To find extraterrestrial life, we must first understand what we’re looking for. This sounds simple; it isn’t.
“We don’t know what life is,” says Cabrol. More specifically, the only type of life we understand to any extent is the life we have here on Earth. And life here is quite complex, from bacteria thriving in underwater volcanic vents to microbes drifting in the upper atmosphere. Earth’s organisms routinely rewrite the rules, so when it comes to extraterrestrial life, it could be familiar to life as we know it—or perhaps shockingly, well, alien.
For now, the primary starting point in the search for life elsewhere is Earth-like chemistry.
“We are made of the most common stuff in the universe, and that’s not by accident,” Cabrol says. If those elements are abundant across the cosmos, then similar biochemistries may be, too. That’s why most searches for life often focus on biosignatures—evidence of current or past life.
On Earth, the hardiest types of life are typically microbial, so it stands to reason that’s the type of alien life we’re most likely to find. But even familiar biochemistries could take unfamiliar forms.
“You can have the same molecule, but their chirality, the geometry of the molecule, might be the opposite,” says Cabrol. The same building blocks don’t always result in the same final product.
Of course, that doesn’t rule out intelligent life existing beyond Earth, and that’s why researchers are also looking for technosignatures, or “things that the environment alone, nature alone, cannot produce,” says Cabrol. That might include chemicals that aren’t known to be produced biologically, unusual heat signatures, or even deliberate signals from an advanced civilization (whether or not we’d be able to identify a signal, let alone understand it, is another question).
Still, Cabrol warns against sensationalism in the search for life. Take, for instance, the hypothesis that interstellar objects like Comet 3I/ATLAS are actually advanced alien technology.
“It’s just fine to have hypotheses and push them until you realize that they are silly,” Cabrol says. As far as current research goes, Comet 3I/ATLAS is, indeed, just a comet. “But having said that, yes, of course. I’m excited,” she adds. “All of a sudden, we have free samples coming from the far end of our galaxy, telling us about the chemistry and the physics of other worlds, places we’re nowhere close to being able to visit anytime soon.”
