>>11208118It pretty much comes down to sample size.
We can land rovers on Mars and maybe put a few melt probes into Europa and Enceladus. In the mean time, we could build massive orbital telescopes capable of thousand-kilometer-per-pixel observations of nearby exoplanet systems, which means we'd have the resolution necessary to study the atmospheric compositions of hundreds if not thousands of planets.
If we look at the absorption peaks of an exoplanet atmosphere and we find anything similar to Earth, that indicates presence of life. I'm not only talking about a few millibar of oxygen, I'm talking about atmospheres with >10% oxygen by volume and detectable amounts of organic molecules like methane. To have lots of oxygen like that without life would require mechanisms that we do not understand, because pretty much nothing other than life can liberate that much oxygen rapidly enough that it can build up to those concentrations. The presence of reducing chemicals in the atmosphere would also prove that the planet itself doesn't simply have a superabundance of oxygen and therefore a decent amount of oxygen in the atmosphere, either.
