EXOPLANETS

EXOPLANETS IS A SHORTENED TERM FOR EXTRASOLAR PLANETS

An extrasolar planet, or exoplanet, is a planet that orbits a star other than the Sun and that is thus beyond the Solar System.

As of 2011 Nov 7 there are 697 planets (573 are planetary systems and 81 are multiple systems) listed in the Extrasolar Planets Encyclopaedia.

THE ASTRONOMY PICTURE OF THE DAY FOR 2010 October 1

Zarmina's World

Illustration Credit & Copyright: Lynette Cook

Explanation: A mere 20 light-years away in the constellation Libra, red dwarf star Gliese 581 has received much scrutiny by astronomers in recent years. Earthbound telescopes had detected the signatures of multiple planets orbiting the cool sun, two at least close to the system's habitable zone -- the region where an Earth-like planet can have liquid water on its surface. Now a team headed by Steven Vogt (UCO Lick), and Paul Butler (DTM Carnagie Inst.) has announced the detection of another planet, this one squarely in the system's habitable zone. Based on 11 years of data, their work offers a very compelling case for the first potentially habitable planet found around a very nearby star. Shown in this artist's illustration of the inner part of the exoplanetary system, the planet is designated Gliese 581g, but Vogt's more personal name is Zarmina's World, after his wife. The best fit to the data indicates the planet has a circular 37 day orbit, an orbital radius of only 0.15 AU, and a mass 3.1 times the Earth's. Modeling includes estimates of a planet radius of 1.5, and gravity at the planet's surface of 1.1 to 1.7 in Earth units. Finding a habitable planet so close by suggests there are many others in our Milky Way Galaxy.

Gliese 581 g

From Wikipedia, the free encyclopedia

Gliese 581 g

The orbits of the Gliese 581 planetary system, with circular orbits, excluding outermost planet f.

Parent star

Gliese 581

Libra

15h 19m 26s

−07° 43′ 20″

10.55

20.3 ± 0.3 ly

(6.2 ± 0.1 pc)

M3V

0.31 M☉

0.29 R☉

3480 ± 48 K

−0.33 ± 0.12

7 – 11 Gyr

(α)

(δ)

(mV)

(m)

(r)

(T)

[Fe/H]

(a)

(e)

(P)

(M)

(K)

0.14601 ± 0.00014[1] AU

0[1]

36.562 ± 0.052[1] d

(0.100 y)

(877 h)

271 ± 48[1]°

1.29 ± 0.19[1] m/s

Physical characteristics

(m sin i)

3.1 ± 0.4[1] M⊕

Discovery information

Discovery date

Discoverer(s)

Detection method

Discovery site

Discovery status

Database references

Gliese 581 g (pronounced /ˈɡliːzə/) or Gl 581 g, is an extrasolar planet, orbiting the red dwarf star Gliese 581, 20.5 light-years (1.94×1014 km) from Earth in the constellation of Libra. It is the sixth planet discovered in the Gliese 581 planetary system and the fourth in order of increasing distance from the star. The planet was discovered by the Lick-Carnegie Exoplanet Survey after a decade of observation. Despite the Gliese 581 system having a "somewhat checkered history of habitable planet claims,"[1] results from the study state that the planet is located in the habitable zone of its parent star, where liquid water is considered a strong possibility.

The discovery of Gliese 581 g was announced in late September 2010. It is believed to be a candidate for the most Earth-like and first Goldilocks planet found outside our solar system, with the greatest potential for harboring life of any found so far. The detection of Gliese 581 g in such a short period of time and at such a close proximity leads astronomers to believe that the proportion of stars with habitable planets may be greater than ten percent.[1]

Discovery

The planet was detected by astronomers in the Lick-Carnegie Exoplanet Survey, led by principal investigator Steven Vogt, professor of astronomy and astrophysics at the University of California, Santa Cruz and co-investigator Paul Butler of the Carnegie Institution of Washington. The discovery was made using radial velocity measurements combining 11 years of data from the HIRES instrument of the Keck 1 telescope and the HARPS instrument of ESO's 3.6m telescope at La Silla Observatory.[1][2]

Gliese 581 g has an orbital period of 37 days, orbiting at a distance of 0.146 AU from its parent star.[1] It is believed to have a mass of 3.1 to 4.3 times that of the Earth and a radius of 1.3 to 2.0 times that of Earth (1.3 to 1.5 times Earth's if predominantly rocky, 1.7 to 2.0 times Earth's if predominantly water ice). Its mass indicates that it is probably a rocky planet with a solid surface. The planet's surface gravity is expected to be in the range of 1.1 to 1.7 times Earth's, enough to hold on to an atmosphere that is likely to be denser than Earth's.[1]

The Lick–Carnegie team explained the results of their research in a paper published in the Astrophysical Journal. Vogt has informally dubbed this planet "Zarmina's World" after his wife's name.[3]

Habitability

Main article: Habitability of red dwarf systems

In an interview with Lisa-Joy Zgorski of the National Science Foundation, Steven Vogt was asked what he thought about the chances of life existing on Gliese 581 g. Vogt was optimistic: "I'm not a biologist, nor do I want to play one on TV. Personally, given the ubiquity and propensity of life to flourish wherever it can, I would say that, my own personal feeling is that the chances of life on this planet are 100%. I have almost no doubt about it."[4] According to the Associated Press interview with Steven Vogt, "Life on other planets doesn't mean E.T. Even a simple single-cell bacteria or the equivalent of shower mold would shake perceptions about the uniqueness of life on Earth."[5]

Temperatures

It is estimated that the average global equilibrium temperature (the temperature in the absence of atmospheric effects) of Gliese 581 g ranges from 209 to 228 K (−64 to −45 °C, or −84 to −49 °F) for Bond albedos (reflectivities) from 0.5 to 0.3 (with the latter being more characteristic of the inner Solar System). Adding an Earth-like greenhouse effect yields an average surface temperature in the range of 236 to 261 K (−37 to −12 °C, or −35 to 10 °F).[1][6] A factor that could potentially give Gliese 581 g a greenhouse effect greater than Earth's is the possibility that the more massive planet also has a more massive atmosphere.[1]

By comparison, Earth's present global equilibrium temperature is 255 K (−18 °C), which is raised to 288 K (15 °C) by greenhouse effects. However, when life evolved early in Earth's history, the Sun's energy output is thought to have been only about 75% of its current value,[8] which would have correspondingly lowered Earth's equilibrium temperature under the same albedo conditions. Yet Earth maintained equable temperatures in that era, perhaps with a more intense greenhouse effect,[9] or a lower albedo,[10] than at present.

Current Martian surface temperatures vary from lows of about −87 °C (−125 °F) during polar winter to highs of up to −5 °C (23 °F) in summer.[11] The wide range is due to the rarefied atmosphere, which cannot store much solar heat, and the low thermal inertia of the soil.[12] Early in its history, a denser atmosphere may have permitted the formation of an ocean on Mars.[13]

Tidal locking

Because of Gliese 581 g's proximity to its parent star, it is predicted to be tidally locked to Gliese 581. Just as the Earth's Moon always presents the same face to the Earth, the length of Gliese 581 g's sidereal day would then precisely match the length of its year, meaning that it would be permanently light on one half and permanently dark on the other half of its surface.[1][14] Tidal locking also means the planet would have no axial tilt and therefore no seasonality in a conventional sense.

With one side of the planet always facing the star, temperatures could range from blazing hot in the light side to freezing cold in the dark side, with continuous Earth-like temperatures imaginable along the terminator (the area between the bright and the dark side), informally known as the twilight zone because of the permanent state of sunrise/sunset-like twilight.

Atmospheric effects

The orbits of planets in the Gliese 581 system as compared to those of our own solar system

G indicates Gliese 581g.

Theoretical models of tidally locked worlds predict that under some conditions, volatile compounds such as water and carbon dioxide, if present, would evaporate in the scorching heat of the sunward side and migrate to the cooler night side, where they would condense to form ice caps. Over time, the entire atmosphere might freeze out on the night side of the planet. Alternatively, an atmosphere massive enough to be stable would circulate the heat more evenly, allowing for a wider habitable area on the surface.[15] For example, Venus has a solar rotation rate approximately 117 times slower than Earth's, producing prolonged days and nights. Despite the uneven distribution of sunlight over time intervals shorter than several months, unilluminated areas of Venus are kept almost as hot as the day side by globally circulating winds.[16] Simulations have shown that an atmosphere containing appropriate levels of greenhouse CO2 and H2O need only be a tenth the pressure of Earth's atmosphere (100 mb) to effectively distribute heat to the night side.[17] However, due to the overpowering light of its star, current technology is unable to determine the atmospheric or surface composition of Gliese 581 g.[18]

The greater mass of Gliese 581 g would tend to compress its atmosphere (i.e., reduce its scale height) relative to Earth's.

A new age of discovery

Keck telescope at twilight

Scientists have monitored only a relatively small number of stars in the search for exoplanets. The discovery of a potentially habitable planet like Gliese 581 g so early in the search might mean that habitable planets are more widely distributed than had been previously believed. According to Vogt, the discovery "implies an interesting lower limit on the fraction of stars that have at least one potentially habitable planet as there are only ~116 known solar-type or later stars out to the 6.3 parsec distance of Gliese 581."[19] This finding foreshadows what Vogt calls a new, second Age of Discovery in exoplanetology:[20]

Confirmation by other teams through additional high-precision RVs would be most welcome. But if GJ 581g is confirmed by further RV scrutiny, the mere fact that a habitable planet has been detected this soon, around such a nearby star, suggests that η⊕ could well be on the order of a few tens of percent, and thus that either we have just been incredibly lucky in this early detection, or we are truly on the threshold of a second Age of Discovery.[1]

If the fraction of stars with potentially habitable planets (η⊕, "eta-Earth") is on the order of a few tens of percent as Vogt proposes, and the Sun's stellar neighborhood is a typical sample of the galaxy, then the discovery of Gliese 581 g in the habitable zone of its star points to the potential of billions of Earth-like planets in our Milky Way galaxy alone.[21]

See also

Notes and references

External links

Wikimedia Commons has media related to: Star system Gliese 581

Wikinews has related news: New planet found in 'Habitable Zone'

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Coordinates: 15h 19m 27s, −07° 43′ 19″

Retrieved from "http://en.wikipedia.org/wiki/Gliese_581_g"

Categories: Extrasolar planets | Libra constellation | Terrestrial planets | Exoplanets discovered in 2010 | Exoplanets detected by radial velocity | Super-Earths | Gliese 581

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2010 August 24 from UniverseToday.com

Artists impression shows the planetary system around the Sun-like star HD 10180.

