Revised Radii of Kepler Stars and Planets Using Gaia Data Release 2

TL;DR

This paper uses Gaia DR2 data to significantly improve the radii estimates of Kepler stars and exoplanets, refining stellar classifications and revealing new insights into planet populations and their distributions.

Contribution

It provides revised stellar radii for Kepler stars with improved precision and recalculates exoplanet sizes, revealing updated features in planet distributions and habitable zone candidates.

Findings

Median radius precision improved to approximately 8%.

Most Kepler targets are main-sequence stars, with fewer subgiants and giants.

Confirmed a radius gap in small, close-in planets at incident fluxes > 200F_⊕.

Abstract

One bottleneck for the exploitation of data from the $Kepler$ mission for stellar astrophysics and exoplanet research has been the lack of precise radii and evolutionary states for most of the observed stars. We report revised radii of 177,911 $Kepler$ stars derived by combining parallaxes from $Gaia$ Data Release 2 with the DR25 $Kepler$ Stellar Properties Catalog. The median radius precision is $\approx$ 8%, a typical improvement by a factor of 4-5 over previous estimates for typical $Kepler$ stars. We find that $\approx$ 67% ($\approx$ 120,000) of all $Kepler$ targets are main-sequence stars, $\approx$ 21% ($\approx$ 37,000) are subgiants, and $\approx$ 12% ($\approx$ 21,000) are red giants, demonstrating that subgiant contamination is less severe than some previous estimates and that Kepler targets are mostly main-sequence stars. Using the revised stellar radii, we recalculate the radii for 2123 confirmed and 1922 candidate exoplanets. We confirm the presence of a gap in the radius distribution of small, close-in planets, but find that the gap is mostly limited to incident fluxes $>$ 200$F_\oplus$ and its location may be at a slightly larger radius (closer to $\approx$ 2$R_\oplus$) when compared to previous results. Further, we find several confirmed exoplanets occupying a previously-described "hot super-Earth desert" at high irradiance, show the relation between gas-giant planet radius and incident flux, and establish a bona-fide sample of eight confirmed planets and 30 planet candidates with $R_{\mathrm{p}}$ $<$ 2$R_\oplus$ in circumstellar "habitable zones" (incident fluxes between 0.25--1.50 $F_\oplus$). The results presented here demonstrate the potential for transformative characterization of stellar and exoplanet populations using $Gaia$ data.