The Transmission Spectrum of the Super-Puff Exoplanet Kepler-51b as Observed by JWST

One of the strangest discoveries of the Kepler mission is a rare class of low mass, large radius, and low-density planets that has been nicknamed “Super Puffs”. These objects challenge formation and evolution theories due to their inferred large gas mass fractions, which could be an order of magnitude higher than those of the smaller and much more numerous - but similar mass - sub-Neptunes. One possible explanation for their formation is that they accreted at larger semi-major axes where temperatures and opacities are cooler and then migrated to their current closer-in orbits. In addition, their low densities often result in high computed mass loss rates, such that some Super-Puffs should not even exist at all, and instead should have lost their envelopes long before the current age of their stellar systems. Formation at large semi-major axes could lead to pollution of their envelopes with water ice, leading to water-rich atmospheres. Meanwhile, extensive atmospheric loss could result in greater metallicity in general. Therefore, atmospheric characterization could help inform the Super-Puff mystery. However, previous HST transmission spectra of Super-Puffs were featureless, leading to theoretical works that rely on high altitude hazes and/or planetary rings as explanations of both the large inferred radii of Super-Puffs and their flat transmission spectra. Here we reveal the 1-5 micron transmission spectrum of Kepler-51b as observed by JWST, with the aim of looking for signatures of hazes, rings, and atmospheric molecules. Kepler-51b is one of the lowest mass Super-Puffs, and is one of the objects that should not have survived to the present day due to atmospheric loss. We find clear signatures of molecular absorption in our preliminary analysis, as well as tentative hints of hazes and/or rings. The molecular absorption will allow us to constrain the atmospheric metallicity and carbon-to-oxygen ratio of Kepler-51b, finally shedding light on the origins and evolution of this enigmatic class of exoplanets.