The James Webb Space Telescope NIRSpec-PRISM Transmission Spectrum of the Super-puff, Kepler-51d

Kepler-51 is a 500 Myr G dwarf hosting three “super-puffs” and one low-mass nontransiting planet. Kepler-51d, the coolest (Teq ∼ 350 K) transiting planet in this system is also one of the lowest-density super-puffs known to date (ρp = 0.038 ± 0.009 g cm−3). With a planetary mass of Mp = 5.6 ± 1.2 M⊕ and a radius of Rp = 9.32 ± 0.18 R⊕, the observed properties of this planet are not readily explained by most planet formation theories. Hypotheses explaining Kepler-51d’s low density range from a substantial H/He envelope comprising >30% its mass, a high-altitude haze layer, to a tilted ring system. To test these hypotheses, we present the NIRSpec-PRISM 0.6–5.3 μm transmission spectrum of Kepler-51d observed by the James Webb Space Telescope. We find a spectrum best fit by a sloped line covering the entire wavelength range. Based on forward modeling and atmosphere retrievals, Kepler-51d likely possesses a low-metallicity atmosphere with high-altitude hazes of submicron particle sizes spanning pressures of 1–100 μbar. However, the spectrum could also be explained by a tilted ring with an estimated lifetime on the order of ∼0.1 Myr. We also investigate the stellar activity of this young Sun-like star, extracting a spot temperature significantly hotter than sunspots and spot covering fractions on the order of 0.1%–10% depending on assumed spot parameters.