Atmospheric escape
Planets losing their air to space, sometimes visibly, sometimes entirely.
A hot planet's upper atmosphere can be heated until the lightest gases exceed escape velocity and stream away. Close-in planets do this dramatically — some trail comet-like tails of hydrogen larger than the star — and over billions of years it can strip a sub-Neptune down to a bare core, which is the leading explanation for the observed gap in planet sizes. Mars lost most of its atmosphere the same way once its magnetic field died, which MAVEN was sent to measure directly.
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Questions
What is Atmospheric escape?
Planets losing their air to space, sometimes visibly, sometimes entirely.
Why does it matter?
Close-in planets do this dramatically — some trail comet-like tails of hydrogen larger than the star — and over billions of years it can strip a sub-Neptune down to a bare core, which is the leading explanation for the observed gap in planet sizes. Mars lost most of its atmosphere the same way once its magnetic field died, which MAVEN was sent to measure directly.