Bayesian statistics and the hunt for ET

One of the goals of the next generation of giant telescopes, now rapidly nearing completion, is to scour the nearby heavens for stars whose planets show signs of life.

If they find even a single such a world, Amedeo Balbi of the University of Rome Tor Vergata and Claudio Grimaldi of the Swiss Federal Institute of Technology Lausanne say, it probably means that the galaxy is teeming with life-bearing worlds.

In fact, the two astrobiologists write in the journal Proceedings of the National Academy of Sciences, such a discovery would indicate that life-bearing worlds may well be more common than pulsars (which are believed to number in the order of 200,000 in the Milky Way galaxy alone).

But if nothing is found, they say, it’s no reason to think we’re alone. It just means there’s no other world with detectable biosignatures close enough for even the best of the upcoming telescopes to detect its biosignatures.

These telescopes include NASA’s James Webb Space Telescope (scheduled for launch in 2021), the ESA’s Atmospheric Remote-sensing Exoplanet Large-survey space telescope (ARIEL, scheduled for launch in 2028), and the ground-based European Extremely Large Telescope (ELT, now under construction on a mountaintop in Chile and expected to be finished in 2025).

But while they are all formidable instruments, Balbi says, they won’t be able to ferret out life signs on more than a fairly small number of nearby worlds.

Exactly how many isn’t yet certain. But, he says, “it is safe to say that even with the best telescopes of the next two decades, we will not be able to study more than a few tens of nearby exoplanets with enough detail to look for evidence of life.”

In order to figure out what detecting such a biosignature might mean, he says, he and Grimaldi used a method called “Bayesian statistics” to figure out how one or more such detections (or none) might affect our understanding of how common life is in the Universe.

“Intuitively,” he says, “we expect that if we find evidence of life on a planet orbiting another star, then life must be common in the Universe. Our study tries to put this in rigorous statistical terms.”

“Right now,” he adds, “we only know one inhabited planet, Earth. So, we know basically nothing about the abundance of life in the Universe. Even just one detection of life elsewhere would change things dramatically.”

Not finding anything, however, won’t change our knowledge as strongly.

“The sample we will be able to study is very small compared to the whole galaxy,” he says. “So if we find something, it has a big impact. If we don’t, there is still the possibility that we have not looked at a large enough sample.”

Confusing the picture slightly, he adds, is the idea of panspermia, which posits that life can survive on rocks chipped off of a habitable world in one planetary system and live long enough to fall to ground in a nearby one, thereby seeding it with life that originated elsewhere.

If so, the distribution of life in the galaxy might be clumpy, meaning that the detection of a biosignature on a nearby exoplanet might simply mean it and Earth life are part of the same clump.

In other words, Balbi says, “We might find ourselves in a region of the galaxy that is more ‘populated’ than others, just because life spread.”

Also, the life signs these telescopes will be seeking don’t mean that if they find them, we’ve found ET. They could equally well mean that all we’ve found is a bunch of bacterial slime.

But, Balbi notes, if life of any type, even microscopic, proves to be common in the Galaxy, it does up the ante that ET is out there, somewhere, waiting to say hello.