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Kepler finds rare multiple planetary system in 'habitable zone'

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February 7, 2011

Artist's impression of Kepler-11 planetary system as three planets transit - observed by N...

Artist's impression of Kepler-11 planetary system as three planets transit - observed by NASA's Kepler spacecraft on Aug 26 2010 (Credit: NASA - Tim Pyle)

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NASA's Kepler space telescope has succeeded in its mission to identify potentially-habitable exoplanets. Kepler has so far observed 156,000 stars in its field of vision and has identified no less than 1235 candidate planets that sit in the “goldilocks zone” (not too close to the star, and not too far away). Of these, scientists at the NASA's Ames Research Center are excited to announce the discovery of the Kepler-11 system – a rare multiple planetary system similar to our own with five planets in the habitable zone.

Of the stars with planetary candidates, 170 show signs of multiple planets, including one known as Kepler-11 that scientists have confirmed has at least six planets. Located around 2000 light years from Earth, it is the fullest, yet most compact planetary system discovered beyond our own. All its planets are larger than Earth, but some are comparable in size to Uranus and Neptune. Known as Kepler 11b to 11g, the innermost exoplanets is ten times closer to its parent star than Earth is to our sun, and the outer planet is still twice as close as we are comparatively, and would orbit between Mercury and Venus in our solar system.

Kepler's planets compared by size to Earth and Jupiter (Credit: NASA - Wendy Stenzel)

"The Kepler-11 planetary system is amazing," said Jack Lissauer, planetary scientist and Kepler science team member at NASA's Ames Research Center. "It’s amazingly compact, it’s amazingly flat, there’s an amazingly large number of big planets orbiting close to their star - we didn’t know such systems could even exist," he continued. Few stars are known to have more than one transiting planet, and Kepler-11 is the first known star to have more than three. So we know that systems like this are not common. There’s certainly far fewer than one percent of stars that have systems like Kepler-11. But whether it’s one in a thousand, one in ten thousand or one in a million, that we don’t know, because we only have observed one of them."

Scientists have been amazed at the varying compositions of confirmed planets with densities varying between iron and something akin to Styrofoam. Our own Earth has a density somewhere between these two. The five inner planets of Kepler 11 are a mixture of rock and gases, with rock making up most of the mass, and gas taking up much of the volume, a composition that might even include water. The high volume of gas provides clues about the formation of the solar system, and suggests that Kepler-11d, e, and f were formed early in its history, within a few million years.

Solar systems are formed when a molecular cloud collapses and forms a star. Disks of gas and dust surround the star, and can be seen around most stars less than one million years old. Few stars more than five million years old have them however, which suggests that planets with a high volume of gas obtained gases from the disk early in a solar system's history before the disk dispersed.

Of the other candidate planets identified since Kepler's launch in March 2009, 68 are about the same size of Earth, 288 a bit bigger; 662 are similar size to Neptune, 165 Jupiter-sized, and 19 are even larger than this. Fifty-four of these sit in the habitable zone, and of these five are Earth-sized. Planet candidates require further observations to confirm they are actual planets.

Planetary candidates identified by Kepler as of Feb 1st 2011 (Credit: NASA - Wendy Stenzel...

"The fact that we’ve found so many planet candidates in such a tiny fraction of the sky suggests there are countless planets orbiting stars like our sun in our galaxy," said William Borucki, Kepler Mission’s science principal investigator. "Kepler can find only a small fraction of the planets around the stars it looks at because the orbits aren’t aligned properly. If you account for those two factors, our results indicate there must be millions of planets orbiting the stars that surround our sun." “It’s looking like the galaxy may be littered with many planets,” said Roger Hunter, Kepler project manager.

The Kepler space observatory has a 96 million pixel camera that identifies orbiting bodies passing by stars by their visible “transit” path. The degree to which the star dims indicates the body and its proximity to the star in a science known as photometry. Temperature is deduced by the star characteristics and the orbital period.

The mission is currently half-way through its expected 3.5 year mission. Kepler will continue searching the sky until at least November 2012, but since transits of planets in the habitable zone of stars similar to our own sun occur about once a year, and three transits are required for verification, it is expected to take three years to locate and verify Earth-size planets orbiting sun-like stars. The more transits are witnessed, the better the sizes and masses can be estimated. The Kepler team additionally use the Spitzer Space Telescope and other ground-based telescopes to make additional observations of objects of interest identified by Kepler.

The team hope that further data from Kepler will allow them to make more precise estimates of planet sizes and masses. They also hope to detect more planets around Kepler-11, either by its transit or by the gravitational pull exerted on the other planets. Perhaps one of these will be even more similar to our own planet.

"Kepler is providing data 100 times better than anyone has ever done before," said Borucki. "It’s exploring a new part of phase space, a new part of the universe that could not be explored without this kind of precision, so it’s producing absolutely beautiful data. We’re seeing the variability of stars like no one has ever seen before. We’re finding planets smaller than anyone has ever seen before, because the data quality is extremely good. The first four months of data have given us an enormous amount of interesting information for the science community to explore and to find the planets among the candidates that we have found," said Borucki. "In the coming years, Kepler’s capabilities will allow us to find Earth-size planets in the habitable zone of other stars. Future missions will be developed to study the composition of planetary atmospheres to determine if they are compatible with the presence of life. The design for these missions depends of Kepler finding whether Earth-size planets in the habitable zone are common or rare."

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