In the news lately, scientists are announcing the finding of potential biosignatures on an exoplanet, but if an exoplanet is not host to “intelligent” life (ie broadcasting to us or able to communicate to us), what would scientists need to confirm its presence?
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I’m hoping some exoplanet specialists will weigh in since it’s a topic that has been much in the news internationally this week.
Realistically, confirming biosignatures is going to be the work of many years. It will take the overlapping detections of several different molecules, at multiple wavelengths each, that are detected in abundances inconsistent with geological or photochemical processes, while at the same time laboratory work and computer modeling of exoplanet atmospheres is improved to match the current data quality provided by telescopes like JWST, or hopefully the Habitable Worlds Observatory in the future. (For example, we don’t have lab spectra of many “biologically relevant” molecules under the conditions found on some exoplanet atmospheres.) The odds are that there’s not going to be a single “Eureka!” detection. (And even all of this may not be enough to say “life” if we don’t really have as solid an understanding of what that means as you might expect! See the answer from u/madz33)
In regards to the recent press on K2-18b, I am not an exoplanet scientist but I do work alongside several that I have talked to about this result. There are some issues to consider in interpreting the publicized paper. While those are public-level articles or threads about it, here are scientific papers about some potential hazards in the analysis for the interested: paper 1, paper 2, paper 3. Needless to say, it’s an active discussion and an exciting time for exoplanet scientists, but caution is warranted and certainty is not going to be had any time soon.
The five most unmistakable ways:
Aliens or an alien probe/spaceship actually visiting our planet/solar system.
Getting a radio (or other) signal that is unmistakably the product of intelligent life, especially if we were able to decipher it and see/hear the broadcast or if it was some other sort of beacon (like a beacon broadcasting a long series of prime numbers)
One of our probes (or human missions) finding life on Mars/Europa/Titan/etc. in the form of actual visible cells/microbes or similar things (or macro life). This would take longer to confirm because microbes could be the result of contamination from Earth.
Finding a planet with an oxygenated atmosphere similar to Earth, at similar temperatures to earth (planets cooking at 2000 C need not apply), with other relevant biosignals. Free molecular oxygen is extremely reactive so a planet having an atmosphere full of the stuff would indicate something very unusual was going on and would be a major indicator of life, and finding other biosignatures would seal the deal.
Direct telescopic observation of an extrasolar planet and seeing vegetation on the surface. This is probably a long ways off.
Basically, macro-effects that are unmistakable evidence of extraterrestrial life, or literally finding extraterrestrial life.
Theoretically you could find a bunch of biosignatures without free molecular oxygen, but I think the further it is away from biochemistries that we understand, the harder it is for us to actually confirm it is life and not some abiological process operating.
Something which is often missing from this discussion is that we simply don’t have a “theory of life” which can be scientifically evaluated. In my research I detect molecules in exoplanet atmospheres which are in disequilibrium, which means the molecule is overabundant when compared with expectations. This disequilibrium is only possible when the source of the molecule is replenishing the supply faster than other processes can remove it. Life on Earth can be functionally thought of as a disequilibrium process which drives certain molecules out of equilibrium, most notably molecular oxygen. However, there are plenty of physical (non biological) processes which can drive different kinds of chemical disequilibrium, in my field it is often from turbulent fluid mixing in the atmosphere.
So even if, in the best case scenario, science can reliably detect (multiple) molecules at disequilibrium abundances, and these molecules can be shown to be consistent with a chemical network which is possible in living organisms, there will always be some uncertainty with respect to the notion that these chemical reactions are because of “life” and not some other organic chemistry which is occurring due to unknown geophysical processes. (Some astronomers even detect organic compounds in interstellar space.)
We can reframe this dilemma in the question of the origin of life on Earth. If you agree with the Hot Spring Hypothesis for the origin of life, (which in my opinion is well evidenced and fascinating) at some point in Earth’s history geophysical processes bootstrapped the production of organic compounds which set the stage for “living matter” to rise out of the abiotic mud. At what point did those processes become “alive?” (This is a kind of “paradox of the heap of sand.”) Maybe there was a sharp phase transition as the living biosphere emerged and crystallized or maybe there was a slow and gradual complexification of the available precursor molecules. Or perhaps the distinction between living and non-living matter is simply an illusion, but this isn’t askTheology.
In the context of remote sensing of biosignature molecules, I don’t think this dilemma has even been meaningfully approached scientifically, even as we evaluate spending billions of dollars on a next generation instrument to search for habitable worlds.
