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Exactly the title. There needs to be a pressure to prevent gases from perpetually expanding, right? Bear with me, I'm not a physicist or studying to be one.

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    $\begingroup$ One word explanation: Gravity. $\endgroup$ – Alfred Centauri Oct 12 '13 at 21:38
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This is probably not a valid stack-exchange question since the answer is available online just by doing a Google search, but the answer is gravity. That's why gas giants are largely gaseous, and the tiny Moon has virtually no atmosphere, and the Earth is somewhere in between.

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  • $\begingroup$ Concise and precise, without too much depth. Good answer! $\endgroup$ – shortstheory Oct 13 '13 at 3:10
  • $\begingroup$ It is a valid question, it's not necessarily easy to google if one doesn't know what keywords to search for. $\endgroup$ – gerrit Oct 13 '13 at 19:10
  • $\begingroup$ what about solar wind and temperature in less gravity planets ? $\endgroup$ – user46925 Jan 18 '16 at 16:13

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