The secret to finding life on other planets is not to look for life as we know it

by Todd Neff

Perhaps one day we’ll send a spacecraft to a rocky planet orbiting Proxima Centauri. And perhaps the first images arriving back from across 4.2 light years of space will feature a purple Proxima Centurian peering straight back into the camera.

In the popular imagination, alien life has tended to focus on the take-me-to-your-leader/humans-as-snacks variety. Those who have been paying attention, though, know that the life we’re most likely to find on Proxima b, Mars or anywhere else will be microscopic. That life might have very little resemblance to the microbes we’re used to here on Earth.

This gives rise to what appears at first to be a scientific problem: the question of how to recognize alien microbes, which astrobiologists assume to be the universe’s most common life form, is to no small degree a philosophical problem. Philosopher Carol Cleland has been a leading voice in helping NASA and the astrobiology community figure out ways not to miss extraterrestrial microbes right under our robotic emissaries’ noses.

Cleland is a professor at the University of Colorado — a space-science powerhouse — and is a member of CU’s Center for Astrobiology as well as director of the new Center for the Study of Origins. She’s wrapping up a book called The Quest for a Universal Theory of Life: Searching for life as we don’t know it. Her key point: because we’re just starting out on this quest, and because we only have a single example of life to work with, we need to keep a really open mind. Rather than firm definitions of what life is or isn’t, she has introduced the idea of “tentative criteria.”

A single example of life, you ask? An elephant would seem to have little in common with a redwood tree, a diatom, or the mold on that sandwich you forgot about in your backpack, after all. Yet all life on Earth emerged from a single common ancestor.

That common ancestor is at the intersection of the blue, red, and black lines. We’re over on the right, hanging out with slime molds, plants and fungi. Besides providing a dose of humility for those of us who consider ourselves to be above slime molds, this tree of life makes clear: we’re quite similar at the biochemical level. Life here is diverse, but it’s ultimately rooted in DNA and RNA-based replication, and protein-based metabolism. This may be the case everywhere. Or it may not be.

“Everybody is still obsessed with defining life,” Cleland said. “They say, well, if you don’t define it, how will you recognize it? My answer is, if you do define life, you won’t recognize alien microbes unless they closely resemble our life.”

This is where tentative criteria come in, she says. With tentative criteria, scientists designing future life-hunting space instruments would do so based on “a suite of characteristics” that individually hint at and collectively point to life, she says. It can’t be too narrow, such as demanding the presence of ribosomes, either. But it also can’t be too broad — say, the ability to concentrate energy from the environment to maintain itself, which some nonbiological systems do.

The idea is to use tentative criteria not to declare “It’s alive!” but rather to spot oddities in an extraterrestrial system that may indicate past or present life. Then you investigate further, ideally with more specialized tools tailored to sorting out additional clues.

“You use tentative criteria to identify anomalies,” Cleland said. “Once you find an anomaly, it doesn’t mean it’s living. It doesn’t mean it’s nonliving. It means it violates our preconceptions about life. And so we should explore it further for the possibility of alien life.”

What are some potential tentative criteria to identify extraterrestrial life? Living creatures on Earth prefer the lighter isotopes of carbon and sulfur. An unexpected concentration of light isotopes of these or other atoms might warrant a deeper look. Living creatures on Earth produce hugely complex molecules — not just DNA and RNA, but also proteins and others. The presence of similar molecular complexity would raise a flag. And, of course, there’s “the presence of a self-sustaining autocatalytic molecular system under conditions you wouldn’t expect,” Cleland added. She may be a philosopher, but she can sound a lot like a scientist.

Whether NASA and others take her advice on using tentative criteria is beyond her control. She’ll forge ahead regardless, she says.

“Hard problems about the natural world really fascinate me,” she said. “As a philosopher of science, I like to find potentially more productive ways of pursuing answers to such questions because I think that people often pursue them the wrong way.”

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