An Eccentric Sub-Neptune Moving Into the Evaporation Desert *

Abstract

Though missions such as Kepler, K2, and TESS have discovered >2000 sub-Neptune and Neptunian planets, there is a dearth of such planets at close-in ( P ≲ 3 days) orbits. This feature, called the Neptune desert or the evaporation desert, is believed to be primarily shaped by planetary migration and photoevaporation. However, this region is not completely devoid of planets—a small number of very hot Neptunes reside within the desert. These planets provide an opportunity to directly probe the effects of migration and photoevaporation. We present confirmation of TOI-5800 b, an eccentric sub-Neptune on an ≈2.6 day period that is likely actively undergoing tidal migration. We use radial velocity measurements from the Carnegie Planet Finder Spectrograph to constrain TOI-5800 b’s mass and eccentricity. We find that it has an unusually high eccentricity (0.39 ± 0.07) for its short orbit. TOI-5800 b is therefore currently experiencing high levels of tidal heating as it moves into the desert. Ranked as a top candidate for transmission and emission spectroscopy within its temperature and radius regime, TOI-5800 b is a prime target for atmospheric characterization with the James Webb Space Telescope. TOI-5800 b presents a unique opportunity to study the atmosphere of a planet undergoing tidal heating and to probe the composition of sub-Neptune planets.