Will we find extraterrestrial life?

My friend Tim asked me this question:

What do you think are the chances that we’ll detect (not necessarily physically encounter, but detect) life on another planet by the end of the century?

I think the odds are quite good, actually.

First, here’s something that I’m pretty confident is true: Within a few decades, we will have figured out how to measure the chemical composition of the atmospheres of other planets.  We’re moving fast in that direction right now, and while it’s a hard technical problem, I don’t see any show-stopping reasons why we can’t do it.  Basically, you have to have telescopes with sharp enough resolution to see the planet separately from its star, and then you just do spectroscopy.

I’ll be very surprised if we haven’t done this to hundreds and hundreds of planets within the next few decades.  We’ll know what molecules are in the atmospheres of those planets.  That means that we’ll detect life if a couple of conditions are satisfied:

1. Extraterrestrial life is not very rare.
2. Extraterrestrial life leaves identifiable chemical signatures in the atmospheres of host planets.

That’s as much as I can say with confidence.  From here on it’s guesswork.  Regarding #2, one important question is what would count as an identifiable signature.  People will naturally look at first for the chemicals that we find in our own atmosphere but that would not be there if there weren’t life.  I think that plain old oxygen (O₂) is one of the main ones here: the oxygen would all be in other forms such as CO₂ if it weren’t constantly replenished by biological processes.  I have no idea whether extraterrestrial life will be based on similar chemistry to ours, so maybe O₂ won’t be the signature we’ll see.  But it does seem likely to me that, if a planet has life on it, there’ll be molecules in its atmosphere that you wouldn’t expect to see in a dead planet, and once we get good at doing spectroscopy, we’ll find them if they’re there.  So I’m not too worried about #2.

#1 is the one nobody knows about.  Is extraterrestrial life found on lots of planets, or is it a one-in-a-trillion shot?  Here, you just have to make your best guess.  Personally, I don’t think it’s likely to be incredibly rare, so once we’re mass-producing spectroscopy of other planets, we’ve got a good shot at finding it.  But that claim is based on no data — it’s a Bayesian prior probability — so feel free to disbelieve me.

I think this life is far more likely to be simple microbes than big intelligent things.  I doubt we’ll be hearing messages from ET any time soon.  That doesn’t mean that I think searches for intelligent life like SETI are a bad idea, though: they’re quite cheap compared to lots of scientific research, and the payoff if they succeed is so huge that I think it’s worth throwing a little bit of resources their way, despite the long odds.