A Large and Variable Leading Tail of Helium in a Hot Saturn Undergoing Runaway Inflation

Abstract

Atmospheric escape shapes the fate of exoplanets, with statistical evidence for transformative mass loss imprinted across the mass–radius–insolation distribution. Here, we present transit spectroscopy of the highly irradiated, low-gravity, inflated hot Saturn HAT-P-67 b. The Habitable Zone Planet Finder spectra show a detection of up to 10% absorption depth of the 10833 Å helium triplet. The 13.8 hr of on-sky integration time over 39 nights sample the entire planet orbit, uncovering excess helium absorption preceding the transit by up to 130 planetary radii in 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. The prominent leading tail serves as direct evidence for dayside mass loss with a strong day-/nightside asymmetry. We see some transit-to-transit variability in the line profile, consistent with the interplay of stellar and planetary winds. We employ one-dimensional Parker wind models to estimate the mass-loss rate, finding values on the order of 2 × 1013 g s−1, with large uncertainties owing to the unknown X-ray and ultraviolet (XUV) flux of the F host star. The large mass loss in HAT-P-67 b represents a valuable example of an inflated hot Saturn, a class of planets recently identified to be rare, as their atmospheres are predicted to evaporate quickly. We contrast two physical mechanisms for runaway evaporation: ohmic dissipation and XUV irradiation, slightly favoring the latter.

Exoplanet Aeronomy: A Case Study of WASP-69 b’s Variable Thermosphere

“…While both Hα and the metastable HeI triplet are observable from ground-based facilities and do not suffer from interstellar extinction, the former has only been detected in among the hottest, most irradiated giant planets (dos Santos 2023). The metastable HeI feature, in contrast, has been detected from a wider variety of planets; this latter probe has emerged as the most widely used over the past several years (e.g., Allart et al 2018Allart et al , 2019Allart et al , 2023Nortmann et al 2018;Salz et al 2018;dos Santos et al 2020;Kasper et al 2020;Spake et al 2021;Vissapragada et al 2022b;Kirk et al 2022;Bennett et al 2023;Guilluy et al 2023Guilluy et al , 2024Zhang et al 2023;Gully-Santiago et al 2024).…”