Kepler-445, Kepler-446 and the Occurrence of Compact Multiples Orbiting Mid-M Dwarf Stars

TL;DR

This study confirms and characterizes multiple small, short-period planets around mid-M dwarf stars Kepler-445 and Kepler-446, revealing that about 21% of such stars host compact planetary systems, with implications for planet formation.

Contribution

The paper provides the first detailed characterization of Kepler-445 and Kepler-446 systems and estimates the occurrence rate of compact multiples around mid-M dwarfs, highlighting their metallicity dependence.

Findings

Approximately 21% of mid-M dwarf stars host compact multiples.

Kepler-445 is metal-rich; Kepler-446 is metal-poor, both with three transiting planets.

Systems support efficient protoplanetary disk metal accretion.

Abstract

We confirm and characterize the exoplanetary systems Kepler-445 and Kepler-446: two mid-M dwarf stars, each with multiple, small, short-period transiting planets. Kepler-445 is a metal-rich ([Fe/H]=+0.25 $\pm$ 0.10) M4 dwarf with three transiting planets, and Kepler-446 is a metal-poor ([Fe/H]=-0.30 $\pm$ 0.10) M4 dwarf also with three transiting planets. Kepler-445c is similar to GJ 1214b: both in planetary radius and the properties of the host star. The Kepler-446 system is similar to the Kepler-42 system: both are metal-poor with large galactic space velocities and three short-period, likely-rocky transiting planets that were initially assigned erroneously large planet-to-star radius ratios. We independently determined stellar parameters from spectroscopy and searched for and fitted the transit light curves for the planets, imposing a strict prior on stellar density in order to remove correlations between the fitted impact parameter and planet-to-star radius ratio for short-duration transits. Combining Kepler-445, Kepler-446 and Kepler-42, and isolating all mid-M dwarf stars observed by Kepler with the precision necessary to detect similar systems, we calculate that 21 $^{+7}_{-5}$ % of mid-M dwarf stars host compact multiples (multiple planets with periods of less than 10 days) for a wide range of metallicities. We suggest that the inferred planet masses for these systems support highly efficient accretion of protoplanetary disk metals by mid-M dwarf protoplanets.