Kepler's Latest Hunting Trips

Torrents of likely new exoplanets are pouring in from NASA's Kepler space telescope - more than 1,200 of them so far, large and small, including weird worlds and systems that no one expected. The Kepler science team unveiled its latest batch of findings on February 2nd, based largely on data from just the first four months (May 12 to Sept. 17, 2009) of Kepler's planned 3½-year mission. The Kepler scientists highlighted two themes. One was the sheer number and variety of likely planets being found, especially small ones, including several small ones in their stars' habitable zones. The team also highlighted one system in particular. A 14th-magnitude star dubbed Kepler-11 seems to have six super-earths and Neptunes transiting it. All are orbiting in nearly the same plane, five of them in compactly nested orbits closer to the star than Mercury is to the Sun. Two or three of these planets have such unexpectedly low average densities that they would float in water.

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.

The Six-Transit System

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).

Combining the masses with the diameters gives each object's average density: about 3, 2, 0.9, 0.5, and 0.7 grams per cubic centimeter. The last three are less dense than water (1.0). Earth's average density, by comparison, is 5.5. They probably have rock-iron cores surrounded by thick envelopes of "ices" (mainly water, methane, and ammonia, either solid or liquid)and gas (mainly hydrogen and helium). In other words, they're miniature giants.Says team member Jonathan Fortney (UC, Santa Cruz), "I think of them as being like a marshmallow with a ball bearing in the center. Most of the mass is in the core, but most of the volume is in the atmosphere. "Says exoplanet researcher David Charbonneau(Harvard-Smithsonian Center for Astrophysics), "It's quite simply one of the most beautiful data sets I have ever seen. "Computer simulations show that the configuration can be gravitationally stable over the star's estimated age of 8 billion years. Surprisingly, none of the planets are locked in orbital resonances with their neighbors. They almost certainly formed farther out and migrated inward, due to gravitational interactions with a massive gas-and-rubble disk when the system was young. The disk would also tend to circularize their orbits, keep the orbital planes well aligned, and help prevent the planets from locking one another into resonances.

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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