James Webb Space Telescope Feed Post

Harvard ADS: An extremely low-density exoplanet spins slow

Paper abstract: We present constraints on the shape of Kepler-51d, which is a super-puff with a mass ~6\,M_\oplus and a radius ~9\,R_\oplus, 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_\perp>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 \gtrsim 33\,hr. In the more plausible situation that the planetary spin is aligned with its orbital direction to within 30^\circ, then its oblateness is <0.08, which corresponds to a dimensionless spin rate <~30\% of the break-up rotation and a dimensional rotation period \gtrsim 53\,hr. 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.