By WILLIAM J. BORUCKI,1 DAVID KOCH,1
GIBOR BASRI,2 TIMOTHY BROWN,3 DOUGLAS CALDWELL,4 EDNA DEVORE,4 EDWARD DUNHAM,5 THOMAS GAUTIER,6 JOHN GEARY,7 RONALD GILLILAND,8 ALAN GOULD,9 STEVE HOWELL,10 JON JENKINS,4 and DAVID LATHAM 7
XNASA Ames Research Center, Moffett Field, CA 94035, USA;
2University of California, Berkeley, CA 94720, USA; [email protected] 3High Altitude Observatory, NCAR, Boulder, CO 80307, USA; [email protected] 4SETI Institute, Mountain View, CA 94043, USA; [email protected]; [email protected]; [email protected] 5Lowell Observatory, Flagstaff, AZ 86001, USA; [email protected] 6Jet Propulsion Laboratory, Pasadena, CA 91109, USA; [email protected]
8Space Telescope Science Institute, Baltimore, MD 21218, USA; [email protected]
9Lawrence Hall of Science, University of California, Berkeley, CA 94720, USA;
10University of California, Riverside, CA 92521, USA; [email protected]
Kepler is a Discovery-class mission designed to determine the frequency of Earth-size and smaller planets in and near the habitable zone (HZ) of spectral type F through M dwarf stars. The instrument consists of a 0.95 m aperture photometer to do high-precision photometry of 100,000 solar-like stars to search for patterns of transits. The depth and repetition time of transits provide the size of the planet relative to the star and its orbital period. Multi-band ground-based observation of these stars is currently underway to estimate the stellar parameters and to choose appropriate targets. With these parameters, the true planet radius and orbit scale—hence the relation to the HZ—can be determined. These spectra are also used to discover the relationships between the characteristics of planets and the stars they orbit. In particular, the association of planet size and occurrence frequency with stellar mass and metallicity will be investigated. At the end of the four-year mission, several hundred terrestrial planets should be discovered with periods between 1-400 days, if such planets are common. A null result would imply that terrestrial planets are rare. Based on the results of the recent Doppler-velocity discoveries, over a thousand giant planets will also be found. Information on the albedos and densities of those giants showing transits will be obtained. The mission is now in Phase C/D development and is scheduled for launch in 2008 into a 372-day heliocentric orbit.
Since the first discoveries of planetary companions around normal stars in 1995, more than 150 such planets have been discovered. At least 5%, and as many as 25%, of solar-like stars show the presence of giant planets (Lineweaver & Grether 2003). These planets are generally very massive, often exceeding that of Jupiter and Saturn. Further, most have semi-major axes less than 1 AU and have high orbital eccentricities. The surprisingly small values for the semi-major axes imply that they form at several AU, but then lose momentum to the accretion disk and spiral inward. It is unclear what processes terminate the inward motion and what fraction of planets fall into the star. However, it is obvious that the inward motion of the giant planets will remove smaller planets by scattering them either into the star or out of the planetary system. It is also possible that the stars not showing the presence of giant planets are devoid of all planets because the giant planets merged with the star after their inward migration. Thus, planetary systems with terrestrial planets might be very rare. However, a recent discovery (Rivera et al. 2005) of a planet with a mass 7.5 times that of the Earth—with a 2.7-day orbital period—indicates that at least some terrestrial planets survive.
Determination of the frequency of terrestrial planets (and their distributions of size and orbital semi-major axes) is needed to increase our understanding of the structure of planetary systems. The Kepler mission is designed to discover hundreds of terrestrial planets in and near the habitable zone (HZ) around a wide variety of stars. For short-period orbits, hundreds of transits will be observed during the four-year mission so that planets as small as Mercury and Mars can be detected. Kepler is a PI-lead mission and was competitively selected as NASA Discovery Mission #10 in December 2001. It is scheduled to launch in June 2008 into an Earth-trailing orbit. A description of the mission and the expected science results are presented.
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