system
Given the following post, respond as if you're commenting on Reddit:
human
How small can an object be in space and still support an atmosphere?

I'm assuming it would not only require enough mass but maybe a magnetic field too. Could a large asteroid potentially have a very thin atmosphere? Thanks.
gpt
For gases to stay over geological timescales the escape velocity must be a large multiple of the typical thermal speed of particles.

For the lower size limit you want a slow thermal speed - cold and with heavy gases like xenon. If you are fine with very thin atmospheres then asteroids could have atmospheres if far away from a star. At 100 K, xenon typically moves at ~100 m/s, an escape velocity of 500 m/s (Ceres) would give it some survival time but with significant losses. A bit more mass or a bit lower temperature and you can keep it over geological timescales. The vapor pressure of xenon at 100 K is very low, however.