Larger Planet Radii Inferred from Stellar "Flicker" Brightness Variations of Bright Planet Host Stars

TL;DR

Using stellar flicker brightness variations to measure stellar surface gravity, this study finds that many bright planet-host stars are larger and more evolved than previously thought, leading to larger inferred planet radii.

Contribution

The paper demonstrates that flicker-based measurements of stellar surface gravity reveal larger stellar and planetary radii, correcting biases in previous estimates.

Findings

Nearly 50% of bright planet-host stars are subgiants.

Stellar radii are 20-30% larger than previous estimates.

Planet radii are correspondingly larger, affecting planet characterization.

Abstract

Most extrasolar planets have been detected by their influence on their parent star, typically either gravitationally (the Doppler method) or by the small dip in brightness as the planet blocks a portion of the star (the transit method). Therefore, the accuracy with which we know the masses and radii of extrasolar planets depends directly on how well we know those of the stars, the latter usually determined from the measured stellar surface gravity, logg. Recent work has demonstrated that the short-timescale brightness variations ("flicker") of stars can be used to measure logg to a high accuracy of ~0.1-0.2 dex (Bastien et al. 2013). Here, we use flicker measurements of 289 bright (Kepmag<13) candidate planet-hosting stars with Teff=4500-6650 K to re-assess the stellar parameters and determine the resulting impact on derived planet properties. This re-assessment reveals that for the brightest planet-host stars, an astrophysical bias exists that contaminates the stellar sample with evolved stars: nearly 50% of the bright planet-host stars are subgiants. As a result, the stellar radii, and hence the radii of the planets orbiting these stars, are on average 20-30% larger than previous measurements had suggested.