The Occurrence and Mass Distribution of Close-in Super-Earths, Neptunes, and Jupiters

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Science  29 Oct 2010:
Vol. 330, Issue 6004, pp. 653-655
DOI: 10.1126/science.1194854

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Closing in on Extraterrestrial Earths

With close to 500 extrasolar planets discovered to date, researchers are starting to estimate the occurrence of low-mass planets to help our understanding of how planets form and evolve. Based on observations of 166 nearby stars with the Keck Telescope, Howard et al. (p. 653) report the occurrence of short-period planets around Sun-like stars as a function of planet mass. Planet formation models predicted that planet occurrence would increase with decreasing mass, such that satellites with masses similar to that of Neptune, and less, would be more common than gas-giant planets like Jupiter. Contrary to predictions, there is no dearth of planets with masses 5 to 30 times that of Earth, implying that the models may need revision. Nevertheless, observations suggest that 23% of Sun-like stars may be orbited by a close-in, terrestrial mass planet.


The questions of how planets form and how common Earth-like planets are can be addressed by measuring the distribution of exoplanet masses and orbital periods. We report the occurrence rate of close-in planets (with orbital periods less than 50 days), based on precise Doppler measurements of 166 Sun-like stars. We measured increasing planet occurrence with decreasing planet mass (M). Extrapolation of a power-law mass distribution fitted to our measurements, df/dlogM = 0.39 M−0.48, predicts that 23% of stars harbor a close-in Earth-mass planet (ranging from 0.5 to 2.0 Earth masses). Theoretical models of planet formation predict a deficit of planets in the domain from 5 to 30 Earth masses and with orbital periods less than 50 days. This region of parameter space is in fact well populated, implying that such models need substantial revision.

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