Credit: ESO/L. Calçada

There is another Sun-like star out there with an intriguing family of planets orbiting about and it could be the closest parallel to our own solar system that astronomers have found yet. European astronomers discovered a planetary system containing at least five planets, orbiting the star HD 10180, with evidence that two other planets may be present. If confirmed, one of those would have the lowest mass ever found.

“We have found what is most likely the system with the most planets yet discovered,” says Christophe Lovis, who led the team. “This remarkable discovery also highlights the fact that we are now entering a new era in exoplanet research: the study of complex planetary systems and not just of individual planets. Studies of planetary motions in the new system reveal complex gravitational interactions between the planets and give us insights into the long-term evolution of the system.”

To make this system even more intriguing, the team also found evidence that the distances of the planets from their star follow a regular pattern, as also seen in our Solar System. “This could be a signature of the formation process of these planetary systems,” said team member Michel Mayor.

HD 10180, is located 127 light years away in the southern constellation of Hydrus. The five confirmed planets are large, about the size of Neptune — between 13 and 25 Earth masses —with orbital periods ranging from between six and 600 days. The planets’ distances from the star ranges from 0.06 and 1.4 times the Earth–Sun distance.

A close-up of the sky around the star HD 10180. Credit: ESO and Digitized Sky Survey 2. Acknowledgment: Davide De Martin

“We also have good reasons to believe that two other planets are present,” said Lovis. One would be a Saturn-like planet (with a minimum mass of 65 Earth masses) orbiting in 2200 days. The other would be the least massive exoplanet ever discovered, with a mass of about 1.4 times that of the Earth. It is very close to its host star, at just 2 percent of the Earth–Sun distance. One “year” on this planet would last only 1.18 Earth-days.

“This object causes a wobble of its star of only about 3 km/hour— slower than walking speed — and this motion is very hard to measure,” says team member Damien Ségransan. If confirmed, this object would be another example of a hot rocky planet, similar to Corot-7b.

The team used the planet-finding HARPS spectrograph, attached to ESO’s 3.6-metre telescope at La Silla, Chile

, and made observations of HD 10180 for six years.

The newly discovered system of planets around HD 10180 is unique in several respects. First of all, with at least five Neptune-like planets lying within a distance equivalent to the orbit of Mars, this system is more populated than our Solar System in its inner region, and has many more massive planets there. Furthermore, the system probably has no Jupiter-like gas giant. In addition, all the planets seem to have almost circular orbits.

With this new announcement, the total number of exoplanets found is 472.

The team’s paper was submitted to Astronomy and Astrophysics (“The HARPS search for southern extra-solar planets. XXVII. Up to seven planets orbiting HD 10180: probing the architecture of low-mass planetary systems” by C. Lovis et al.).

Source: ESO

Related posts:

  1. Other Solar System

  2. First Image of Another Multi-Planet Solar System

  3. Similar Solar System Discovered

  4. Researchers Find a Planet, Right Where They Expected

  5. Diagram of the Solar System

  6. Three Neptunes Orbiting Another Star

  7. New Planetary System has Familiar Feel

  8. Beyond the Solar System

  9. Second Smallest Exoplanet Found

  10. Metal Poor Star Found With Planets

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From WIKIPEDIA at: http://en.wikipedia.org/wiki/%25s

For Lists of Extrasolar Planets, see List of Extrasolar Planets, List of Extrasolar Planet Extremes,

List of Unconfirmed Exoplanets.

Planet Fomalhaut b (inset against Fomalhaut's interplanetary dust cloud) imaged by the Hubble Space Telescope's coronagraph (NASA photo)

HR 8799 (center blob) with infrared images of planets HR 8799d (bottom), HR 8799c (upper right), and HR 8799b (upper left)

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Exoplanet (9 Jupiter Masses) Caught on the Move Around Beta Pictoris

2010 June 10

This composite image represents the close environment of Beta Pictoris as seen in near infrared light. This very faint environment is revealed after a very careful subtraction of the much brighter stellar halo. The outer part of the image shows the reflected light on the dust disc, as observed in 1996 with the ADONIS instrument on ESO's 3.6 m telescope; the inner part is the innermost part of the system, as seen at 3.6 microns with NACO on the Very Large Telescope. The newly detected source is more than 1000 times fainter than Beta Pictoris, aligned with the disc, at a projected distance of 8 times the Earth-Sun distance. Both parts of the image were obtained on ESO telescopes equipped with adaptive optics.

Date

Source

For the first time, astronomers have been able to directly follow the motion of an exoplanet as it moves from one side of its host star to the other. The planet has the smallest orbit so far of all directly imaged exoplanets, lying almost as close to its parent star as Saturn is to the Sun. Scientists believe that it may have formed in a similar way to the giant planets in the Solar System. Because the star is so young, this discovery proves that gas giant planets can form within discs in only a few million years, a short time in cosmic terms.

Only 12 million years old, or less than three-thousandths of the age of the Sun, Beta Pictoris is 75% more massive than our parent star. It is located about 60 light-years away towards the constellation of Pictor (the Painter) and is one of the best-known examples of a star surrounded by a dusty debris disc [1]. Earlier observations showed a warp of the disc, a secondary inclined disc and comets falling onto the star. “Those were indirect, but tell-tale signs that strongly suggested the presence of a massive planet, and our new observations now definitively prove this,” says team leader Anne-Marie Lagrange. “Because the star is so young, our results prove that giant planets can form in discs in time-spans as short as a few million years.”

Recent observations have shown that discs around young stars disperse within a few million years, and that giant planet formation must occur faster than previously thought. Beta Pictoris is now clear proof that this is indeed possible.

The team used the NAOS-CONICA instrument (or NACO [2]), mounted on one of the 8.2-metre Unit Telescopes of ESO's Very Large Telescope (VLT), to study the immediate surroundings of Beta Pictoris in 2003, 2008 and 2009. In 2003 a faint source inside the disc was seen (eso0842), but it was not possible to exclude the remote possibility that it was a background star. In new images taken in 2008 and spring 2009 the source had disappeared! The most recent observations, taken during autumn 2009, revealed the object on the other side of the disc after a period of hiding either behind or in front of the star (in which case it is hidden in the glare of the star). This confirmed that the source indeed was an exoplanet and that it was orbiting its host star. It also provided insights into the size of its orbit around the star.

Images are available for approximately ten exoplanets, and the planet around Beta Pictoris (designated “Beta Pictoris b”) has the smallest orbit known so far. It is located at a distance between 8 and 15 times the Earth-Sun separation — or 8-15 Astronomical Units — which is about the distance of Saturn from the Sun. “The short period of the planet will allow us to record the full orbit within maybe 15-20 years, and further studies of Beta Pictoris b will provide invaluable insights into the physics and chemistry of a young giant planet’s atmosphere,” says student researcher Mickael Bonnefoy.

The planet has a mass of about nine Jupiter masses and the right mass and location to explain the observed warp in the inner parts of the disc. This discovery therefore bears some similarity to the prediction of the existence of Neptune by astronomers Adams and Le Verrier in the 19th century, based on observations of the orbit of Uranus.

“Together with the planets found around the young, massive stars Fomalhaut and HR8799, the existence of Beta Pictoris b suggests that super-Jupiters could be frequent byproducts of planet formation around more massive stars,” explains Gael Chauvin, a member of the team.

Such planets disturb the discs around their stars, creating structures that should be readily observable with the Atacama Large Millimeter/submillimeter Array (ALMA), the revolutionary telescope being built by ESO together with international partners.

A few other planetary candidates have been imaged, but they are all located further from their host star than Beta Pictoris b. If located in the Solar System, they all would lie close to or beyond the orbit of the furthest planet, Neptune. The formation processes of these distant planets are likely to be quite different from those in our Solar System and in Beta Pictoris.

“The recent direct images of exoplanets — many made by the VLT— illustrate the diversity of planetary systems,” says Lagrange. “Among those, Beta Pictoris b is the most promising case of a planet that could have formed in the same way as the giant planets in our Solar System.”

Notes

[1] Debris discs are composed of dust resulting from collisions among larger bodies such as planetary embryos or asteroids. They are larger versions of the zodiacal dust band in our Solar System. The disc around Beta Pictoris was the first to be imaged and is now known to extend up to about 1000 times the distance between the Earth and the Sun.

[2] NACO is an adaptive optics instrument attached to ESO’s Very Large Telescope, located in Chile. Thanks to adaptive optics, astronomers can remove most of the blurring effect of the atmosphere and obtain very sharp images.

More information

This research was presented in a paper to appear this week in Science Express (“A Giant Planet Imaged in the disk of the Young Star Beta Pictoris,” by A.-M. Lagrange et al.).

The team is composed of A.-M. Lagrange, M. Bonnefoy, G. Chauvin, D. Ehrenreich, and D. Mouillet (Laboratoire d'Astrophysique de l'Observatoire de Grenoble, Université Joseph Fourier, CNRS, France), D. Apai (Space Telescope Science Institute, Baltimore, USA), A. Boccaletti, D. Gratadour, D. Rouan, and S. Lacour (LESIA, Observatoire de Paris-Meudon, France), and M. Kasper (ESO).

ESO, the European Southern Observatory, is the foremost intergovernmental astronomy organisation in Europe and the world’s most productive astronomical observatory. It is supported by 14 countries: Austria, Belgium, the Czech Republic, Denmark, France, Finland, Germany, Italy, the Netherlands, Portugal, Spain, Sweden, Switzerland and the United Kingdom. ESO carries out an ambitious programme focused on the design, construction and operation of powerful ground-based observing facilities enabling astronomers to make important scientific discoveries. ESO also plays a leading role in promoting and organising cooperation in astronomical research. ESO operates three unique world-class observing sites in Chile: La Silla, Paranal and Chajnantor. At Paranal, ESO operates the Very Large Telescope, the world’s most advanced visible-light astronomical observatory and VISTA, the world’s largest survey telescope. ESO is the European partner of a revolutionary astronomical telescope ALMA, the largest astronomical project in existence. ESO is currently planning a 42-metre European Extremely Large optical/near-infrared Telescope, the E-ELT, which will become “the world’s biggest eye on the sky”.

