[cletus]

I'm not sure what the point of this is. In the last 60+ years we've obviously learned a lot and widened our definition of where life on Earth can live (eg subsea volcanic vents, in rocks in Antarctica). We obviously don't know the limits of biochemistry and thus in what environments life can evolve and where it can continue to live where it exists. We're certainly not going to be detecting a world full of microbes from light years away.

What we're primarily interested in is detecting tecnological life. At stellar distances this really means detecting evidence of spacefaring civilization. This means practially the same thing because the gap between initial technology and spacefaring technology is a cosmic blink of an eye.

And those signatures will transcend biochemistry and what type of world you originate on barring some major glaring error in our understanding of physics (eg thermodynamics being violated).

The reason is that life will ultimately come down to energy and mass. Both of these mean there is pressure to expand and to expand is to become visible. You can argue that expansion isn't inevitable in all cases and you might be correct but it doesn't matter. It matters only if all civilizations remain small and/or hidden. And the odds of that go down as the number of civilizations go up.

I am of course talking about Dyson Swarms, a cloud of orbitals around a star. To put this in perspective, a full Dyson Swarm around our Sun would give each of the 8 billion people on EArth living area roughly equivalent in size to Africa and each person would have a million times more energy than the entire of humanity currently uses.

Such megastructures would be very obvious from a great distance too (ie due to the IR signature of dissipating heat). Likewise, there would be no hiding from such a civilization.

[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.

[analog31]

I think we're quickly moving towards living in a sort of poor man's Dyson sphere. More and more of our communications are being enclosed in optical fibers that leak virtually no radiation. Our free space comms are increasingly designed to operate on diminishing amounts of power, and to be practically indistinguishable from noise. We may be a dark planet by a couple centuries from the invention of radio.

Chemical changes to our atmosphere might be detectable from afar.

[Zigurd]

That "We're certainly not going to be detecting a world full of microbes from light years away" is debatable.

Changes that are not geological, like Earth's great oxygenation event could be vivid enough to be detectable among exoplanets.