Abstract
Extremely low-density planets (“super-puffs”) are a small but intriguing subset of the transiting planet population. With masses in the super-Earth range (1 - 10 $M\oplus)andradiiakintothoseofgiantplanets(>4R\oplus),theirlargeenvelopesmayhavebeenaccretedbeyondthewatersnowlineandmanyappeartobesusceptibletocatastrophicmassloss.Boththepresenceofwaterandtheimportanceofmasslosscanbeexploredusingtransmissionspectroscopy.Here,wepresentnewHubblespacetelescopeWFC3spectroscopyandupdatedKeplertransitdepthmeasurementsforthesuper−puffKepler−79d.Wedonotdetectanymolecularabsorptionfeaturesinthe1.1−1.7μmWFC3bandpass,andthecombinedKeplerandWFC3dataareconsistentwithaflat−linemodel,indicatingthepresenceofaerosolsintheatmosphere.WecomparetheshapeofKepler−79dstransmissionspectrumtopredictionsfromamicrophysicalhazemodelthatincorporatesanoutwardparticlefluxduetoongoingmassloss.Wefindthatphotochemicalhazesofferanattractiveexplanationfortheobservedpropertiesofsuper−puffslikeKepler−79d,astheysimultaneouslyrenderthenear−infraredspectrumfeaturelessandreducetheinferredenvelopemass−lossratebymovingthemeasuredradius(opticaldepthunitysurfaceduringtransit)tolowerpressures.Werevisitthebroaderquestionofmass−lossratesforsuper−puffsandfindthattheageestimatesandmass−lossratesforthemajorityofsuper−puffscanbereconciledifhazesmovethephotospherefromthetypicallyassumedpressureof∼10mbarto\sim 10\ \mu $ bar.