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An extrasolar planet, or exoplanet, is a planet that orbits a star other than the Sun and that is thus beyond the Solar System. As of 2011 Nov 7 there are 697 planets (573 are planetary systems and 81 are multiple systems) listed in the Extrasolar Planets Encyclopaedia.[1] The vast majority of them have been detected through radial velocity observations and other indirect methods rather than actual imaging. [1] Most of those currently known are giant planets thought to resemble Jupiter; however, substantial sampling bias exists since more massive planets are much easier to detect with current technology. Of the first 464 known exoplanets, 112 have estimated masses. A few relatively lightweight exoplanets, only a few times more massive than Earth, have now been detected and projections suggest that planets of roughly Earth-like mass will eventually be found to outnumber extrasolar gas giants.[2]

Extrasolar planets became a subject of scientific investigation in the mid-19th century. Many astronomers supposed that such planets existed, but they had no way of knowing how common they were or how similar they might be to the planets of our solar system. The first confirmed radial velocity detection was made in 1995, revealing a gas giant planet in a four-day orbit around the nearby G-type star 51 Pegasi. The frequency of detections has tended to increase on an annual basis since then.[1] It is estimated that at least 10% of sun-like stars have planets, and the true proportion may be much higher.[3] The discovery of extrasolar planets sharpens the question of whether some might support extraterrestrial life.[4]

Currently Gliese 581 d, the fourth planet of the red dwarf star Gliese 581 (approximately 20 light years from Earth), appears to be the best example yet discovered of a possible terrestrial exoplanet that orbits within the habitable zone surrounding its star. Although initial measurements suggested that Gliese 581 d resided outside the so-called "Goldilocks Zone", additional measurements place it within.[5]

History of Detection

Retracted Discoveries

Unconfirmed until 1992, extrasolar planets had been assumed possible for a long period of time. In the 16th century the Italian philosopher Giordano Bruno, an early supporter of Copernicus' theory that the Earth and other planets orbit the sun, put forward the view that the fixed stars are really suns like our own, with planets going round them. The same possibility is mentioned in Isaac Newton's "General Scholium" (1713): "And if the fixed Stars are the centers of other like systems, these, being form'd by the like wise counsel, must be all subject to the dominion of One" (trans. Motte 1729).

Our solar system compared with the system of 55 Cancri

Claims about detection of exoplanets have been made from the 19th century. Some of the earliest involve the binary star 70 Ophiuchi. In 1855 Capt. W. S. Jacob at the East India Company's Madras Observatory reported that orbital anomalies made it "highly probable" that there was a "planetary body" in this system.[6] In the 1890s, Thomas J. J. See of the University of Chicago and the United States Naval Observatory stated that the orbital anomalies proved the existence of a dark body in the 70 Ophiuchi system with a 36-year period around one of the stars.[7] However, Forest Ray Moulton soon published a paper proving that a three-body system with those orbital parameters would be highly unstable.[8] During the 1950s and 1960s, Peter van de Kamp of Swarthmore College made another prominent series of detection claims, this time for planets orbiting Barnard's Star.[9] Astronomers now generally regard all the early reports of detection as erroneous.

In 1991, Andrew Lyne, M. Bailes and S.L. Shemar claimed to have discovered a pulsar planet in orbit around PSR 1829-10, using pulsar timing variations.[10] The claim briefly received intense attention, but Lyne and his team soon retracted it.[11]

Confirmed Discoveries

Our inner solar system superimposed behind the orbits of the planets HD 179949 b, HD 164427 b, Epsilon Reticuli Ab, and Mu Arae b (all parent stars are in the center)

The first published discovery to have received subsequent confirmation was made in 1988 by the Canadian astronomers Bruce Campbell, G. A. H. Walker, and S. Yang.[12] Their radial-velocity observations suggested that a planet orbited the star Gamma Cephei. They remained cautious about claiming a true planetary detection, and widespread skepticism persisted in the astronomical community for several years about this and other similar observations. It was mainly because the observations were at the very limits of instrumental capabilities at the time. Another source of confusion was that some of the possible planets might instead have been brown dwarfs, objects that are intermediate in mass between planets and stars. The following year, additional observations were published that supported the reality of the planet orbiting Gamma Cephei,[13] though subsequent work in 1992 raised serious doubts.[14] Finally, in 2003, improved techniques allowed the planet's existence to be confirmed.[15]

In early 1992, radio astronomers Aleksander Wolszczan and Dale Frail announced the discovery of planets around another pulsar, PSR 1257+12.[16] This discovery was quickly confirmed, and is generally considered to be the first definitive detection of exoplanets. These pulsar planets are believed to have formed from the unusual remnants of the supernova that produced the pulsar, in a second round of planet formation, or else to be the remaining rocky cores of gas giants that survived the supernova and then decayed into their current orbits.

On October 6, 1995, Michel Mayor and Didier Queloz of the University of Geneva announced the first definitive detection of an exoplanet orbiting an ordinary main-sequence star (51 Pegasi).[17] This discovery was made at the Observatoire de Haute-Provence and ushered in the modern era of exoplanetary discovery. Technological advances, most notably in high-resolution spectroscopy, led to the detection of many new exoplanets at a rapid rate. These advances allowed astronomers to detect exoplanets indirectly by measuring their gravitational influence on the motion of their parent stars. Additional extrasolar planets were eventually detected by observing the variation in a star's apparent luminosity as an orbiting planet passed in front of it.

To date[update], 422 exoplanets are listed in the Extrasolar Planets Encyclopaedia, including a few that were confirmations of controversial claims from the late 1980s.[1] The first system to have more than one planet detected was Upsilon Andromedae. Forty-four such multiple-planet systems are known as of December 2009. Among the known exoplanets are four pulsar planets orbiting two separate pulsars. Infrared observations of circumstellar dust disks also suggest the existence of millions of comets in several extrasolar systems.

Detection Methods

Main article: Methods of detecting extrasolar planets

Planets are extremely faint light sources compared to their parent stars. At visible wavelengths, they usually have less than a millionth of their parent star's brightness. In addition to the intrinsic difficulty of detecting such a faint light source, the parent star causes a glare that washes it out.

For those reasons, current telescopes can only directly image exoplanets under exceptional circumstances. Specifically, it is most likely to be possible when the planet is especially large (considerably larger than Jupiter), widely separated from its parent star, and hot so that it emits intense infrared radiation.

The vast majority of known extrasolar planets have been discovered through indirect methods:

Diagram showing how an exoplanet orbiting a larger star could produce changes in position and velocity of the star as they orbit their common center of mass.

    • Astrometry: Astrometry consists of precisely measuring a star's position in the sky and observing the ways in which that position changes over time. If the star has a planet, then the gravitational influence of the planet will cause the star itself to move in a tiny circular or elliptical orbit around the common center of mass (see animation on the right). Because the motion of the star is so small, this method has not yet been very productive at detecting exoplanets.

    • Radial velocity or Doppler method: As the star moves in its small orbit around the system's center of mass, its velocity also changes. Variations in the star's radial velocity - that is, the speed with which it moves towards or away from Earth — can be deduced from displacements in the star's spectral lines due to the Doppler effect. Extremely small radial-velocity variations can be detected, down to roughly 1 m/s. This has been by far the most productive method of discovering exoplanets.

    • Transit method: If a planet crosses (or transits) in front of its parent star's disk, then the observed brightness of the star drops by a small amount. The amount by which the star dims depends on its size and on the size of the planet. This has been the second most productive method of detection, though confirmation from another method is usually considered necessary.

    • Gravitational microlensing: Microlensing occurs when the gravitational field of a star acts like a lens, magnifying the light of a distant background star. Possible planets orbiting the foreground star can cause detectable anomalies in the lensing event light curve. This method has resulted in only a few planetary detections, but it has the advantage of being especially sensitive to planets at large separations from their parent stars.

    • Pulsar timing: A pulsar (the small, ultradense remnant of a star that has exploded as a supernova) emits radio waves extremely regularly as it rotates. Slight anomalies in the timing of its observed radio pulses can be used to track changes in the pulsar's motion caused by the presence of planets.

    • Eclipsing binary: If a planet has a large orbit that carries it around both members of an eclipsing double star system, then the planet can be detected through small variations in the timing of the stars' eclipses of each other. As of December 2009, two planets have been found by this method.

    • Circumstellar disks: Disks of space dust surround many stars, and this dust can be detected because it absorbs ordinary starlight and re-emits it as infrared radiation. Features in dust disks may suggest the presence of planets.

Almost all known extrasolar planet candidates have been found using ground-based telescopes. However, many of the methods can yield better results if the observing telescope is located above the restless atmosphere. COROT (launched in December 2006) and Kepler, (launched in March 2009) are the only active space missions dedicated to extrasolar planet search. Hubble Space Telescope and MOST have found or confirmed a few planets. There are many planned or proposed space missions such as New Worlds Mission, Darwin, Space Interferometry Mission, Terrestrial Planet Finder, and PEGASE.

Definition

Main article: Definition of planet

According to the International Astronomical Union's working definition of "planet," a planet must orbit a star.[18] However, the current IAU definition for planet only accounts for our own solar system and all extrasolar planets were excluded from this definition for now.[19] The "working" definition for extrasolar planets was established in 2001 (and last modified in 2003) with the following criteria:

    1. Objects with true masses below the limiting mass for thermonuclear fusion of deuterium (currently calculated to be 13 Jupiter masses for objects of solar metallicity) that orbit stars or stellar remnants are "planets" (no matter how they formed). The minimum mass/size required for an extrasolar object to be considered a planet should be the same as that used in our solar system.

    2. Substellar objects with true masses above the limiting mass for thermonuclear fusion of deuterium are "brown dwarfs", no matter how they formed nor where they are located.

    3. Free-floating objects in young star clusters with masses below the limiting mass for thermonuclear fusion of deuterium are not "planets", but are "sub-brown dwarfs" (or whatever name is most appropriate).

