Air clings to small planets after all

Small, rocky worlds once thought bare may have habitable atmospheres

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One of the most important features of extrasolar planets is whether they have an atmosphere or not. For astrobiology in particular, small, rocky planets blanketed with an atmosphere offer the best hope in the search for life beyond Earth.

The two key factors affecting whether a planet can retain an atmosphere over billions of years are: its escape velocity, which affects how well its gravity is able to hold on to a gas envelope; and the cumulative amount of sunlight it has received from its star (instellation). High instellation, particularly of extreme X-rays, acts to strip away gas molecules.