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

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${}_{-0.12}^{+0.16}{R}_{\oplus }$ 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 $6.5\pm 1.6M_{\oplus }$, and a mean density of $6.0_{-2.7}^{+3.0}$~g~cm${}^{-3}$.

24 pages, 14 figures, accepted to AJ