There have also been reports of free-floating planetary-mass objects (ones not orbiting any star), sometimes called "rogue planets" or "interstellar planets". Such objects are not discussed in this article since they are outside the working definition of "planet". Some of these may have formed as a planet around a star, but were subsequently ejected from that planetary system.

Nomenclature

The system used in the scientific literature for naming exoplanets is almost the same as the system used for naming binary stars. The only modification is that a lowercase letter is used for the planet instead of the uppercase letter used for stars. A lowercase letter is placed after the star name, starting with "b" for the first planet found in the system (for example, 51 Pegasi b); "a" is skipped to help prevent confusion with the primary star. The next planet found in the system would be labeled with the next letter in the alphabet. For instance, any more planets found around 51 Pegasi would be catalogued as "51 Pegasi c" and then "51 Pegasi d", and so on. If two planets are discovered at about the same time, the closer one to the star gets the next letter, followed by the farther planet. However, in some cases, a smaller planet is found closer to the star than other previously known planets, causing the letter order to not follow the order of the planets from the star. For example, in the Gliese 876 system, the most recently discovered planet is referred to as Gliese 876 d, despite the fact that it is closer to the star than Gliese 876 b and Gliese 876 c. At present, the planet 55 Cancri f, which is the fifth planet found in the 55 Cancri system, is the only planet to have "f" in its name, the highest letter currently in use.

The 55 Cancri solar system contains 5 confirmed planets, more than any other extrasolar star system, and may contain more. The planet 55 Cancri f (pictured in this artist's conception in the foreground) is currently the only planet with the designation "f".

If a planet orbits one member of a multiple-star system, then an uppercase letter for the star will be followed by a lowercase letter for the planet. Examples include the planets 16 Cygni Bb and 83 Leonis Bb. However, if the planet orbits the primary star of the system, and the secondary stars were either discovered after the planet or are relatively far form the primary star and planet, then the uppercase letter is usually omitted. For example, Tau Boötis b orbits in a binary system, but because the secondary star was both discovered after the planet and very far from the primary star and planet, the term "Tau Boötis Ab" is rarely if ever used.

Only two planetary systems have planets that are named unusually. Before the discovery of 51 Pegasi b in 1995, two pulsar planets (PSR B1257+12 B and PSR B1257+12 C) were discovered from pulsar timing of their dead star. Since there was no official way of naming planets at the time, they were called "B" and "C" (similar to how planets are named today). However, uppercase letters were used, most likely because of the way binary stars were named. When a third planet was discovered, it was designated PSR B1257+12 A (simply because the planet was closer than the other two).[20]

An alternate nomenclature, often seen in science fiction, uses Roman numerals in the order of planets' positions from the star. (This is inspired by an old system for naming moons of the outer planets, such as "Jupiter IV" for Callisto.) But for the above reasons, such a system has proven impractical for scientific use. To use our solar system as an example, Jupiter would most likely be the first planet discovered, and Saturn the second; but, as the terrestrial planets would not be easily detected, Jupiter and Saturn would be called "Sol I" and "Sol II" by science-fiction nomenclature, and need to be renamed "Sol V" and "Sol VI" if all four terrestrial planets are discovered later. In contrast, by the current system, even if the terrestrial planets were found, Jupiter and Saturn would remain "Sol b" and "Sol c" and not need renaming.

Finally, some planets have received unofficial names comparable to those of planets in the Solar System. The most noted planets that have been given names include: Osiris (HD 209458 b), Bellerophon (51 Pegasi b), and Methuselah (PSR B1620-26 b). The International Astronomical Union (IAU) currently has no plans to officially assign such names to extrasolar planets, considering it impractical.[21]

General Properties

Number of Stars with Planets

Planet-search programs have discovered planets orbiting a substantial fraction of the stars they have looked at. However, the total fraction of stars with planets is uncertain because of observational selection effects. The radial-velocity method and the transit method (which between them are responsible for the vast majority of detections) are most sensitive to large planets on small orbits. For that reason, many known exoplanets are "hot Jupiters": planets of roughly Jupiter-like mass on very small orbits with periods of only a few days. It is now known that 1% to 1.5% of sunlike stars possess such a planet, where "sunlike star" refers to any main-sequence star of spectral classes F, G, or K without a close stellar companion.[3] It is further estimated that 3% to 4.5% of sunlike stars possess a giant planet with an orbital period of 100 days or less, where "giant planet" means a planet of at least thirty Earth masses.[22]

The fraction of stars with smaller or more distant planets remains difficult to estimate. Extrapolation does suggest that small planets (of roughly Earth-like mass) are more common than giant planets. It also appears that planets on large orbits may be more common than ones on small orbits. Based on such extrapolation, it is estimated that up to 20% of sunlike stars may have at least one giant planet.[22] It is believed that at least 40% of solar-type stars have low-mass planets.[23]

Regardless of the exact fraction of stars with planets, the total number of exoplanets must be very large. Since our own Milky Way Galaxy has at least 100 billion stars, it must also contain billions of planets if not hundreds of billions of them.

Characteristics of Planet-hosting Stars

Most known exoplanets orbit stars roughly similar to our own Sun, that is, main-sequence stars of spectral categories F, G, or K. One reason is simply that planet search programs have tended to concentrate on such stars. But even after taking this into account, statistical analysis indicates that lower-mass stars (red dwarfs, of spectral category M) are either less likely to have planets or have planets that are themselves of lower mass and hence harder to detect.[24][22] Recent observations by the Spitzer Space Telescope indicate that stars of spectral category O, which are much hotter than our Sun, produce a photo-evaporation effect that inhibits planetary formation.[25]

Stars are composed mainly of the light elements hydrogen and helium. They also contain a small fraction of heavier elements such as iron, and this fraction is referred to as a star's metallicity. Stars of higher metallicity are much more likely to have planets, and the planets they have tend to be more massive than those of lower-metallicity stars.[3] It has also been shown that stars with planets are more likely to be deficient in lithium.[26]

Orbital Parameters

All extrasolar planets discovered through 2010-01-05, with detection method indicated (radial velocity = dark blue, transit = dark green, timing = dark purple, astrometry = dark yellow, direct imaging = dark red, microlensing = dark orange, pulsar timing = purple) and our solar system planets (gray circles) provided for reference.

Most known extrasolar planet candidates have been discovered using indirect methods and therefore only some physical and orbital parameters can be determined. For example, out of the six independent parameters that define an orbit, the radial-velocity method can determine four: semi-major axis, eccentricity, longitude of periastron, and time of periastron. Two parameters remain unknown: inclination and longitude of the ascending node.

Many exoplanets have orbits with very small semi-major axes, and are thus much closer to their parent star than any planet in our own solar system is to the Sun. That fact, however, is mainly due to observational selection: The radial-velocity method is most sensitive to planets with small orbits. Astronomers were initially very surprised by these "hot Jupiters", but it is now clear that most exoplanets (or, at least, most high-mass exoplanets) have much larger orbits, some located in habitable zones where suitable for liquid water and life.[22] It appears plausible that in most exoplanetary systems, there are one or two giant planets with orbits comparable in size to those of Jupiter and Saturn in our own solar system.

The eccentricity of an orbit is a measure of how elliptical (elongated) it is. Most exoplanets with short orbital periods (of 20 days or less) have near-circular orbits of very low eccentricity. That is believed to be due to the effect of tidal circularization, in which gravitational interaction between two bodies gradually reduces orbital eccentricity over time. By contrast, most known exoplanets with longer orbital periods have quite eccentric orbits. This is not an observational selection effect, since a planet can be detected about a star equally well regardless of the eccentricity of its orbit. The prevalence of elliptical orbits is a major puzzle, since current theories of planetary formation strongly suggest planets should form with circular (that is, non-eccentric) orbits. One possible theory is that small companions such as T dwarfs (methane-bearing brown dwarfs) can hide in such solar systems and can cause the orbits of planets to be extreme.[27] This is also an indication that our own solar system may be unusual, since all of its planets except for Mercury do have near-circular orbits.[3] However, it has been suggested that some of the high eccentricity values reported for exoplanets may be overestimates, since simulations show that many observations are also consistent with two planets on circular orbits. Planets reported as single moderately eccentric planets have a ~15% chance of being part of such a pair.[28]

Mass Distribution

This planetary habitability chart shows where life might exist on extrasolar planets based on our own solar system and life on Earth.

When a planet is found by the radial-velocity method, its orbital inclination i is unknown. The method is unable to determine the true mass of the planet, but rather gives its minimum mass Msini. In a few cases an apparent exoplanet may actually be a more massive object such as a brown dwarf or red dwarf. However, statistically the factor of sini takes on an average value of π/4≈0.785 and hence most planets will have true masses fairly close to the minimum mass.[22] Furthermore, if the planet's orbit is nearly perpendicular to the sky (with an inclination close to 90°), the planet can also be detected through the transit method. The inclination will then be known, and the planet's true mass can be found. Also, astrometric observations and dynamical considerations in multiple-planet systems can sometimes be used to constrain a planet's true mass.

The vast majority of exoplanets detected so far have high masses. As of December 2009, all but twenty-four of them have more than ten times the mass of Earth.[1] Many are considerably more massive than Jupiter, the most massive planet in the Solar System. However, these high masses are in large part due to an observational selection effect: all detection methods are much more likely to discover massive planets. This bias makes statistical analysis difficult, but it appears that lower-mass planets are actually more common than higher-mass ones, at least within a broad mass range that includes all giant planets. In addition, the fact that astronomers have found several planets only a few times more massive than Earth, despite the great difficulty of detecting them, indicates that such planets are fairly common.[3] According to 2008 data from the HARPS (High Accuracy Radial velocity Planet Searcher) spectrograph instrument in Chile, about one star in 14 may have gas giant planets, while one in three probably has rocky planets of below 30 Earth masses.[29]

Temperature and Composition

It is possible to estimate the temperature of an exoplanet based on the intensity of the light it receives from its parent star. For example, the planet OGLE-2005-BLG-390Lb is estimated to have a surface temperature of roughly -220° C (roughly 50 K). However, such estimates may be off because they depend on the planet's usually unknown albedo, and because factors such as the greenhouse effect may introduce unknown complications. A few planets have had their temperature measured by observing the variation in infrared radiation as the planet moves around in its orbit and is eclipsed by its parent star. For example, the planet HD 189733b has been found to have an average temperature of 1205±9 K (932±9° C) on its dayside and 973±33 K (700±33° C) on its nightside.[30]

If a planet is detectable by both the radial-velocity and the transit methods, then both its true mass and its radius can be found. The planet's density can then be calculated. Planets with low density are inferred to be composed mainly of hydrogen and helium, while planets of intermediate density are inferred to have water as a major constituent. A planet of high density is believed to be rocky, like Earth and the other terrestrial planets of the Solar System.