Over the years we have found many things that people have claimed was proof of life. As examples:
Pulsars were thought to be ET beacons.
Viking Mars landers had experiments to determine metabolism in the soil and one of them got a positive result.
Further analysis of these came up with more mundane explanations.
The problem is that we should first decide “What would proof if life be?” and then search for it, rather than find an anomalous result and see if life explains it.
Of course that means that we actually know what “life” is and in truth we do not. Sure we can define life as we see it here, but that is very limited.
Short of a radio signal that announces the presence of intelligent life, the road to confirming life on another planet will be long.
The easiest way is to look for the light spectrum that contains molecules that could not have come about in any other way than from biological processes. Once you find a candidate planet, you need repeated observations over time, ideally with different instruments all confirming the presence.
You then need to systematically eliminate any potential for error, or alternate explanation. You just keep doing observations and trying to test alternate theories until every other possibility is so remote that life becomes the only plausible explanation. By then, most scientists will accept that some form of life exists there.
But even then, nothing will be 100%. We could launch a much more powerful telescope in the future that changes everything. The only way to get total confirmation would be to actually visit the planet and see. (Baring the life there reaching out to us)
If you observe that very specific wavelengths of light are being absorbed in large quantities on that planet, especially if they are the the most abundant wavelength being emitted by star of that system. It’s a very strong indicator of photosynthesis. The most abundant wavelength of light from our star is red light, that is why most chlorophyll is green, because it absorbs red light.
Realistically, they would need to examine it directly. Life isn’t a strict definition yet, we don’t all agree on what it is. There are characteristics we do agree on: it has to self-replicate, it has to exchange energy with it’s environment etc. Without actually watching the organism in question, we probably couldn’t confirm all of these criteria.
We might be able to see some evidence of life (motion, a change in population etc) from orbit, but without watching it in a lab, I doubt we could satisfy all the criteria.
Scientist would need what they are finding today. Again, and again (results need to replicate robustly), and then some different completely independent biosignatures, and those again and again, and then some more, etc.
There probably won’t ever be the ONE discovery that in and of itself proves the existence of life on another planet, no matter how much headline writers want that to be true. Science is an incremental process. And when direct evidence is so hard to come by, even more so.
Just look at how long it’s taking us to confirm liquid water on prehistoric mars. There was never the ONE discovery that confirmed it. Is was an ever-growing abundunance of individual pieces of evidence, each of which is not foolproof (or water proof so to speak ;-)), each of which could be explained away. But the mass of all of it together convinces us that the hypothesis is more likely to be true than not. And that’s taken us decades. In fact, we’re still kind of in the middle of it and there are still some scientists who are not convinced yet.
Finding life on an alien planet will be the same. They will find some planets with weak biosignatures, then some more, then they will find the first planet that exhibits two different biosignatures and so on. And that will accumulate until it is more likely than not that life exists out there. And even then, it will at first be some kind of “Well, it’s incredibly unlikely that ALL of these planets have biosignatures on accident, but I don’t know for sure about THAT specific one”. And it will take more time and more study and more tiny, incremental progress until we can finally smugly say “Yes, of course there’s life. It’s over there! We’ve known that for decades, haven’t you been paying attention!?” with all the weight of hindsight 😉
I don’t think that’s a thing can be done with planet atmospheres. Europa has an atmosphere consisting almost solely of molecular oxygen, and Mercury has a high percentage of oxygen in its atmosphere, for instance. Oxygen is a deadly poison for many life forms, even on Earth.
I would look for any lighting at night that isn’t due to volcanic action. Whether it be artificial lighting or forest fires, it would still be a sign of life.
I would also look for narrowband optical absorption. On Earth that’s a sign of chlorophyll.
afaik, most “life on other planets” is talking about non-intelligent life at this point.
But most methods I’ve seen worked on the basis of verifying the existence of chemical compounds and elements that, to our knowledge, can only be created through biological processes.
If we can verify through the spectrographical analysis of the atmosphere of a planet that gases exist, that to our knowledge could only have been created through biological processes, the conclusion is that biological life must exist there.
The same would be true for advanced life that has technology, since those processes would also create compounds that would not be created in nature on its own.
What finding would it take for scientists to confirm life on another planet?
to find a living creature – getting there, observe and evaluate
all “biosignals” possibly indicating life from the distance are just “potential”
Scientists would need multiple, independent lines of evidence (like specific atmospheric gases in disequilibrium, narrowband spectral absoprtion patterns, and seasonal changes) that can’t be explained by any known non-biological process – basically a “beyond reasonable doubt” approach rather than a single smoking gun.