A Plateau in the Planet Population Below Twice the Size of Earth

TL;DR

This study uses a new detection algorithm and Kepler data to analyze small planets, revealing a plateau in occurrence at 2 Earth-radii that suggests different formation processes for smaller planets.

Contribution

It introduces TERRA, a new transit search algorithm optimized for small planets, and provides the first detailed occurrence rate distribution showing a plateau at 2 Earth-radii.

Findings

Occurrence rises from 5.7 to 2 Earth-radii

Occurrence plateaus below 2 Earth-radii

Approximately 15.1% of stars host 1-2 Earth-radii planets

Abstract

We carry out an independent search of Kepler photometry for small transiting planets with sizes 0.5--8.0 times that of Earth and orbital periods between 5 and 50 days, with the goal of measuring the fraction of stars harboring such planets. We use a new transit search algorithm, TERRA, optimized to detect small planets. We restrict our stellar sample to include the 12,000 stars having the lowest photometric noise in the Kepler survey. We report 129 planet candidates having radii less than 6 Earth-radii found in 3 years of Kepler photometry. Forty-seven of these candidates are not in Batalha et al. (2012). We gather Keck HIRES spectra for the majority of these targets leading to precise stellar radii and hence precise planet radii. We inject synthetic dimmings from mock transiting planets into the actual Kepler photometry and analyze that photometry with TERRA to assess detection completeness. We compute the occurrence of planets as a function of planet radius and period, correcting for the detection completeness. The resulting distribution of planet sizes exhibits a power law rise in occurrence from 5.7 Earth-radii down to 2 Earth-radii, as found in Howard et al. (2012). That rise clearly ends at 2 Earth-radii. The occurrence of planets is consistent with constant from 2 Earth-radii toward 1 Earth-radius. This unexpected plateau in planet occurrence at 2 Earth-radii suggests distinct planet formation processes for planets above and below 2 Earth-radii. We find 15.1% of solar type stars---roughly one in six---has a 1--2 Earth-radii planet with P = 5--50 days.