Spectroscopic measurements can be used to study a transiting planet's atmospheric composition.[31] Water vapor, sodium vapor, methane, and carbon dioxide have been detected in the atmospheres of various exoplanets in this way.

Another line of information about exoplanetary atmospheres comes from observations of orbital phase functions. Extrasolar planets have phases similar to the phases of the Moon. By observing the exact variation of brightness with phase, astronomers can calculate particle sizes in the atmospheres of planets.

Stellar light becomes polarized when it interacts with atmospheric molecules, which could be detected with a polarimeter. So far, one planet has been studied by polarimetry.

Unanswered Questions

Many unanswered questions remain about the properties of exoplanets, such as the details of their composition and the likelihood of possessing moons. The recent discovery that several surveyed exoplanets lacked water showed that there is still much more to be learned about the properties of exoplanets.[citation needed] Another question is whether they might support life. Several planets do have orbits in their parent star's habitable zone, where it should be possible for Earth-like conditions to prevail. Most of those planets are giant planets more similar to Jupiter than to Earth; if these planets have large moons, the moons might be a more plausible abode of life.

Various estimates have been made as to how many planets might support simple life or even intelligent life. For example, Dr. Alan Boss of the Carnegie Institution of Science estimates there may be a “hundred billion” terrestrial planets in our Milky Way Galaxy, many with simple lifeforms. He further believes there could be thousands of civilizations in our galaxy. Recent work by Duncan Forgan of Edinburgh University has also tried to estimate the number of intelligent civilizations in our galaxy. The research suggested there could be thousands of them.[32] However, due to the great uncertainties regarding the origin and development of life and intelligence, all such estimates must be regarded as extremely speculative. Detection of life (other than an advanced civilization) at interstellar distances is a tremendously challenging technical task that will not be feasible for many years, even if such life is commonplace.

Notable Discoveries

Exoplanets, by year of discovery, through 2010-01-05.

1996 to 2006

1996, 47 Ursae Majoris b

This Jupiter-like planet was the first long-period planet discovered, orbiting at 2.11 AU from the star with the eccentricity of 0.049. There is a second companion that orbits at 3.39 AU with the eccentricity of 0.220 ± 0.028 and a period of 2190 ± 460 days.

1998, Gliese 876 b

The first planet found that orbits around a red dwarf star (Gliese 876). It orbits closer to the star than Mercury is to the Sun. More planets have subsequently been discovered closer to the star.[33]

1999, Upsilon Andromedae

The first multiple-planetary system to be discovered around a main sequence star. It contains three planets, all of which are Jupiter-like. Planets b, c, d were announced in 1996, 1999, and 1999 respectively. Their masses are 0.687, 1.97, and 3.93 MJ; they orbit at 0.0595, 0.830, and 2.54 AU respectively.[34] In 2007 their inclinations were determined as non-coplanar.

1999, HD 209458 b

This exoplanet, originally discovered with the radial-velocity method, became the first exoplanet to be seen transiting its parent star. The transit detection conclusively confirmed the existence of the planets suspected to be responsible for the radial velocity measurements.[35]

2001, HD 209458 b

Astronomers using the Hubble Space Telescope announced that they had detected the atmosphere of HD 209458 b. They found the spectroscopic signature of sodium in the atmosphere, but at a smaller intensity than expected, suggesting that high clouds obscure the lower atmospheric layers.[36] In 2008 the albedo of its cloud layer was measured, and its structure modeled as stratospheric.

2001, Iota Draconis b

The first planet discovered around the giant star Iota Draconis, an orange giant. This provides evidence for the survival and behavior of planetary systems around giant stars. Giant stars have pulsations that can mimic the presence of planets. The planet is very massive and has a very eccentric orbit. It orbits on average 27.5% further from its star than Earth does from the Sun.[37] In 2008 the system's origin would be traced to the Hyades cluster, alongside Epsilon Tauri.

Artist's impression of the pulsar planet PSR B1620-26 b (discovered in 2003); it is over 12.5 billion years old, making it the oldest known extrasolar planet.

2003, PSR B1620-26 b

On July 10, using information obtained from the Hubble Space Telescope, a team of scientists led by Steinn Sigurdsson confirmed the oldest extrasolar planet yet. The planet is located in the globular star cluster M4, about 5,600 light years from Earth in the constellation Scorpius. This is one of only three planets known to orbit around a stellar binary; one of the stars in the binary is a pulsar and the other is a white dwarf. The planet has a mass twice that of Jupiter, and is estimated to be 13 billion years old.[38]

2004, Mu Arae c

In August, a planet orbiting Mu Arae with a mass of approximately 14 times that of the Earth was discovered with the European Southern Observatory's HARPS spectrograph. Depending on its composition, it is the first published "hot Neptune" or "super-Earth".[39]

Infrared image of 2M1207 (bluish) and 2M1207b (reddish). The two objects are separated by less than one arc second in Earth's sky. Image taken using the European Southern Observatory's 8.2 m Yepun Very Large Telescope

2004, 2M1207 b

The first planet found around a brown dwarf. The planet is also the first to be directly imaged (in infrared). According to an early estimate, it has a mass 5 times that of Jupiter; other estimates give slightly lower masses. It was originally estimated to orbit at 55 AU from the brown dwarf. The brown dwarf is only 25 times as massive as Jupiter. The temperature of the gas giant planet is very high (1250 K), mostly due to gravitational contraction.[40] In late 2005, the parameters were revised to orbital radius 41 AU and mass of 3.3 Jupiters, because it was found that the star is closer to Earth than was originally believed. In 2006, a dust disk was found around 2M1207, providing evidence for active planet formation.[41]

2005, Gliese 876 d

In June, a third planet orbiting the red dwarf star Gliese 876 was announced. With a mass estimated at 7.5 times that of Earth, it may be rocky in composition. The planet orbits at 0.021 AU with a period of 1.94 days.[42]

2005, HD 149026 b

In July, a planet with the largest core known was announced. The planet, HD 149026 b, orbits the star HD 149026, and has a core that was then estimated to be 70 Earth masses (as of 2008, 80-110), accounting for at least two-thirds of the planet's mass.[43]

Artist's impression of the planet OGLE-2005-BLG-390Lb (with surface temperature of approximately −220 °C), orbiting its star 20,000 light years (189.2 exameters or 117.5 quadrillion miles) from Earth; this planet was discovered with gravitational microlensing.

2006, OGLE-2005-BLG-390Lb

On January 25, the discovery of OGLE-2005-BLG-390Lb was announced. This is the most distant and probably the coldest exoplanet found to date. It is believed that it orbits a red dwarf star around 21,500 light years from Earth, towards the center of the Milky Way galaxy. It was discovered using gravitational microlensing, and is estimated to have a mass of 5.5 times that of Earth. Prior to this discovery, the few known exoplanets with comparably low masses had only been discovered in orbits very close to their parent stars, but this planet is estimated to have a relatively wide separation of 2.6 AU from its parent star.[44][45]

2006, HD 69830

Has a planetary system with three Neptune-mass planets. It is the first triple planetary system without any Jupiter-like planets discovered around a Sun-like star. All three planets were announced on May 18 by Lovis. All three orbit within 1 AU. The planets b, c and d have masses of 10, 12 and 18 times that of Earth, respectively. The outermost planet, d, appears to be in the habitable zone, shepherding the asteroid belt.[46]

2007 to 2010

2007, HD 209458 b and HD 189733 b

On February 21, 2007, NASA and Nature released news that HD 209458 b and HD 189733 b were the first two extrasolar planets to have their atmospheric spectra directly observed.[47][48] This has long been seen as the first mechanism by which extrasolar but non-intelligent life forms could be searched for. A group of investigators led by Dr. Jeremy Richardson of NASA's Goddard Space Flight Center were first to publication, in the February 22 issue of Nature. Richardson et al. spectrally measured HD 209458 b's atmosphere in the range of 7.5 to 13.2 micrometres. The results defied theoretical expectations in several ways. The spectrum had been predicted to have a peak at 10 micrometres which would have indicated water vapor in the atmosphere, but such a peak was absent, indicating no detectable water vapor. Another, unpredicted peak was observed at 9.65 micrometres, which the investigators attributed to clouds of silicate dust, a phenomenon not previously observed. Another unpredicted peak occurred at 7.78 micrometres, which the investigators did not have an explanation for. A team led by Carl Grillmair of NASA's Spitzer Science Center made the observations of HD 189733 b, and their results were pending publication in Astrophysical Journal Letters at the time of the news release. On July 11, 2007, the findings by the Spitzer Science Center were published in the Nature: Spectral imprints of water vapor were found by the Spitzer Space Telescope, thus representing the first solid evidence of water on an extrasolar planet.[49]

2007, Gliese 581 c

A team of astronomers led by Stephane Udry used the HARPS instrument on the European Southern Observatory's 3.6-meter telescope to discover this exoplanet by means of the radial velocity method.[50] The team calculated that the planet could support liquid water and possibly life.[51] However, subsequent habitability studies[52][53] indicate that the planet likely suffers from a runaway greenhouse effect similar to Venus, rendering the presence of liquid water impossible. These studies suggest that the third planet in the system, Gliese 581 d, is more likely to be habitable. Seth Shostak, a senior astronomer with the SETI institute, stated that two unsuccessful searches had already been made for radio signals from extraterrestrial intelligence in the Gliese 581 system.[51]

2007, Gliese 436 b

This planet was one of the first Neptune-mass planets discovered, in August 2004. In May 2007, a transit was found, revealed as the smallest and least massive transiting planet yet at 22 times that of Earth. Its density is consistent with a large core of an exotic form of solid water called "hot ice", which would exist, despite the planet's high temperatures, because the planet's gravity causes water to be extremely dense.[54]

