Earth-size exoplanets in comfortable orbits are common

NASA’s Kepler telescope was launched into orbit in March 2009. Its principal task has been to continually monitor 2 × 10⁵ stars in our galactic neighborhood for the slight dimming that might indicate an exoplanet transiting across a star’s disk. Now an independent team has reported an analysis of 3½ years of Kepler photometric data in search of Sun-like stars harboring Earth-size planets in orbits that might permit liquid water. To estimate the true abundance of such planets, the team had to correct the observed distribution of some 600 planets found orbiting Sun-like stars for observational biases that favor giant planets closely orbiting their stars with periods of days or weeks. The team concluded that 22 ± 8% of Sun-like stars harbor at least one Earth-size planet orbiting in the “habitable zone” that allows liquid water in the absence of strong greenhouse effects or other planetary peculiarities. That estimate would put the nearest habitable-zone exoplanet within about a dozen light-years of Earth—close enough for detailed follow-up with modest telescopes. The bias-corrected distribution of planet sizes, with Earth-size planets dominating over Jupiter-like gas giants, favors a much debated planet-formation scenario in which the accretion of a rocky core precedes the accumulation of a gas envelope from the star’s protoplanetary disk. The loss of a crucial gyroscopic motor earlier this year has now diverted Kepler to tasks with less stringent requirements for long-term pointing stability. (E. A. Petigura, A. W. Howard, G. W. Marcy, Proc. Natl. Acad. Sci. USA, in press .)—Bertram Schwarzschild