21) (PDF). IAU Circular No. 8723 - Satellites of Pluto. Press release. Retrieved on 2007-02-12. 
  73. ^ F. R. Ward; RM Canup (August 25 2006). "Forced Resonant Migration of Pluto's Outer Satellites by Charon". Science 313 (5790): 1107–1109. doi:10.1126/science.1127293. PMID 16825533.;313/5790/1107. Retrieved on 12 February 2007. 
  74. ^ H. A. Weaver; S. A. Stern, M. J. Mutchler, A. J. Steffl, M. W. Buie, W. J. Merline, J. R. Spencer, E. F. Young and L. A. Young (February 23 2006). "Discovery of two new satellites of Pluto" (subscription required). Nature 439 (7079): 943–945. doi:10.1038/nature04547. Retrieved on 3 April 2007. 
  75. ^ a b Andrew J. Steffl; S. Alan Stern (2007). "First Constraints on Rings in the Pluto System" (subscription required). The Astronomical Journal 133 (4): 1485–1489. doi:10.1086/511770. astro-ph/0608036. 
  76. ^ A.J. Steffl; M.J. Mutchler, H.A. Weaver, S.A.Stern, D.D. Durda, D. Terrell, W.J. Merline, L.A. Young, E.F. Young, M.W. Buie, J.R. Spencer (2006). "New Constraints on Additional Satellites of the Pluto System". The Astronomical Journal 132 (2): 614–619. doi:10.1086/505424. 
  77. ^ "Pluto's Orbit". NASA New Horizons (2007). Retrieved on 2007-03-26.
  78. ^ "Colossal Cousin to a Comet?". New Horizons. Retrieved on 2006-06-23.
  79. ^ Neil deGrasse Tyson (1999). "Space Topics: Pluto Top Ten: Pluto Is Not a Planet". The Planetary Society. Retrieved on 2006-06-23.
  80. ^ "Neptune's Moon Triton". The Planetary Society. Retrieved on 2007-03-26.
  81. ^ David Jewitt (2004). "The Plutinos". University of Hawaii. Retrieved on 2007-03-26.
  82. ^ "Voyager Frequently Asked Questions". Jet Propulsion Laboratory (January 14, 2003). Retrieved on 2006-09-08.
  83. ^ Dava Sobel (1993). "The last world". Discover magazine. Retrieved on 2007-04-13.
  84. ^ Dr. David R. Williams (2005). "Pluto Kuiper Express". NASA Goddard Space Flight Center. Retrieved on 2007-03-26.
  85. ^ Robert Roy Britt (2003). "Pluto Mission a Go! Initial Funding Secured". Retrieved on 2007-04-13.
  86. ^ Dr. Alan Stern (2006). "Happy 100th Birthday, Clyde Tombaugh". Southwest Research Institute. Retrieved on 2007-04-13.
  87. ^ "New Horizons, Not Quite to Jupiter, Makes First Pluto Sighting". The John Hopkins University Applied Physics Laboratory (2006-11-28). Retrieved on 2007-03-20.
  88. ^ R.W. Robinett (2001). "Spacecraft Artifacts as Physics Teaching Resources" (PDF). Department of Physics, The Pennsylvania State University. Retrieved on 2007-03-26.
  89. ^ "Space Topics: Voyager- The Golden Record". Planetary Society. Retrieved on 2007-03-26.
  90. ^ Allison M. Heinrichs (2006). "Dwarfed by comparison". Pittsburgh Tribune-Review. Retrieved on 2007-03-26.
  91. ^ David L. Clark and David E. Hobart (2000). "Reflections on the Legacy of a Legend" (PDF). Retrieved on 2007-08-09.
  92. ^ Michael E. Brown and Chadwick A. Trujillo (2006). "Direct Measurement of the Size of the Large Kuiper Belt Object (50000) Quaoar". The American Astronomical Society. Retrieved on 2007-03-26.
  93. ^ W. M. Grundy, K. S. Noll, D. C. Stephens. "Diverse Albedos of Small Trans-Neptunian Objects". Lowell Observatory, Space Telescope Science Institute. Retrieved on 2007-03-26.
  94. ^ "Hubble Finds 'Tenth Planet' Slightly Larger Than Pluto". Hubblesite (2006). Retrieved on 2007-03-26.
  95. ^ "NASA-Funded Scientists Discover Tenth Planet". Jet Propulsion Laboratory (2005). Retrieved on 2007-02-22.
  96. ^ Steven Soter (2006-08-16). "What is a Planet?". Retrieved on 2006-08-24. submitted to The Astronomical Journal, August 16, 2006
  97. ^ Mike Brown (2006). "The discovery of 2003 UB313, the 10th planet.". California Institute of Technology. Retrieved on 2006-05-25.
  98. ^ "Astronomer Responds to Pluto-Not-a-Planet Claim". (February 2, 2001). Retrieved on 2006-09-08.
  99. ^ "IAU 2006 General Assembly: Resolutions 5 and 6" (PDF), IAU (August 24, 2006). 
  100. ^ International Astronomical Union (News Release - IAU0603) (2006-08-24). IAU 2006 General Assembly: Result of the IAU Resolution votes. Press release. Retrieved on 2008-06-15. 
  101. ^ Steven Soter (2007). "What is a Planet?". Department of Astrophysics, American Museum of Natural History. Retrieved on 2007-02-21.
  102. ^ "IAU 2006 General Assembly: Result of the IAU Resolution votes", IAU (August 24, 2006). 
  103. ^ "Plutoid chosen as name for Solar System objects like Pluto". International Astronomical Union (News Release - IAU0804) (June 11, 2008, Paris). Retrieved on 2008-06-11.
  104. ^ Central Bureau for Astronomical Telegrams, International Astronomical Union (2006). "Circular No. 8747" (PDF). Retrieved on 2007-02-23.
  105. ^ Robert Roy Britt (August 24, 2006). "Pluto Demoted: No Longer a Planet in Highly Controversial Definition", Retrieved on 8 September 2006. 
  106. ^ Sal Ruibal (January 6, 1999). "Astronomers question if Pluto is real planet", USA Today. 
  107. ^ Robert Roy Britt (November 21, 2006). "Why Planets Will Never Be Defined", Retrieved on 1 December 2006. 
  108. ^ Robert Roy Britt (August 24, 2006). "Scientists decide Pluto’s no longer a planet", MSNBC. Retrieved on 8 September 2006. 
  109. ^ a b David Shiga (August 25, 2006). "New planet definition sparks furore", Retrieved on 8 September 2006. 
  110. ^ Marc W. Buie (September 2006). "My response to 2006 IAU Resolutions 5a and 6a". Lowell Observatory. Retrieved on 2007-03-20.
  111. ^ Dennis Overbye (2006-08-24). "Pluto Is Demoted to ‘Dwarf Planet’", The New York Times. Retrieved on 20 March 2007. 
  112. ^ DeVore, Edna (September 7, 2006). "Planetary Politics: Protecting Pluto", Retrieved on 8 September 2006. 
  113. ^ C. Holden (2007). "Rehabilitating Pluto" (PDF). Science 315: 1643. doi:10.1126/science.315.5819.1643c. Retrieved on 13 April 2007. 
  114. ^ ""Pluto's still the same Pluto", (October 21, 2006). Retrieved on 1 November 2006. 
  115. ^ ""'Planet' Pluto: America's 'Eternal Embarrassment' (translated by Watching", The Beijing News (August 28, 2006). Retrieved on 8 October 2007. 
  116. ^ JPL Photojournal Jan 24, 2008, accessed 2008-02-14
  117. ^ JR Minkel (April 10, 2008). "Is Rekindling the Pluto Planet Debate a Good Idea?". Scientific American. Retrieved on 2008-04-13.
  118. ^ "“Plutoed” Voted 2006 Word of the Year" (PDF). American Dialect Society (January 5, 2007). Retrieved on 2007-01-07.
  119. ^ "Pluto's revenge: 'Word of the Year' award" (HTML). CNN (January 7, 2007). Retrieved on 2007-01-20.
  120. ^ "'Plutoed' chosen as '06 Word of the Year" (HTML). Associated Press (Jan. 8, 2007). Retrieved on 2007-01-10.