2007, TrES-4

The largest-diameter and lowest-density exoplanet to date, TrES-4 is 1.7 times Jupiter's diameter but only 0.84 times its mass, giving it a density of just 0.2 grams per cubic centimeter—about the same as balsa wood. It orbits its primary closely and is therefore quite hot, but stellar heating alone does not appear to explain its large size.[55]

2008, OGLE-2006-BLG-109Lb and OGLE-2006-BLG-109Lc

On February 14, the discovery of the, until now, most similar Jupiter-Saturn planetary system constellation was announced, with the ratios of mass, distance to their star and orbiting time similar to that of Jupiter-Saturn. This can be important for possible life in a solar system as Jupiter and Saturn have a stabilizing effect to the habitable zone by sweeping away large asteroids from the habitable zone.[56]

An artist's conception of extrasolar planet HD 189733 b

2008, HD 189733 b

On March 20, follow up studies to the first spectral analyses of an extrasolar planet were published in the scientific journal Nature, announcing evidence of an organic molecule found on an extrasolar planet for the first time. In 2007 water vapor was already detected in the spectrum of HD 189733 b, but new analyses showed not only water vapor, but also methane existing in the atmosphere of the giant gas planet. Although conditions on HD 189733 b are too harsh to harbor life, it still is the first time a key molecule for organic life was found on an extrasolar planet.[57]

2008, HD 40307

On June 16, Michel Mayor announced a confirmed planetary system with three super-Earths orbiting this K-type star. Their masses are between 4 to 9 Earth masses and with periods between 4 to 20 days. It is speculated that this may be the first multi-planetary system without any known gas giants. All three terrestrial planets were discovered by the HARPS spectrograph in La Silla, Chile.[58] These three worlds were amongst the first seven confirmed of a panel of 45 candidate planets detected by the HARPS spectrograph on May 28, 2008. The discoveries represented a significant increase in the numbers of known super-earths. Based on this, astronomers now suggest that such low-mass planets may outnumber the Jupiter-like planets by 3 to 1.[2] While more data are needed to confirm the remaining candidates, some news media picked up the story.

2008, Fomalhaut b

On November 13, NASA and the Lawrence Livermore National Laboratory announced the discovery of an extrasolar planet orbiting just inside the debris ring of the A class star Fomalhaut (Alpha Piscis Austrini). This was the first extrasolar planet to be directly imaged by an optical telescope.[59] The mass of Fomalhaut b is estimated to be 3 times the mass of Jupiter.[60][61] Based on the planet's unexpected brightness at visible wavelengths, the discovery team suspects it is surrounded by its own large disk or ring that may be a satellite system in the process of formation.

2008, HR 8799

On November 13, the same day as Fomalhaut b, the discovery of three planets orbiting HR 8799 was announced. This was the first direct image of multiple planets. Christian Marois of the National Research Council of Canada's Herzberg Institute of Astrophysics and his team used the Keck and Gemini telescopes in Hawaii. The Gemini images allowed the international team to make the initial discovery of two of the planets with data obtained on October 17, 2007. Then, on October 25, 2007, and in the summer of 2008 the team confirmed this discovery and found a third planet orbiting even closer to the star with images obtained at the Keck II telescope. A review of older data taken in 2004 with the Keck II telescope revealed that the three planets were visible on these images. Their masses and separation are approximately 10 MJ @ 24 AU, 10 MJ @ 38 AU and 7 MJ @ 68 AU.[61][62]

2009, COROT-7b

On February 3, the European Space Agency announced the discovery of a planet orbiting the star COROT-7. Although the planet orbits its star at a distance less than 0.02 AU, its diameter is estimated to be around 1.7 times that of Earth, making it the smallest super-Earth yet measured. Due to its extreme closeness to its parent star, it is believed to have a molten surface at a temperature of 1000–1500 °C.[63] It was discovered by the French COROT satellite.

2009, Gliese 581 e

On April 21, the European Southern Observatory announced the discovery of a fourth planet orbiting the star Gliese 581. The planet orbits its parent star at a distance of less than 0.03 AU and has a minimum mass estimated at 1.9 times that of Earth. As of December 2009, this is the lightest known extrasolar planet to orbit a main-sequence star.[5]

2009, 30 planets

On October 19, it was announced that 30 new planets were discovered, all were detected by radial velocity method. It is the most planets ever announced in a single day during the exoplanet era. October 2009 now holds the most planets discovered in a month, breaking the record set in June 2002 and August 2009, during which 17 planets were discovered.

2009, 61 Virginis

On December 14, three planets (one is super-Earth and two are Neptune-mass planets) were discovered. Also a super-Earth planet and two unconfirmed planets around HD 1461 were discovered. These discoveries indicated that low-mass planets that orbit around nearby stars are very common. 61 Virginis is the first star like the Sun to host the super-Earth planets.[64]

2009, GJ 1214 b

On December 16, a super-Earth planet was discovered by transit. The determination of density from mass and radius suggest that this planet may be an ocean planet composed of 75% water and 25% rock. Some of the water on this planet should be in the exotic form of ice VII. This is the first planet discovered by MEarth Project, which is used to look for transits of super-Earth planets crossing the face of M-type stars.[65]

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See also: Stars with extrasolar planets • Extrasolar planet extremes • Unconfirmed exoplanets

Retrieved from "http://en.wikipedia.org/wiki/Extrasolar_planet"

Categories: Extrasolar planets | Lists of planets | SETI | Types of planet

UPDATE BELOW

An extrasolar planet, or exoplanet, is a planet outside the Solar System. As of 30 September 2010, astronomers have announced confirmed detections of 492 such planets.[1] The vast majority have been detected through radial velocity observations and other indirect methods rather than actual imaging.[1] Most are giant planets thought to resemble Jupiter; this partly reflects a sampling bias in that more massive planets are easier to observe with current technology. Several relatively lightweight exoplanets, only a few times more massive than Earth, have also been detected and projections suggest that these will eventually be found to outnumber giant planets.[2][3] It is now known that a substantial fraction of stars have planetary systems, including at least around 10% of sun-like stars. (The true proportion may be much higher.)[4] It follows that billions of exoplanets must exist in our own galaxy alone. There also exist planets that orbit brown dwarfs and free floating planets that do not orbit any parent body at all, though as a matter of definition it is unclear if either of these should be referred to by the term "planet."

Extrasolar planets became an object of scientific investigation in the nineteenth century. Many astronomers supposed that they existed, but there was no way of knowing how common they were or how similar they might be to the planets of our solar system. The first confirmed detection was made in 1992, with the discovery of several terrestrial-mass planets orbiting the pulsar PSR B1257+12.[5] The first confirmed detection of an exoplanet orbiting a main-sequence star was made in 1995, when a giant planet, 51 Pegasi b, was found in a four-day orbit around the nearby G-type star 51 Pegasi. The frequency of detections has tended to increase on an annual basis since then.[1]

The discovery of extrasolar planets has intensified interest in the possibility of extraterrestrial life.[6] As of September 2010[update], Gliese 581 g, fourth planet of the red dwarf star Gliese 581, appears to be the best known example of a possibly terrestrial exoplanet orbiting within the habitable zone that surrounds its star.[7]

History of detection

Retracted discoveries

Unconfirmed until 1992, extrasolar planets had long been a subject of discussion and speculation. In the sixteenth century the Italian philosopher Giordano Bruno, an early supporter of the Copernican theory that the Earth and other planets orbit the Sun, put forward the view that the fixed stars are similar to the Sun and are likewise accompanied by their own planets.[8] In the eighteenth century the same possibility was mentioned by Isaac Newton in the "General Scholium" that concludes his Principia. Making a comparison to the Sun's planets, he wrote "And if the fixed stars are the centers of similar systems, they will all be constructed according to a similar design and subject to the dominion of One." [9]

Claims of exoplanet detections have been made since the nineteenth century. Some of the earliest involve the binary star 70 Ophiuchi. In 1855 Capt. W. S. Jacob at the East India Company's Madras Observatory reported that orbital anomalies made it "highly probable" that there was a "planetary body" in this system.[10] In the 1890s, Thomas J. J. See of the University of Chicago and the United States Naval Observatory stated that the orbital anomalies proved the existence of a dark body in the 70 Ophiuchi system with a 36-year period around one of the stars.[11] However, Forest Ray Moulton soon published a paper proving that a three-body system with those orbital parameters would be highly unstable.[12] During the 1950s and 1960s, Peter van de Kamp of Swarthmore College made another prominent series of detection claims, this time for planets orbiting Barnard's Star.[13] Astronomers now generally regard all the early reports of detection as erroneous.[14]

In 1991, Andrew Lyne, M. Bailes and S.L. Shemar claimed to have discovered a pulsar planet in orbit around PSR 1829-10, using pulsar timing variations.[15] The claim briefly received intense attention, but Lyne and his team soon retracted it.[16]

Confirmed discoveries

The first published discovery to have received subsequent confirmation was made in 1988 by the Canadian astronomers Bruce Campbell, G. A. H. Walker, and S. Yang.[17] Although they remained cautious about claiming a true planetary detection, their radial-velocity observations suggested that a planet orbited the star Gamma Cephei. Partly because the observations were at the very limits of instrumental capabilities at the time, widespread skepticism persisted in the astronomical community for several years about this and other similar observations. Another source of confusion was that some of the possible planets might instead have been brown dwarfs, objects that are intermediate in mass between planets and stars. The following year, however, additional observations were published that supported the reality of the planet orbiting Gamma Cephei,[18] though subsequent work in 1992 raised serious doubts.[19] Finally, in 2002, improved techniques allowed the planet's existence to be confirmed.[20]

In early 1992, radio astronomers Aleksander Wolszczan and Dale Frail announced the discovery of planets around another pulsar, PSR 1257+12.[5] This discovery was quickly confirmed, and is generally considered to be the first definitive detection of exoplanets. These pulsar planets are believed to have formed from the unusual remnants of the supernova that produced the pulsar, in a second round of planet formation, or else to be the remaining rocky cores of gas giants that survived the supernova and then decayed into their current orbits.

