The Frequency of Low-Mass Exoplanets. III. Toward eta-Earth at Short Periods

TL;DR

This study estimates the occurrence rate of low-mass, short-period exoplanets around solar-type stars using high-precision radial velocity data, highlighting a high prevalence of such planets and implications for the eta-Earth estimate.

Contribution

It provides the first detailed occurrence rate estimates for terrestrial-mass planets at short periods, emphasizing the abundance of low-mass planets in close orbits.

Findings

18.5% of stars host planets with mass <10 Earth masses at periods <50 days

1.5% of stars host giant planets (>100 Earth masses) at similar periods

High prevalence of low-mass planets challenges existing formation models

Abstract

Determining the occurrence rate of terrestrial-mass planets (m_p < 10M_earth) is a critically important step on the path towards determining the frequency of Earth-like planets (eta-Earth), and hence the uniqueness of our Solar system. Current radial-velocity surveys, achieving precisions of 1 m/s, are able to detect terrestrial-mass planets and provide meaningful estimates of their occurrence rate. We present an analysis of 67 solar-type stars from the Anglo-Australian Planet Search specifically targeted for very high-precision observations. When corrected for incompleteness, we find that the planet occurrence rate increases sharply with decreasing planetary mass. Our results are consistent with those from other surveys: in periods shorter than 50 days, we find that 1.5% of stars host a giant (m_p > 100M_earth) planet, and that 18.5% of stars host a planet with m_p < 10M_earth. The preponderance of low-mass planets in short-period orbits is in conflict with formation simulations in which the majority of terrestrial-mass planets reside at larger orbital distances. This work gives a hint as to the size of eta-Earth, but to make meaningful predictions on the frequency of terrestrial planets in longer, potentially habitable orbits, low-mass terrestrial planet searches at periods of 100-200 days must be made an urgent priority for ground-based Doppler planet searches in the years ahead.