[nwallin]

> We're certainly not going to be detecting a world full of microbes from light years away.

JWST would be able to detect an oxygen rich atmosphere like ours, which is a sure sign of life. Oxygen rich atmospheres are not a stable equilibrium; without all the plants and plankton on Earth, the oxygen in the atmosphere would all recombine, and Earth would be left with an atmosphere with no oxygen but additional CO2.

Atmospheres can either be CO2 rich or methane rich; they can't have significant amounts of both CO2 and methane. They don't exist in equilibrium; either a planet will preferentially convert CO2 to methane or methane to CO2, and there won't be any left of the other.

There are a handful of other atmospheric biosignatures that JWST can detect. Chemical compositions that are a sure sign of life; chemical signatures that are conclusive or strong indicators that life is present, even if we can't classify or describe it. Classifying exoplanets' atmospheres is a primary mission goal of JWST; it's what it's designed to do, and it's doing it right now.

[nwallin]

It's too late to edit my post, but the point of the CO2/methane thing is that if we found a planet that had lots of both CO2 and methane in its atmosphere, that is an atmosphere that exists in a non-stable equilibrium, which implies life.