system
Given the following post, respond as if you're commenting on Reddit:
human
Can presence of molecular oxygen in an exo planet reliably indicate existence of life there?

Can a planet have a lot of oxygen in its atmosphere without living organisms producing it? If not would looking for planets with lots of oxygen in the atmosphere be a way of guessing if a distant exo planet has life?
gpt
Molecular oxygen is usually an indication that a planet's atmospheric chemistry is in serious disequilibrium. O2 is extremely reactive with a lot of stuff around it - after all, it's what makes stuff burn. The fact that it exists at all on a planet suggests that something must be actively making it, and at a rate more quickly than everything else on the planet would oxidize.

This makes an especially compelling case for life when substances that are easily oxidized are found in the atmosphere, such as methane. Even without that, though, there's still usually plenty of other stuff on a planet's surface that can oxidize. For example, in the presence of free oxygen, iron will quickly turn into iron oxide (a.k.a. rust, as is the case on the surface of Mars).

Of course, this does assume that chlorophyll-containing organisms are responsible for the oxygen disequilibrium. It's entirely possible there could be an entire ecosystem of life on some planet that doesn't rely on oxygen as a waste product, so life cannot be ruled out on a planet without free oxygen. It's just that is we do detect it, it's very likely to indicate that's it's biological in origin.