On October 6, 1995, Michel Mayor and Didier Queloz of the University of Geneva announced the first definitive detection of an exoplanet orbiting an ordinary main-sequence star (51 Pegasi).[21] This discovery, made at the Observatoire de Haute-Provence, ushered in the modern era of exoplanetary discovery. Technological advances, most notably in high-resolution spectroscopy, led to the detection of many new exoplanets at a rapid rate. These advances allowed astronomers to detect exoplanets indirectly by measuring their gravitational influence on the motion of their parent stars. Additional extrasolar planets were eventually detected by observing the variation in a star's apparent luminosity as an orbiting planet passed in front of it.

To date[update], 492 exoplanets are listed in the Extrasolar Planets Encyclopaedia, including a few that were confirmations of controversial claims from the late 1980s.[1] The first system to have more than one planet detected was PSR 1257+12; the first confirmed to have multiple planets orbiting a main-sequence star was Upsilon Andromedae. Fifty-five such multiple-planet systems are known as of September 2010[update]. Among the known exoplanets are four pulsar planets orbiting two separate pulsars. Infrared observations of circumstellar dust disks also suggest the existence of millions of comets in several extrasolar systems.

Detection methods

Main article: Methods of detecting extrasolar planets

Planets are extremely faint light sources compared to their parent stars. At visible wavelengths, they usually have less than a millionth of their parent star's brightness. It is extremely difficult to detect such a faint light source, and furthermore the parent star causes a glare that tends to wash it out.

Direct image of exoplanets around the star HR8799 using a vector vortex coronagraph on a 1.5m portion of the Hale telescope

For the above reasons, telescopes have directly imaged no more than about ten exoplanets. This has only been possible for planets that are especially large (usually much larger than Jupiter) and widely separated from their parent star. Most of the directly imaged planets have also been very hot, so that they emit intense infrared radiation; the images have then been made at infrared rather than visible wavelengths, in order to reduce the problem of glare from the parent star.

A team of researchers from NASA's Jet Propulsion Laboratory has recently demonstrated a technique for blocking a star's light with a vector vortex coronagraph, thus enabling direct detections to be made much more easily. The researchers are hopeful that many new planets may be imaged using this technique.[22][23] Another promising approach is nulling interferometry.[24]

At the moment, however, the vast majority of known extrasolar planets have only been detected through indirect methods. The following are the indirect methods that have proven useful:

As a planet orbits a star, the star also moves in its own small orbit around the system's center of mass. Variations in the star's radial velocity — that is, the speed with which it moves towards or away from Earth — can be detected from displacements in the star's spectral lines due to the Doppler effect. Extremely small radial-velocity variations can be observed, down to roughly 1 m/s. This has been by far the most productive method of discovering exoplanets. It has the advantage of being applicable to stars with a wide range of characteristics.

If a planet crosses (or transits) in front of its parent star's disk, then the observed brightness of the star drops by a small amount. The amount by which the star dims depends on its size and on the size of the planet, among other factors. This has been the second most productive method of detection, though it suffers from a substantial rate of false positives and confirmation from another method is usually considered necessary.

TTV is a variation on the transit method where the variations in transit of one planet can be used to detect another. The first planetary candidate found this way was exoplanet WASP-3c, using WASP-3b in the WASP-3 system by Rozhen Observatory, Jena Observatory, and Toruń Centre for Astronomy.[25] The new method can potentially detect Earth sized planets or exomoons.[25]

Microlensing occurs when the gravitational field of a star acts like a lens, magnifying the light of a distant background star. Planets orbiting the lensing star can cause detectable anomalies in the magnification as it varies over time. This method has resulted in only a few planetary detections, but it has the advantage of being especially sensitive to planets at large separations from their parent stars.

Astrometry consists of precisely measuring a star's position in the sky and observing the changes in that position over time. The motion of a star due to the gravitational influence of a planet may be observable. Because that motion is so small, however, this method has not yet been very productive at detecting exoplanets.

A pulsar (the small, ultradense remnant of a star that has exploded as a supernova) emits radio waves extremely regularly as it rotates. If planets orbit the pulsar, they will cause slight anomalies in the timing of its observed radio pulses. Four planets have been detected in this way, around two different pulsars. The first confirmed discovery of an extrasolar planet was made using this method.

If a planet has a large orbit that carries it around both members of an eclipsing double star system, then the planet can be detected through small variations in the timing of the stars' eclipses of each other. As of December 2009, two planets have been found by this method.

Disks of space dust surround many stars, and this dust can be detected because it absorbs ordinary starlight and re-emits it as infrared radiation. Features in the disks may suggest the presence of planets.

Most extrasolar planet candidates have been found using ground-based telescopes. However, many of the methods can work more effectively with space-based telescopes that avoid atmospheric haze and turbulence. COROT (launched December 2006) and Kepler (launched March 2009) are the two currently active space missions dedicated to searching for extrasolar planets. Hubble Space Telescope and MOST have also found or confirmed a few planets. There are also several planned or proposed space missions geared towards exoplanet observation, such as New Worlds Mission, Darwin, Space Interferometry Mission, Terrestrial Planet Finder and PEGASE.

Definition

The official definition of "planet" used by the International Astronomical Union (IAU) only covers the Solar System and thus takes no stance on exoplanets.[26][27] As of April 2010, the only definitional statement issued by the IAU that pertains to exoplanets is a working definition issued in 2001 and modified in 2003.[28] This definition contains the following criteria:

    • Objects with true masses below the limiting mass for thermonuclear fusion of deuterium (currently calculated to be 13 Jupiter masses for objects of solar metallicity) that orbit stars or stellar remnants are "planets" (no matter how they formed). The minimum mass/size required for an extrasolar object to be considered a planet should be the same as that used in our solar system.

    • Substellar objects with true masses above the limiting mass for thermonuclear fusion of deuterium are "brown dwarfs", no matter how they formed nor where they are located.

    • Free-floating objects in young star clusters with masses below the limiting mass for thermonuclear fusion of deuterium are not "planets", but are "sub-brown dwarfs" (or whatever name is most appropriate).

This article follows the above working definition. Therefore it only discusses planets that orbit stars or brown dwarfs. (There have also been several reported detections of planetary-mass objects, sometimes called "rogue planets," that do not orbit any parent body.[29] Some of these may have once belonged to a star's planetary system before being ejected from it.)

However, it should be noted that the IAU's working definition is not universally accepted. One alternate suggestion is that planets should be distinguished from brown dwarfs on the basis of formation. It is widely believed that giant planets form through core accretion, and that process may sometimes produce planets with masses above the deuterium fusion threshold;[30][31] massive planets of that sort may have already been observed.[32] This viewpoint also admits the possibility of sub-brown dwarfs, which have planetary masses but form like stars from the direct collapse of clouds of gas.

Nomenclature

The system used in the scientific literature for naming exoplanets is almost the same as the system used for naming binary stars. The only modification is that a lowercase letter is used for the planet instead of the uppercase letter used for stars. The lowercase letter is placed after the star name, starting with "b" for the first planet found in the system (for example, 51 Pegasi b); "a" is skipped to avoid any confusion with the primary star. The next planet found in the system is labeled with the next letter in the alphabet. For instance, any more planets found around 51 Pegasi will be catalogued as "51 Pegasi c" and "51 Pegasi d", and so on. If two planets are discovered at the same time, the closer one to the star gets the next letter, followed by the farther planet. In a few cases a planet has been found closer to the star than other previously known planets, so that the letter order does not follow the order of the planets from the star. For example, in the 55 Cancri system, the most recently discovered planet is referred to as 55 Cancri f, despite the fact that it is closer to the star than 55 Cancri d. As of August 2010 the highest letter in use is "h", applied to the planet HD 10180 h.

If a planet orbits one member of a multiple-star system, then an uppercase letter for the star will be followed by a lowercase letter for the planet. Examples include the planets 16 Cygni Bb and 83 Leonis Bb. However, if the planet orbits the primary star of the system, and the secondary stars were either discovered after the planet or are relatively far form the primary star and planet, then the uppercase letter is usually omitted. For example, Tau Boötis b orbits in a binary system, but because the secondary star was both discovered after the planet and very far from the primary star and planet, the term "Tau Boötis Ab" is rarely if ever used.

Only two planetary systems have planets that are named unusually. Before the discovery of 51 Pegasi b in 1995, two pulsar planets (PSR B1257+12 B and PSR B1257+12 C) were discovered from pulsar timing of their dead star. Since there was no official way of naming planets at the time, they were called "B" and "C" (similar to how planets are named today). However, uppercase letters were used, most likely because of the way binary stars were named. When a third planet was discovered, it was designated PSR B1257+12 A (simply because the planet was closer than the other two).[33]

An alternate nomenclature, often seen in science fiction, uses Roman numerals in the order of planets' positions from the star. (This is inspired by an old system for naming moons of the outer planets, such as "Jupiter IV" for Callisto.) But such a system has proven impractical for scientific use. To use our solar system as an example, Jupiter would most likely be the first planet discovered, and Saturn the second; but, as the terrestrial planets would not be easily detected, Jupiter and Saturn would be called "Sol I" and "Sol II" by science-fiction nomenclature, and need to be renamed "Sol V" and "Sol VI" if all four terrestrial planets are discovered later. In contrast, by the current system, even if the terrestrial planets were found, Jupiter and Saturn would remain "Sol b" and "Sol c" and not need renaming.

Finally, several planets have received unofficial names comparable to those of planets in the Solar System. Among the notable planets that have been given such names are Osiris (HD 209458 b), Bellerophon (51 Pegasi b), and Methuselah (PSR B1620-26 b). The International Astronomical Union (IAU) currently has no plans to officially assign names of this sort to extrasolar planets, considering it impractical.[34]

General properties

Number of stars with planets

Most of the discovered extrasolar planets lie within 300 light years of the Solar System.

