Clara Sousa-Silva: The fingerprints of life beyond Earth | TED

92,105 views ・ 2021-09-03

TED


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It's my job to find aliens, so as you might guess,
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I spent a lot of time thinking about them,
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imagining little green women
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meeting up with their friends, commuting to work.
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And that gets me thinking about alien astronomers
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trying to find us in their night sky.
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If these alien astronomers had looked in our direction
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in the last century,
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they would have been really excited to detect unequivocal signs of technology.
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But what if they looked ten thousand years ago,
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before we showed signs of civilization?
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Would they shrug in disappointment and go look elsewhere for life?
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No, and neither should we.
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Instead, we can look for other signs of life.
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For example, if those alien astronomers had looked our way 10,000 years ago,
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they might notice that even without signs of civilization,
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we still looked slightly unusual.
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For one, we have a thick and temperate atmosphere,
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but more notably,
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we have suspiciously large amounts of oxygen in the atmosphere.
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This would be a really encouraging sign of life for my alien colleagues
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because the composition of the Earth's atmosphere
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can only sustain itself through a biological cycle.
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So can we do the same to them?
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I'm certainly trying.
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I am a quantum astrochemist,
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which means I study the quantum interactions
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between molecules and light in space.
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We can't see these molecules or even the planets they're on.
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But when life from a star goes through an alien atmosphere,
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each molecule within it leaves a unique fingerprint
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in the starlight that I can see from here.
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And I look for the fingerprints
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of molecules that could be associated with life,
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or biosignatures,
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like complex pollutants or oxygen.
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In the context of Earth, oxygen is a wonderful biosignature,
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but oxygen is not that hard to make.
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So, for example, if our sun had different levels of radiation
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or if our oceans were evaporating from a runaway greenhouse effect,
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then large amounts of oxygen could accumulate in our atmosphere
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without biology,
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and then oxygen would be a false positive for life.
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So maybe oxygen won't be the solution to finding life beyond Earth,
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but then what is?
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Well, my specialty is to look for unusual molecules
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that have fewer false positives for life
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because they're so difficult to make that they're rarely made spontaneously.
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And my favorite of those unusual molecules is phosphine.
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When I first started working on phosphine about a decade ago,
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people had a hard time thinking of it as a biosignature at all.
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Instead, it was known
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for being this horrific, foul-smelling molecule
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that messes with life's ability to use oxygen,
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making it a really effective killer.
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Because of this fatal interaction with oxygen metabolism,
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phosphine is used widely as a pesticide
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and sadly for the same reason it was used many times in chemical warfare.
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Phosphine can be made in the lab,
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and it's also produced in the extreme environments
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found inside gas giants like Jupiter and Saturn.
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But on rocky planets like the Earth, it is rarely created accidentally.
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So we don't really expect to find phosphine on Earth at all.
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And yet we do.
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We find it in small amounts throughout the globe,
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and in some places in strangely large quantities,
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places like swamps and rice fields and lake beds
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and the excrements and guts of most animals.
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And what all of these ecosystems have in common
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is that they all host organisms that are not reliant on oxygen,
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so phosphine can't hurt them.
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Indeed, phosphine seems to be safely and enthusiastically produced
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in all of these oxygen-poor ecosystems.
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So I figured, maybe other planets with life less reliant on oxygen than ours
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could also have phosphine, but as a really popular biosignature.
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And here's the best thing about phosphine.
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Because it's so hard to make on rocky planets like the Earth,
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it has almost no false positives for life.
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So I started considering what telescopes we would need
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to detect phosphine on planets in our galactic neighborhood.
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Because if we did, I predicted it could only mean life.
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I was imagining a distant planet,
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a oxygen-poor tropical paradise
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with a phosphine-rich biosphere that we might one day be able to detect.
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But turns out phosphine was a little more exciting
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than I had initially envisaged
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because a few months after I finished this work
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an astronomer, Jane Greaves, reached out to me
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asking for help with interpreting a telescope signal seen here in white.
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Then months later, another signal, seen here in orange,
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that seemed to indicate that phosphine might be present not on a distant planet,
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but right next door, on the clouds of Venus.
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So did we do it?
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Did we find life beyond Earth? We don't know.
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These Venus observations were noisy and preliminary,
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so we still need to confirm, without a doubt,
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that the signal is real,
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and if it is, we need to make sure
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it's not another molecule mimicking phosphine's fingerprint.
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And if it is unambiguously phosphine,
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we still need to figure out what or who is making it,
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because maybe it's true
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that life is the best explanation for the presence of phosphine
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on a planet like Venus.
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But maybe that's wrong
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and there's an exotic but not biological way of making phosphine
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that no one has thought of yet.
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Either way, as much as I love phosphine,
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I don't think that's how we'll find life.
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The detection of life will likely not come from a single molecule.
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No matter how special it is.
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We'll have to detect a whole biosphere producing a complex network of gases
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that together form a message that reads: “We’re alive!”
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As the Venus story shows,
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the detection of life will likely be uncertain,
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but Venus is the perfect lab for us to test our theories of biospheres
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and how to interpret them.
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If we learn to understand the atmosphere of Venus
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and the message it contains,
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then we can find out if we got it right by going there and checking.
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And we'll do that at the end of the decade.
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But this will not be the last time
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that we have the discovery of a biosignature
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on a potentially habitable planet,
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and next time we won't be able to just go there and check.
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So my biggest concern
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is not that we will fail to find a habitable planet in our lifetimes.
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My biggest concern is that we'll point our very expensive telescopes
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directly at an inhabited planet
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and just not know we did it.
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But I am determined to not miss life.
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So, yes, I will look for the unambiguous but quite unlikely signs of technology
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like complex pollutants.
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And I'll look for the pleasant and familiar
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but potentially misleading signs of life, like oxygen.
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And of course, I'll keep looking
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for the strange and scary biosignatures like phosphine.
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But crucially, I will look for all the molecules
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that can together paint a holistic picture of a biosphere.
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All of this so that one day we'll know life when we see it.
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Thank you.
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