External links

Sister project Wikimedia Commons has media related to: Pluto
Find more about Pluto on Wikipedia's sister projects:

Definitions from Wiktionary
Textbooks from Wikibooks
Quotations from Wikiquote
Source texts from Wikisource
Images and media from Commons
News stories from Wikinews

Learning resources from Wikiversity

v  d  e

Charon · Nix · Hydra

Pluto and its three natural satellites as seen by the Hubble Space Telescope




Other topics

v  d  e
Minor planets navigator

(134339) 5628 T-3       ·       (134340) Pluto       ·       (134341) 1979 MA

v  d  e
Plutoids (trans-Neptunian dwarf planets) and Plutoid candidates

Kuiper belt
Orcus · 2003 AZ84 · Ixion · Pluto · 2005 RN43 · 2002 MS4 · 2002 UX25 · Varuna · Haumea  · 2002 TX300 · 2005 UQ513 · Quaoar · Makemake · 2003 MW12 · 2002 AW197

Scattered disc

See also Triton · List of trans-Neptunian objects · List of objects in the Solar System (by radius · by mass)
For a pronunciation guide, see Centaur and TNO pronunciation. Plutoids are bolded, candidates are not.

v  d  e
Trans-Neptunian objects

Pronunciation guide
Objects in italics are unnumbered.




Other resonant

Scattered disc


Natural satellites

1 Also classified as a dwarf planet.
2 Although currently classified as an SDO, Eris and Sedna may be part of the inner Oort Cloud.
v  d  e
The Solar System

The SunMercuryVenusThe MoonEarthPhobos and DeimosMarsCeresThe asteroid beltJupiterMoons of JupiterSaturnMoons of SaturnUranusMoons of UranusMoons of NeptuneNeptuneMoons of PlutoPlutoMoons of HaumeaHaumeaMakemakeThe Kuiper BeltDysnomiaErisThe Scattered DiscThe Oort Cloud

The Sun · Mercury · Venus · Earth · Mars · Ceres · Jupiter · Saturn · Uranus · Neptune · Pluto · Haumea · Makemake · Eris

See also astronomical objects, the Solar System's list of objects, sorted by radius or mass, and the Solar System Portal
Featured article