The discovery and mass measurement of a new ultra-short-period planet: EPIC~228732031b

TL;DR

This paper reports the discovery of a new ultra-short-period planet, EPIC 228732031b, with detailed measurements of its mass, radius, and density, contributing to the understanding of such extreme exoplanets.

Contribution

It presents the first mass measurement of EPIC 228732031b using two independent methods, demonstrating the planet's properties and adding to the catalog of ultra-short-period planets.

Findings

Planet EPIC 228732031b has a radius of 1.81 R_⊕ and a period of 8.9 hours.

The planet's mass is measured at 6.5 ± 1.6 M_⊕.

The mean density of the planet is approximately 6.0 g/cm³.

Abstract

We report the discovery of a new ultra-short-period planet and summarize the properties of all such planets for which the mass and radius have been measured. The new planet, EPIC~228732031b, was discovered in {\it K2} Campaign 10. It has a radius of 1.81$^{+0.16}_{-0.12}~R_{\oplus}$ and orbits a G dwarf with a period of 8.9 hours. Radial velocities obtained with Magellan/PFS and TNG/HARPS-N show evidence for stellar activity along with orbital motion. We determined the planetary mass using two different methods: (1) the "floating chunk offset" method, based only on changes in velocity observed on the same night, and (2) a Gaussian process regression based on both the radial-velocity and photometric time series. The results are consistent and lead to a mass measurement of $6.5 \pm 1.6~M_{\oplus}$, and a mean density of $6.0^{+3.0}_{-2.7}$~g~cm$^{-3}$.