There are a lot of hurdles in detecting extraterrestrial life, and that's not even counting the possibility that it might not exist.
Honestly, I don't think that last stumbling block is all that likely, and it's not just because proving we're not alone in the universe has been one of my dearest wishes since I was six years old and watching the original Lost in Space. Since astronomer Frank Drake came up with his famous Drake equation in 1961, which breaks down the likelihood of extraterrestrial intelligence into seven individual parameters (each with its own, independent probability), the estimates of the values of those parameters have done nothing but increase. As only one example, one of the parameters is f(p) -- the fraction of stars that have planetary systems. When Drake first laid out his equation, astronomers had no certainty at all about f(p). They were working off a sample size of one; we know the Solar System exists because we live in it. But was its formation a fluke? Were stars with planets extremely uncommon?
No one knew.
Now, exoplanet discovery has become so routine that it barely even makes the news any more. The first exoplanet around a main-sequence star -- 51 Pegasi b -- was discovered in 1995. Since then, astronomers have found 5,297 exoplanets, with new ones being announced literally every week. It seems like damn near a hundred percent of stable main-sequence stars have planetary systems, and most of them have at least one planet in the "Goldilocks zone," where the temperatures are conducive to the presence of liquid water.
Even setting aside my hopes regarding aliens, the sheer probability of their existence has, from a purely mathematical standpoint based upon the current state of our knowledge of the universe, improved significantly.
But this still leaves us with a problem: how do we find it? The distances even to the nearest stars are insurmountable unless someone comes up with warp drive. (Where are you, Zefrem Cochrane?) So we're left with remote sensing -- looking for biosignatures. The most obvious biosignature would be a radio transmission that's clearly from intelligent life, such as the one Ellie Arroway found in Contact; but it bears keeping in mind that through almost all of the Earth's 3.7-billion-odd years it's been inhabited by living creatures, it would have been entirely silent. Alien astronomers looking from their home worlds toward the Earth would not have heard so much as a whisper. It's only since we started using radio waves to transmit signals, a century ago, that we'd be detectable that way; and given how much transmission is now done via narrow-beam satellite and fiber optics cables rather than simple wide-range broadcast, it's entirely possible that once the technology improves Earth will go silent once again. There may only be a short period during which a technological civilization is producing signals that are potentially detectable from a long way away.
So the question remains: how could we determine if an exoplanet had life?
The tentative answer is to look for other kinds of biosignatures, and the most obvious one is chemicals that "shouldn't be there" -- in other words, that would not form naturally unless there were life there producing them through its metabolic processes. This, too, is not a simple task. Not only is there the technological challenge of detecting what's in a distant exoplanet's atmosphere (something we're getting a lot better at, as spectroscopy improves), there's the deeper question of how we know what should be there. If we find an odd chemical in a planet's atmosphere, how do we know if it was made by life, or by some exotic (but abiotic) chemistry based on the planet's composition and conditions?
We've gotten caught this way before; three years ago, scientists discovered traces of a chemical called phosphine in the atmosphere of Venus, and a lot of us -- myself included -- got our hopes up that it might be a biosignature of something alive in the clouds of our hostile sister planet. The consensus now is that it isn't -- the amounts are vanishingly small, and any phosphine on Venus is a product of its wild convection and bizarre atmospheric makeup. So once we detect a chemical on an exoplanet, is there a way to do a Drake-equation-style estimate of its likelihood of forming abiotically?
Astrobiologist Leroy Cronin, of the University of Glasgow, has proposed an answer, based on something he calls "assembly theory." Assembly theory, significantly, doesn't rely on any kind of analogy to terrestrial life. Cronin and others are now trying to figure out strategies to find life as we don't know it -- living creatures that might be based upon extremely different chemistry.
What he's done is given us a purely mathematical way to index chemicals according to how many independent steps it takes to create them from simple, pre-existing building blocks. This molecular assembly number, Cronin says, is directly proportional to its likelihood of being created by a living thing. As a simple analogy, he shows how you would find the molecular assembly number for the word abracadabra:
- add a + b;
- add ab + r;
- add abr + a;
- add abra + c;
- add abrac + a;
- add abraca + d;
- add abracad + abra (we'd already created abra in step three).