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

June 2025 • 2025AJ....169..336W

Authors • Wang, Gavin • Balmer, William O. • Pueyo, Laurent • Thorngren, Daniel • Schmidt, Stephen P. • Wang, Le-Chris • Schlaufman, Kevin C. • Stefánsson, Guðmundur • Rustamkulov, Zafar • Sing, David K.

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 ρp=0.061−0.021+0.020gcm−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.

Links

• SIMBAD https://simbad.u-strasbg.fr/simbad/sim-ref?querymethod=bib&simbo=on&submit=submit+bibcode&bibcode=2025AJ....169..336W
• PDF https://iopscience.iop.org/article/10.3847/1538-3881/adcec9/pdf
• PREPRINT http://arxiv.org/abs/2504.13997
• DATA https://doi.org/10.17909/4c8f-f930
• DATA https://doi.org/10.26093/cds/vizier
• DATA https://irsa.ipac.caltech.edu/bibdata/2025/W/2025AJ....169..336W.html
• ELECTR https://doi.org/10.3847/1538-3881/adcec9