Planet-search programs have discovered planets orbiting a substantial fraction of the stars they have looked at. However, the total fraction of stars with planets is uncertain because of observational selection effects. The radial-velocity method and the transit method (which between them are responsible for the vast majority of detections) are most sensitive to large planets on small orbits. For that reason, many known exoplanets are "hot Jupiters": planets of roughly Jupiter-like mass on very small orbits with periods of only a few days. It is now known that 1% to 1.5% of sunlike stars possess such a planet, where "sunlike star" refers to any main-sequence star of spectral classes F, G, or K without a close stellar companion.[4] It is further estimated that 3% to 4.5% of sunlike stars possess a giant planet with an orbital period of 100 days or less, where "giant planet" means a planet of at least thirty Earth masses.[35]

The fraction of stars with smaller or more distant planets remains difficult to estimate. Extrapolation does suggest that small planets (of roughly Earth-like mass) are more common than giant planets. It also appears that planets on large orbits may be more common than ones on small orbits. Based on such extrapolation, it is estimated that perhaps 20% of sunlike stars have at least one giant planet while at least 40% may have planets of lower mass.[35][36][37]

Regardless of the exact fraction of stars with planets, the total number of exoplanets must be very large. Since our own Milky Way Galaxy has at least 200 billion stars, it must also contain billions of planets if not hundreds of billions of them.

Characteristics of planet-hosting stars

The Morgan-Keenan spectral classification

Most known exoplanets orbit stars roughly similar to our own Sun, that is, main-sequence stars of spectral categories F, G, or K. One reason is simply that planet search programs have tended to concentrate on such stars. But even after taking this into account, statistical analysis indicates that lower-mass stars (red dwarfs, of spectral category M) are either less likely to have planets or have planets that are themselves of lower mass and hence harder to detect.[35][38] Recent observations by the Spitzer Space Telescope indicate that stars of spectral category O, which are much hotter than our Sun, produce a photo-evaporation effect that inhibits planetary formation.[39]

Stars are composed mainly of the light elements hydrogen and helium. They also contain a small fraction of heavier elements such as iron, and this fraction is referred to as a star's metallicity. Stars of higher metallicity are much more likely to have planets, and the planets they have tend to be more massive than those of lower-metallicity stars.[4] It has also been shown that stars with planets are more likely to be deficient in lithium.[40]

Orbital parameters

Scatterplot showing masses and orbital periods of all extrasolar planets discovered through 2010-10-03, with colors indicating method of detection:

For reference, Solar System planets are marked as gray circles.

Most known extrasolar planet candidates have been discovered using indirect methods and therefore only some physical and orbital parameters can be determined. For example, out of the six independent parameters that define an orbit, the radial-velocity method can determine four: semi-major axis, eccentricity, longitude of periastron, and time of periastron. Two parameters remain unknown: inclination and longitude of the ascending node.

Many exoplanets have orbits with very small semi-major axes, and are thus much closer to their parent star than any planet in our own solar system is to the Sun. That fact, however, is mainly due to observational selection: The radial-velocity method is most sensitive to planets with small orbits. Astronomers were initially very surprised by these "hot Jupiters", but it is now clear that most exoplanets (or, at least, most high-mass exoplanets) have much larger orbits, some located in habitable zones where suitable for liquid water and life.[35] It appears plausible that in most exoplanetary systems, there are one or two giant planets with orbits comparable in size to those of Jupiter and Saturn in our own solar system. Giant planets with substantially larger orbits are now known to be rare, at least around sun-like stars.[41]

The eccentricity of an orbit is a measure of how elliptical (elongated) it is. Most exoplanets with orbital periods of 20 days or less have near-circular orbits of very low eccentricity. That is believed to be due to tidal circularization, an effect in which the gravitational interaction between two bodies gradually reduces their orbital eccentricity. By contrast, most known exoplanets with longer orbital periods have quite eccentric orbits. (As of July 2010, 55% of such exoplanets have eccentricities greater than 0.2 while 17% have eccentricities greater than 0.5.[1]) This is not an observational selection effect, since a planet can be detected about equally well regardless of the eccentricity of its orbit. The prevalence of elliptical orbits is a major puzzle, since current theories of planetary formation strongly suggest planets should form with circular (that is, non-eccentric) orbits.[14]

The prevalence of eccentric orbits may also indicate that our own solar system is somewhat unusual, since all of its planets except for Mercury have near-circular orbits.[4]

However, it has recently been suggested that some of the high eccentricity values reported for exoplanets may be overestimates, since simulations show that many observations are also consistent with two planets on circular orbits. Planets reported as single moderately eccentric planets have a ~15% chance of being part of such a pair.[42] This misinterpretation is especially likely if the two planets orbit with a 2:1 resonance. One group of astronomers has concluded that "(1) around 35% of the published eccentric one-planet solutions are statistically indistinguishable from planetary systems in 2:1 orbital resonance, (2) another 40% cannot be statistically distinguished from a circular orbital solution" and "(3) planets with masses comparable to Earth could be hidden in known orbital solutions of eccentric super-Earths and Neptune mass planets."[43]

A combination of astrometric and radial velocity measurements has revealed that some planetary systems differ from our Solar System by containing planets whose orbital planes are significantly tilted relative to each other.[44] Research has now also shown that more than half of hot Jupiters have orbital planes substantially misaligned with their parent stars' rotation. A substantial fraction even have retrograde orbits, meaning that they orbit in the opposite direction from the star's rotation.[45] Andrew Cameron of the University of St Andrews stated, “The new results really challenge the conventional wisdom that planets should always orbit in the same direction as their stars spin." [46] Rather than a planet's orbit having been disturbed, it may be that the star itself flipped over early in their system's formation due to interactions between the star's magnetic field and the planet-forming disc.[47]

Mass distribution

When a planet is found by the radial-velocity method, its orbital inclination i is unknown. The method is unable to determine the true mass of the planet, but rather gives its minimum mass M sini. In a few cases an apparent exoplanet may actually be a more massive object such as a brown dwarf or red dwarf. However, statistically the factor of sini takes on an average value of π/4≈0.785 and hence most planets will have true masses fairly close to the minimum mass.[35] Furthermore, if the planet's orbit is nearly perpendicular to the sky (with an inclination close to 90°), the planet can also be detected through the transit method. The inclination will then be known, and the planet's true mass can be found. Also, astrometric observations and dynamical considerations in multiple-planet systems can sometimes be used to constrain a planet's true mass.

The vast majority of exoplanets detected so far have high masses. As of January 2010, all but twenty-five of them have more than ten times the mass of Earth.[1] Many are considerably more massive than Jupiter, the most massive planet in the Solar System. However, these high masses are in large part due to an observational selection effect: all detection methods are much more likely to discover massive planets. This bias makes statistical analysis difficult, but it appears that lower-mass planets are actually more common than higher-mass ones, at least within a broad mass range that includes all giant planets. In addition, the fact that astronomers have found several planets only a few times more massive than Earth, despite the great difficulty of detecting them, indicates that such planets are fairly common.[4]

The results from the first 43 days of Kepler mission "imply that small candidate planets with periods less than 30 days are much more common than large candidate planets with periods less than 30 days and that the ground-based discoveries are sampling the large-size tail of the size distribution"[3]

Temperature and composition

Comparison of sizes of planets with different compositions

It is possible to estimate the temperature of an exoplanet based on the intensity of the light it receives from its parent star. For example, the planet OGLE-2005-BLG-390Lb is estimated to have a surface temperature of roughly -220°C (roughly 50 K). However, such estimates may be substantially in error because they depend on the planet's usually unknown albedo, and because factors such as the greenhouse effect may introduce unknown complications. A few planets have had their temperature measured by observing the variation in infrared radiation as the planet moves around in its orbit and is eclipsed by its parent star. For example, the planet HD 189733b has been found to have an average temperature of 1205±9 K (932±9°C) on its dayside and 973±33 K (700±33°C) on its nightside.[48]

If a planet is detectable by both the radial-velocity and the transit methods, then both its true mass and its radius can be found. The planet's density can then be calculated. Planets with low density are inferred to be composed mainly of hydrogen and helium, while planets of intermediate density are inferred to have water as a major constituent. A planet of high density is believed to be rocky, like Earth and the other terrestrial planets of the Solar System.

Spectroscopic measurements can be used to study a transiting planet's atmospheric composition.[49] Water vapor, sodium vapor, methane, and carbon dioxide have been detected in the atmospheres of various exoplanets in this way. The technique might conceivably discover atmospheric characteristics that suggest the presence of life on an exoplanet, but no such discovery has yet been made.

Another line of information about exoplanetary atmospheres comes from observations of orbital phase functions. Extrasolar planets have phases similar to the phases of the Moon. By observing the exact variation of brightness with phase, astronomers can calculate particle sizes in the atmospheres of planets.

Stellar light becomes polarized when it interacts with atmospheric molecules, which could be detected with a polarimeter. So far, one planet has been studied by polarimetry.

Unanswered questions

This planetary habitability chart shows where life might exist on extrasolar planets based on our own solar system and life on Earth.

Many unanswered questions remain about the properties of exoplanets. One puzzle is that many transiting exoplanets are much larger than expected given their mass, meaning that they have surprisingly low density. Several theories have been proposed to explain this observation, but none have yet been widely accepted among astronomers.[50] Another question is how likely exoplanets are to possess moons. No such moons have yet been detected, but they may be fairly common.

Perhaps the most interesting question about exoplanets is whether they might support life. Several planets do have orbits in their parent star's habitable zone, where it should be possible for liquid water to exist and for Earth-like conditions to prevail. Most of those planets are giant planets more similar to Jupiter than to Earth; if any of them have large moons, the moons might be a more plausible abode of life. Gliese 581 g is thought to be a rocky planet orbiting in the middle of its star's habitable zone. Gliese 581 g was discovered in September 2010 and is believed to be the most "Earth-like" planet discovered to date.

Various estimates have been made as to how many planets might support simple life or even intelligent life. For example, Dr. Alan Boss of the Carnegie Institution of Science estimates there may be a "hundred billion" terrestrial planets in our Milky Way Galaxy, many with simple lifeforms. He further believes there could be thousands of civilizations in our galaxy. Recent work by Duncan Forgan of Edinburgh University has also tried to estimate the number of intelligent civilizations in our galaxy. The research suggested there could be thousands of them.[51] However, due to the great uncertainties regarding the origin and development of life and intelligence, all such estimates must be regarded as extremely speculative. Apart from the scenario of an extraterrestrial civilization that is emitting powerful signals, the detection of life at interstellar distances is a tremendously challenging technical task that will not be feasible for many years, even if such life is commonplace.

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