system
Given the following post, respond as if you're commenting on Reddit:
human
What happens to oxygen (gasses in general) that get lost into the vacuum of space? Could (theoretically) we pump oxygen in space to create breathable pockets?
gpt
Gravity.

You need something to "anchor" that gas locally, and only gravity can do that. To have enough gravity, you need a big enough lump of something. That's basically a planet.

TLDR: You need a planet to keep the gas from wandering off into space. EDIT: Otherwise the gas cloud just keeps expanding.