Small planets observed to orbit closely around their host star fall into two main populations:

1. those with radii smaller than 1.5 Earth radii, thought to be primarily rocky cores with little or no remaining atmosphere, and
2. those with radii larger than 2 Earth radii, thought to retain some of their hydrogen and helium atmospheres.

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What causes the difference between these two populations? We think that all close-in exoplanets are sculpted by the energetic radiation of their host stars. This radiation can erode away the primordial atmospheres — and for the smallest planets, this will leave only their rocky cores behind.

As we work to understand the detailed physics of this photoevaporation, it would be helpful to be able to directly watch a planet’s atmosphere escaping in this way. In a new study, scientist Megan Mansfield (University of Chicago) and collaborators present just the thing: observations of the escaping atmosphere of the exoplanet HAT-P-11b.

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