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The Kepler mission revealed a class of planets known as ''super-puffs,'' withmasses only a few times larger than Earth's but radii larger than Neptune,giving them very low mean densities. All three of the known planets orbitingthe young solar-type star Kepler 51 are super-puffs. The Kepler 51 systemthereby provides an opportunity for a comparative study of the structures andatmospheres of this mysterious class of planets, which may provide clues abouttheir formation and evolution. We observed two transits each of Kepler 51b and51d with the Wide Field Camera 3 (WFC3) on the Hubble Space Telescope.Combining new WFC3 transit times with re-analyzed Kepler data and updatedstellar parameters, we confirmed that all three planets have densities lowerthan 0.1 g/cm$^{3}$. We measured the WFC3 transmission spectra to befeatureless between 1.15 and 1.63 $\mu$m, ruling out any variations greaterthan 0.6 scale heights (assuming a H/He dominated atmosphere), thus showing nosignificant water absorption features. We interpreted the flat spectra as theresult of a high-altitude aerosol layer (pressure $<$3 mbar) on each planet.Adding this new result to the collection of flat spectra that have beenobserved for other sub-Neptune planets, we find support for one of the twohypotheses introduced by Crossfield and Kreidberg (2017), that planets withcooler equilibrium temperatures have more high-altitude aerosols. We stronglydisfavor their other hypothesis that the H/He mass fraction drives theappearance of large amplitude transmission features.

The Featureless Transmission Spectra of Two Super-Puff Planets