TOI-220 $b$: a warm sub-Neptune discovered by TESS

TL;DR

This paper reports the discovery and detailed characterization of TOI-220 b, a warm sub-Neptune exoplanet with precisely measured mass and radius, orbiting a bright, old K dwarf star, and discusses its possible internal structure and atmospheric composition.

Contribution

First detailed characterization of TOI-220 b, including precise mass, radius, and internal structure analysis based on combined TESS and radial velocity data.

Findings

TOI-220 b has a mass of 13.8 Earth masses and radius of 3.03 Earth radii.

The planet orbits its star every 10.69 days.

Internal structure analysis suggests a steam atmosphere and supercritical water layer.

Abstract

In this paper we report the discovery of TOI-220 $b$, a new sub-Neptune detected by the Transiting Exoplanet Survey Satellite (TESS) and confirmed by radial velocity follow-up observations with the HARPS spectrograph. Based on the combined analysis of TESS transit photometry and high precision radial velocity measurements we estimate a planetary mass of 13.8 $\pm$ 1.0 M$_{Earth}$ and radius of 3.03 $\pm$ 0.15 R$_{Earth}$, implying a bulk density of 2.73 $\pm$ 0.47 $\textrm{g cm}^{-3}$. TOI-220 $b$ orbits a relative bright (V=10.4) and old (10.1$\pm$1.4 Gyr) K dwarf star with a period of $\sim$10.69 d. Thus, TOI-220 $b$ is a new warm sub-Neptune with very precise mass and radius determinations. A Bayesian analysis of the TOI-220 $b$ internal structure indicates that due to the strong irradiation it receives, the low density of this planet could be explained with a steam atmosphere in radiative-convective equilibrium and a supercritical water layer on top of a differentiated interior made of a silicate mantle and a small iron core.