Exoplanets, -moons, -comets

Effect Of Tidal Gravity And Planetary Rotation On The Retrieved Atmospheric Abundances Of Close-in Exoplanets

By Keith Cowing
Status Report
astro-ph.EP
June 18, 2026
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Effect Of Tidal Gravity And Planetary Rotation On The Retrieved Atmospheric Abundances Of Close-in Exoplanets
A schematic cross section of a planet along the plane of the terminator showing the accelerations acting on an atmospheric parcel (light grey). The atmospheric extent corresponding to the scenario with rotation and tidal effects included is illustrated by a light orange elliptical annulus (outer). The atmospheric scale heights in the diagram have been exaggerated for visibility. The atmospheric extent corresponding to the scenario without the rotation and tidal effects included is illustrated by a darker orange circular annulus (inner). The atmospheric parcel is subjected to both centrifugal and tidal accelerations. The centrifugal acceleration acts perpendicular to the axis of rotation, while the tidal acceleration arises from the differential gravitational pull of the host star. The net radial component of these accelerations opposes the planetary gravitational acceleration đť‘”. In the quantitative analysis presented in this paper, the atmospheric annulus relevant to transmission spectroscopy is approximated as circular. — astro-ph.EP

Most modern atmospheric retrievals adopt the simplifying assumption that the planetary atmosphere endures no planetary rotation and stellar tidal effect.

However, for tidally locked close-in exoplanets, the gravitational influence of the host star and the rapid rotation of the planet can significantly modify the effective gravity, leading to changes in the atmospheric scale height and mixing ratios of molecular abundances.

In this work, we develop a combined framework to include these rotation and tidal effects into a forward and retrieval model to study how they affect the molecular abundances of close-in exoplanets. We specifically apply our model to the planet WASP-12b, observed with HST, and WASP-39b, observed with JWST, and investigate how atmospheric retrieval parameters change when tidal and centrifugal corrections to gravity are included.

The forward model calculation for strongly affected gravity due to tidal and rotation effects in WASP-12b shows an increment in transit depth in the range of 150-500 ppm for major molecules in the atmosphere, whereas for WASP-39b with small gravity reduction shows variations of 60- 180 ppm.

The atmospheric retrievals for WASP-12b using HST and WASP-39b using JWST transmission spectra with and without effective gravity corrections show an increment in the retrieved molecular abundances. A systematic study by reducing the effective gravity by 20%, 30% and an extreme value 50% for WASP-39b shows increasing changes in the inferred log-mixing ratios of various molecules.

Our results show a similar trend with non-isothermal P-T profiles, but cloudy models suppress the combined effect of rotation and tidal gravity.

K. Arnav, Gopal Hazra

Comments: 10 pages, 6 figures, under review, comments are welcome
Subjects: Earth and Planetary Astrophysics (astro-ph.EP); Solar and Stellar Astrophysics (astro-ph.SR)
Cite as: arXiv:2606.18100 [astro-ph.EP] (or arXiv:2606.18100v1 [astro-ph.EP] for this version)
https://doi.org/10.48550/arXiv.2606.18100
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Submission history
From: Gopal Hazra
[v1] Tue, 16 Jun 2026 16:01:26 UTC (3,010 KB)
https://arxiv.org/abs/2606.18100

Astrobiology, exoplanet,

Keith Cowing

Biologist, Explorers Club Fellow, ex-NASA Space Biologist and Payload integrator, Editor of NASAWatch.com and Astrobiology.com, Lapsed climber, Explorer, Synaesthete, Former Challenger Center board member 🖖🏻

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