A Revised Density Estimate for the Largest Known Exoplanet, HAT-P-67 b

Abstract

Low-density ( ρ _p < 0.1 g cm ^−3 ) hot Saturns are expected to quickly (<100 Myr) lose their atmospheres owing to stellar irradiation, explaining their rarity. HAT-P-67 b seems to be an exception, with ρ _p < 0.09 g cm ^−3 and maintaining its atmosphere to well after 1 Gyr. We present a photometric and spectroscopic follow-up of HAT-P-67 b to determine how it avoided mass loss. HAT-P-67 b orbits a V = 10.1 evolved F-type star in a 4.81-day orbit. We present new radial velocity observations of the system from the NEID spectrograph on the WIYN 3.5 m Telescope from a follow-up campaign robust to stellar activity. We characterize the activity using photometry and activity indicators, revealing a stellar rotation period (5.40 ± 0.09 days) near HAT-P-67 b’s orbital period. We mitigate the stellar activity using a constrained quasi-periodic Gaussian process through a joint fit of archival ground-based photometry, TESS photometry, and our NEID observations, obtaining a planetary mass of M _p = 0.45 M _J ± 0.15 M _J . Combined with a radius measurement of R _p = 2.140 R _J ± 0.025 R _J , this yields a density of ${\rho }_{p}=0.06{1}_{-0.021}^{+0.020}\,{\rm{g}}\,{{\rm{cm}}}^{-3}$ , making HAT-P-67 b the second-lowest-density hot giant known to date. We find that the recent evolution of the host star caused mass loss for HAT-P-67 b to only recently occur. The planet will be tidally disrupted/engulfed in ∼150–500 Myr, shortly after losing its atmosphere. With rapid atmospheric mass loss, a large helium leading tail, and upcoming observations with the Hubble Space Telescope, HAT-P-67 b is an exceptional target for future studies, for which an updated mass measurement provides important context.

Keywords

• Exoplanet astronomy
• Exoplanet tides
• Exoplanets
• Extrasolar gaseous giant planets
• Radial velocity
• Stellar activity