The Kepler Mission: A Photometric Mission to Determine the Frequency of Inner Planets Near the Habitability Zone of Solar-like Stars

W. Borucki and D. Koch
NASA Ames Research Center, Moffett Field, CA

E. Dunham
Lowell Observatory, Flagstaff, AZ

W. Cochran
Univ. of Texas, Austin, TX

D. Cullers and J. Jenkins
SETI Institute, Mountain View, CA

T. Brown
High Altitude Observatory, NCAR, Boulder, CO

G. Marcy
San Francisco State Univ., San Francisco, CA

Kepler is a mission designed to detect and characterize Earth-sized planets around solar-like stars. The sizes of the planets are determined from the decrease in light from a star that occurs during planetary transits, and the orbital period is determined from the repeatability of the transits. The orbital radius and the nearness of the planet to the habitability zone is estimated from ancillary measurements of the stellar mass and brightness. Such measurements determine the spacing of planets, their distribution of size with orbital distance, and their variation with stellar type and multiplicity. Because thousands of stars must be continually monitored to detect the transits, extensive information on the stars can be obtained on their rotation rates and activity cycles. Observations of p-mode oscillations also provides information on age and metallicity.

These goals are accomplished by continuously and simultaneously monitoring a single field of 140,000 stars for evidence of brightness changes caused by transits of Earth-sized or larger planets. To obtain the high precision needed to find planets as small as the Earth and Venus, a wide-field-of-view Schmidt telescope with an array of CCD detectors at its focal plane must be located outside the Earth's atmosphere. Both SMM (Solar Maximum Mission) and SOHO observations of the low-level variability of the Sun (~1:100,000) on the time scales of a transit (4 to 16 hours), and our laboratory measurements of the photometric precision of CCDs (1:100,000) show that the detection of planets as small as the Earth is practical. The probability for detecting transits is quite favorable for planets in inner orbits. If other planetary systems are similar to our own, then approximately 1% of those systems will show transits. About 50% of the field stars will be F, G, and K main sequence dwarfs for which transits by small planets will be detectable. If most of these stars have planetary systems similar to that of our solar system, nearly 300 planetary systems should be detected. If, instead, most of the planetary systems contain planetary cores (i.e., planets twice the size of the Earth, but without the mass to attract a massive hydrogen-helium atmosphere) in inner orbits, then some 600 planetary systems should be discovered. Presuming other systems have orbits with small relative inclinations, then about 10% of these systems should also show transits by a second planet. However, if no other planetary systems exist with small inner planets, then the Kepler Mission will still discover some 1400 giant inner planets from their reflected light assuming that their frequency is as large as the results of Marcy and Butler indicate.