An accurate mass determination for Kepler-1655b, a moderately-irradiated world with a significant volatile envelope

TL;DR

This paper confirms Kepler-1655b as a Neptune-sized planet with a significant gas envelope, providing an accurate mass measurement that suggests it is not Earth-like and highlighting its position in the transition between rocky and gas-rich planets.

Contribution

The study provides the first precise mass measurement of Kepler-1655b using high-resolution spectroscopy, confirming its gaseous nature and refining its position in the planet population.

Findings

Kepler-1655b has a mass of 5.0+3.1/-2.8 Mearth.

Kepler-1655b's radius is 2.213+/-0.082 Rearth.

The planet's mass excludes an Earth-like composition with 98% confidence.

Abstract

We present the confirmation of a small, moderately-irradiated (F = 155 +/- 7 Fearth) Neptune with a substantial gas envelope in a P=11.8728787+/-0.0000085-day orbit about a quiet, Sun-like G0V star Kepler-1655. Based on our analysis of the Kepler light curve, we determined Kepler-1655b's radius to be 2.213+/-0.082 Rearth. We acquired 95 high-resolution spectra with TNG/HARPS-N, enabling us to characterize the host star and determine an accurate mass for Kepler-1655b of 5.0+3.1/-2.8 Mearth via Gaussian-process regression. Our mass determination excludes an Earth-like composition with 98\% confidence. Kepler-1655b falls on the upper edge of the evaporation valley, in the relatively sparsely occupied transition region between rocky and gas-rich planets. It is therefore part of a population of planets that we should actively seek to characterize further.