system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why space vacuum does not suck out our atmosphere?
gpt
Atmosphere is just a layer of gas on the surface of the planet. Their composition varies a lot; Venus' is incredibly thick, but Moon's is so sparse that it weights just a few meager tons.

But why does Earth have one? There just was enough gases to form one. Gases are lighter than solids or liquids, so when gas is produced one way or another it starts to climb above the heavier materials.

We _currently_ have an atmosphere because Earth has enough gravity to keep it from floating away and a strong enough magnetic field to keep solar radiation from eating it up.