Abstract

The statistical dearth of small planets revealed by Kepler and Keck suggests that transformational photoevaporation may be a compulsory stage in the evolution of planets between 1.5 and 2 Earth radii. A search for actively evaporating planets has ensued. The Habitable Zone Planet Finder (HPF) Spectrograph Exospheres program aims to expand the search across a diverse range of planet and host star properties via the 10830 Ångstrom Helium triplet, a sensitive probe of active atmospheric escape. Here we present a detection of >10% absorption depth during transit spectroscopy of a highly irradiated, low-gravity, hot Saturn. The 13.8 hours of on-sky integration time over 41 HET tracks provides enough out-of-transit phase coverage as to reveal comparably deep absorption preceding the transit, evincing a large leading tail. This configuration can be understood as the escaping material overflowing its small Roche lobe, and advecting most of the gas into the stellar—and not planetary---rest frame, consistent with the Doppler velocity structure seen in the Helium line profiles. We, therefore, attribute the leading tail to Keplerian shear, with some mix of preferential mass loss from dayside heating and/or a dayside pile-up into a headwind of weak stellar wind.  The observation of a profoundly evaporating highly irradiated hot Saturn confirms the prediction of a "sweet spot" for runaway evaporation from the Ohmic Dissipation theory put forth by Batygin, Stevenson, and Bodenheimer in 2011, before the dearth of such objects was widely known. This connection naturally explains the lack of inflated sub-Saturns later seen by Thorngren and Fortney as a consequence of the ephemeral lifetimes of this category.