1,235 Likely Worlds
The new data release brings Kepler's total to 1,235 planet candidates apparently transiting stars. Team member Geoff Marcy (University of California, Berkeley) estimates that "90% to 95% of these candidates are bona fide planets." This compares to 513 exoplanets discovered by all other projects since 1993. Of the new candidates, dozens are roughly Earth-diameter or smaller. Add steam member Daniel Fabrycky (University of California, Santa Cruz), "There area ton of multiple-planet candidates: 115doubles, 45 triples, 8 quadruples, 1 quintuple, and 1 sextet. "Sixty-eight are roughly the diameter of Earth. The total includes 288 more with super-Earth diameters, 662 in the Neptune class, 165 about the size of Jupiter, and 19 significantly larger than Jupiter. Of the 68 roughly Earth-size bodies, five are within the habitable zones of their host stars: the not-too-hot, not-too cold region where liquid water could lie exposed on the surface under modest atmospheric pressure, as on Earth. Forty nine other worlds within habitable zones range from about twice Earth's diameter to larger than Jupiter. Overall, when you consider the unlikelihood that an object will transit its star at all as seen from our line of sight, the statistics indicate that some 20% of stars are closely orbited by planets Earth-size and up.
Of the 156,000 stars that Kepler is watching, the team drew particular attention to Kepler-11, a near-copy of the Sun about 2,000 light-years away under the eastern wing of Cygnus. Six small bodies orbit it with periods from 10 to 118 days. The amount of the star's light that each blocks during its transits tells its size. Counting out from the star, they have 2.0, 3.2, 3.4,4.5, 2.6, and 3.7 Earth diameters. The six worlds are too lightweight, and the star is too far and faint, for astronomers to measure their masses by the gravitational wobbles that they induce in the star.
But in a tour de force of celestial mechanics, the Kepler team measured the slight delays and speedups in their observed transit times due to their gravitational influences on one another. The team was able to untangle all the interactions. The transit-timing variations yielded masses for the five inner bodies: 4, 13, 6, 8, and 2 Earth masses.
Left: On August 26, 2010, the Kepler spacecraft observed the Sun-like star Kepler-11 being transited by three of its six planets simultaneously, as depicted in this artist's concept. Right: As of its latest data release, NASA’s Kepler science team had identified 1,235 planet candidates (yellow dots), far more than the number of transiting exoplanets known prior to the mission (red dots).
The planets generally become less dense the farther they orbit the star, a relationship that hardly comes as a surprise. The star's heat and wind are likely to be slowly stripping the atmospheres away,with the innermost planets suffering the most. "This is exactly the kind of system you want in order to study this mass-loss process," says Fortney. "Six planets around the same star - it's ideal for comparative planetary science."
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