The Discovery and Mass Measurement of a New Ultra-short-period Planet: EPIC 228732031b

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 K2 Campaign 10. It has a radius of 1.81(-0.12)(+0.16) R-circle plus and orbits a G dwarf with a period of 8.9 hr. 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. +/- 1.6 M-circle plus and a mean density of 6.0(-2.7)(+3.0) g cm(-3).