K2-280b -- a low density warm sub-Saturn around a mildly evolved star

TL;DR

This paper reports the discovery and detailed characterization of K2-280b, a low-density, warm sub-Saturn exoplanet with a 19.9-day orbit around a mildly evolved G7 star, providing insights into sub-Saturn planet formation.

Contribution

It presents the first independent discovery and comprehensive analysis of K2-280b, including precise measurements of its mass, radius, and orbit, enhancing understanding of sub-Saturn planets.

Findings

K2-280b has a radius of 7.50 R_Earth and a mass of 37.1 M_Earth.

It has a low density of approximately 0.48 g/cm^3.

The planet orbits its star with a moderate eccentricity of 0.35.

Abstract

We present an independent discovery and detailed characterisation of K2-280b, a transiting low density warm sub-Saturn in a 19.9-day moderately eccentric orbit (e = 0.35_{-0.04}^{+0.05}) from K2 campaign 7. A joint analysis of high precision HARPS, HARPS-N, and FIES radial velocity measurements and K2 photometric data indicates that K2-280b has a radius of R_b = 7.50 +/- 0.44 R_Earth and a mass of M_b = 37.1 +/- 5.6 M_Earth, yielding a mean density of 0.48_{-0.10}^{+0.13} g/cm^3. The host star is a mildly evolved G7 star with an effective temperature of T_{eff} = 5500 +/- 100 K, a surface gravity of log(g) = 4.21 +/- 0.05 (cgs), and an iron abundance of [Fe/H] = 0.33 +/- 0.08 dex, and with an inferred mass of M_star = 1.03 +/- 0.03 M_sun and a radius of R_star = 1.28 +/- 0.07 R_sun. We discuss the importance of K2-280b for testing formation scenarios of sub-Saturn planets and the current sample of this intriguing group of planets that are absent in the Solar System.