We present constraints on the shape of Kepler-51d, which is a super-puff with a mass ∼6M⊕ and a radius ∼9R⊕, based on detailed modeling of the transit light curve from JWST NIRSpec. The projected shape of this extremely low-density planet is consistent with being spherical, and a projected oblateness f⊥>0.2 can be excluded regardless of the spin obliquity angles. If this is taken as the limit on the true shape of the planet, Kepler-51d is rotating at ≲50% of its break-up spin rate, or its rotation period is ≳33hr. In the more plausible situation that the planetary spin is aligned with its orbital direction to within 30∘, then its oblateness is <0.08, which corresponds to a dimensionless spin rate ≲30% of the break-up rotation and a dimensional rotation period ≳53hr. This seems to contradict the theoretical expectation that planets with such low masses may be spinning near break-up. We point out the usefulness of the stellar mean density and the orbital eccentricity in constraining the shape of the transiting planet, so planets with well-characterized host and orbital parameters are preferred in the detection of planetary oblateness with the JWST transit method.
@article{arxiv.2410.07977,
title = {An extremely low-density exoplanet spins slow},
author = {Quanyi Liu and Wei Zhu and Kento Masuda and Jessica E. Libby-Roberts and Aaron Bello-Arufe and Caleb I. Canas},
journal= {arXiv preprint arXiv:2410.07977},
year = {2024}
}