SCIENCE'S NEW FRONTIER
As scientists look forward to the discovery of the 1000th confirmed exo-solar planet, will life ever be found beyond the earth? Martin Hickes reports.
Some 400 hundred years ago, the Italian friar, astronomer and philosopher Giordano Bruno was burned at the stake for suggesting that there was evidence that the earth revolved around the sun and that the universe possessed an infinite number of habitable worlds.
After his death, he gained considerable fame, particularly among 19th- and early 20th-century commentators who regarded him as a martyr for free thought and modern scientific ideas.
Today, while life has yet to be discovered on other worlds, the number of confirmed planets which orbit other stars outside our solar system, perhaps remarkably, is slowly clocking up to the 900 mark, with the 1,000th milestone expected to be achieved within the next year.
Beyond this actual number, more than 2,700 candidate planets have been earmarked for confirmation, making the total number of so-called exo-solar planets i.e. those beyond our known solar system, adding up to around 3,000.
And even beyond this figure, NASA’s Kepler mission, launched back in 2009, has to date detected over 18,000 additional ‘candidate’ planets – likely worlds beyond our own - including, perhaps crucially, 262 ‘habitable’ ones.
While the $600m Kepler mission has no remit to discover life per se, more a focus on possible habitable planets, the holy grail of this new astronomic realm remains a ‘new earth’ – a planet which could sustain life around a distant star; or ultimately the discovery of alien life on an extant one.
NASA’s Planet Quest web site, a division of the Jet Propulsion Laboratory, is counting down (or up) the new 1000th planet milestone, and features a special ticker tracking the number of confirmed exo-planets discovered.
A host of ‘new planet’ watching sites - and planet watchers – are also helping to capture the public’s imagination for other worlds.
NASA says: “The centuries-old quest for other worlds like our Earth has been rejuvenated by the intense excitement and popular interest surrounding the discovery of hundreds of planets orbiting other stars.
“There is now clear evidence for substantial numbers of three types of exoplanets; gas giants, hot-super-Earths in short period orbits, and ice giants.
“The challenge now is to find terrestrial planets (i.e., those one half to twice the size of the Earth), especially those in the habitable zone of their stars where liquid water and possibly life might exist.
“The Kepler Mission, is specifically designed to survey a portion of our region of the Milky Way galaxy to discover dozens of Earth-size planets in or near the ‘habitable zone’ and determine how many of the billions of stars in our galaxy have such planets.
“Results from this mission will allow us to place our solar system within the continuum of planetary systems in the Galaxy.”
Distant planets are too small to be pinpointed by ground based telescopes.
The first confirmed exo solar planet detection came in 1992, with the discovery of several terrestrial-mass planets orbiting a pulsar, a special type of star.
But the first confirmed detection of an exo-planet orbiting a ‘main-sequence star’ was made in 1995, when a giant planet was found in a four-day orbit around 51 Pegasi.
Due to improved observational techniques, the rate of detections has increased rapidly since then.
The vast majority have been detected through indirect methods such as radial velocity measurements, which measure perturbations in the orbit of a star caused by a nearby candidate planet.
Other techniques include measuring the ‘dip’ in light from parent star when a candidate planet passes in front of such.
Earlier this month, the Kepler team announced the discovery of the smallest planet yet found around a star like our Sun.
Dubbed Kepler-37b, the planet is slightly larger than our moon, measuring about one-third the size of Earth, which made its detection a challenge.
The research team used asteroseismology, the study of the interior of distant stars, to measure the radius of the host star Kepler-37 to three percent accuracy, which translates to exceptional accuracy in the planet's size.
Kepler-37 is the smallest star where this technique has been used to precisely measure a star’s size and, therefore the size of the planet.
March 2013 also marks the fourth anniversary of the Kepler launch.
Along its journey, it has found planets and planetary systems that made astronomy firsts.
These include: Kepler-10b, the first rocky planet outside our solar system; Kepler-11, an intriguing and tightly compacted six-planet system; Kepler-16b, the first double-star planet; Kepler-22b, Kepler's first habitable zone planet around a sun-like star; and Kepler-37b, a very small planet the size of Earth's moon.
Since the last Kepler catalog was released in February 2012, the number of candidates discovered in the Kepler data has increased by 20 percent and now totals 2,740 potential planets.
The most dramatic increases have been seen in the number of Earth-size and super Earth-size candidates discovered, which grew by 43 and 21 percent respectively.
On January 7, 2013, astronomers from the Kepler Mission space observatory announced the discovery of KOI-172.02, an Earth-like exoplanet candidate orbiting a star similar to our Sun in the habitable zone and possibly a "prime candidate to host alien life".
Kepler has been approved for extension through 2016. The extension provides four additional years to find Earth-size planets in the habitable zone -- the region in a planetary system where liquid water could exist on the surface of the orbiting planet -- around sun-like stars in our galaxy.
Astronomers at the Harvard-Smithsonian Center for Astrophysics (CfA) this year used Kepler's data to estimate that "at least 17 billion" Earth-sized exo-planets could well reside in the Milky Way Galaxy.
Scientists analysing the Kepler data reckon 17 per cent of stars have a planet 0.8 - 1.25 times the size of Earth.
About one-fourth of stars have a super-Earth (1.25 - 2 times the size of Earth) in an orbit of 150 days or less. The same fraction of stars has a ‘mini-Neptunes’.
Larger planets are much less common. Only about 3 per cent of stars have a ‘large Neptune’ (4 - 6and only 5 per cent of stars have a gas giant (6 - 22 times the size of the Earth) in an orbit of 400 days or less.
As astronomers continue to scan the heavens, it may well be the case that Bruno was right all along.