A Revised Density Estimate for the Largest Known Exoplanet, HAT-P-67 b
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| Main Authors: | , , , , , , , , , |
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| Format: | Preprint |
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2025
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| _version_ | 1866909595491893248 |
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| author | 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. |
| author_facet | 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. |
| contents | Low-density ($ρ< 0.1 \rm{~g~cm^{-3}}$) hot Saturns are expected to quickly ($<100$ Myr) lose their atmospheres due to stellar irradiation, explaining their rarity. HAT-P-67 b seems to be an exception, with $ρ< 0.09 \rm{~g~cm^{-3}}$ 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 3.5m Telescope from a follow-up campaign robust to stellar activity. We characterize the activity using photometry and activity indicators, revealing a stellar rotation period ($5.40\pm0.09$ d) 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 $M_p = 0.45 \pm 0.15~M_{\rm J}$. Combined with a radius measurement of $R_p=2.140 \pm 0.025~R_{\rm J}$, this yields a density of $ρ_p = 0.061^{+0.020}_{-0.021} \rm{~g~cm^{-3}}$, making HAT-P-67 b the second lowest-density hot giant known to date. We find 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 $\sim 150-500$ 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. |
| format | Preprint |
| id |
arxiv_https___arxiv_org_abs_2504_13997 |
| institution | arXiv |
| publishDate | 2025 |
| record_format | arxiv |
| spellingShingle | A Revised Density Estimate for the Largest Known Exoplanet, HAT-P-67 b 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. Earth and Planetary Astrophysics Low-density ($ρ< 0.1 \rm{~g~cm^{-3}}$) hot Saturns are expected to quickly ($<100$ Myr) lose their atmospheres due to stellar irradiation, explaining their rarity. HAT-P-67 b seems to be an exception, with $ρ< 0.09 \rm{~g~cm^{-3}}$ 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 3.5m Telescope from a follow-up campaign robust to stellar activity. We characterize the activity using photometry and activity indicators, revealing a stellar rotation period ($5.40\pm0.09$ d) 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 $M_p = 0.45 \pm 0.15~M_{\rm J}$. Combined with a radius measurement of $R_p=2.140 \pm 0.025~R_{\rm J}$, this yields a density of $ρ_p = 0.061^{+0.020}_{-0.021} \rm{~g~cm^{-3}}$, making HAT-P-67 b the second lowest-density hot giant known to date. We find 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 $\sim 150-500$ 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. |
| title | A Revised Density Estimate for the Largest Known Exoplanet, HAT-P-67 b |
| topic | Earth and Planetary Astrophysics |
| url | https://arxiv.org/abs/2504.13997 |