I need a way for a spacecraft to detect absolutely irrefutable proof of organic, but not intelligent, life on a planet, most likely through spectroscopy or another similar concept. The spacecraft is not in orbit around the planet (heliocentric and heading toward the star), but is in the same system. Assume a solar twin, but the planet could be of any format required, even a moon of another planet hosting the life if necessary. For plot reasons, I need this to not be something easily noticeable at first.

The spectroscope used for detection is probably a little bit behind modern day, but if there's some concept needed to stretch the capabilities I'd accept that too. If it helps with the timeframe, the delay in the realization should be at least a few days, but the data is going to be printed out on physical paper - no computer screens or automatic analysis tools available.

In short, what chemical compound(s) detectable on a planet from a spacecraft would be irrefutable, but not immediately obvious, proof of life?

See my previous question for additional context on the technology restrictions. (You might also be able to deduce the plot motivations I have for asking this question.)

  • $\begingroup$ The previous question you link to gives a range of possible technologies, it doesn't clarify anything related to this question as far as I can see. To save this question being closed as fishing for ideas (or story-based), could you give us explicit details of the technology available and the type of life you're expecting to not find with them... What are the systematised procedures for determining the criteria of the presence of life? Perhaps see the Wikipedia article on biosignature for a start to your researches. $\endgroup$ Feb 2 at 14:41
  • $\begingroup$ Cont.: Also MERMOZ. $\endgroup$ Feb 2 at 14:43
  • $\begingroup$ 'Within 1AU' doesn't really help. What distance exactly would they be at when they detect life? Detecting life on the moon from earth would be very different to detecting life on Mars (both of which are 'within 1 AU'). Ranging from atmospheric spectral analysis to noticing things have moved about. $\endgroup$
    – JeffUK
    Feb 2 at 14:43
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    $\begingroup$ Edited to clarify I'm looking for what sorts of chemical compounds might fit the bill - spectroscopy was the term I was looking for but could not remember or find immediate reference to for some reason. $\endgroup$ Feb 2 at 14:46
  • $\begingroup$ @JeffUK I gave that range just to be clear it wasn't from another solar system but also wasn't parked in direct orbit around the planet. The exact range is not super important to me but if it changes the answer I can accommodate. I'll make another edit to explain this. $\endgroup$ Feb 2 at 14:47

4 Answers 4


Dimethyl Sulfide

In September and October 2023, there was news that the James Webb telescope had found this compound on a faraway planet. This is what gives the sea its particular smell, and there are basically only two sources of this substance on Earth: microbes and chemical plants.

If there is either of these sources on the planet (K2-18b, right at the bottom of Leo), then we'll have found life in another planet. Most likely it would be microbial, as the planet is a mini-neptunian hydrogen hell. But it may also be that we'll end up discovering a non-biological, non-artificial source for dimethyl sulfide as well.

There is also still a one in 50 or 60-something chance that the detection of dimethyl sulfide was an error, and some data that James Webb collected just a few days ago to confirm or deny that is still being processed.

But suppose we get confirmation it's biological. Imagine that, alien life found in our lifetime, and it's not fiction!

Anyway, your probe could be in a similar situation, where it has detected dimethyl sulfide through spectography. But it only had one good observation, so it needs a few more passes to confirm that it got the spectographic signature right.

  • $\begingroup$ Sorry for the long delay in accepting, but this is exactly the sort of thing I was looking for! I can sort out the plot elements behind missing it initially myself, but I didn't know what would be missed. Thank you! $\endgroup$ Feb 21 at 21:29

I'm going to back up my comment with an answer, that still potentially fits.

Visual cues

Your astronauts take a series of photographs of the planet. None of them show anything extraordinary, the planet would have to be a mostly barren rock for them to not spot life immediately, so assume life only exists in the seas.

Only careful comparison of photographs reveals a visual change in the water that could only be an algae-like bloom, a shoal of proto-fish, or a really fricking big whale, etc. Something you mightn't 'detect' immediately. If comparing the photos over time isn't in your protocol, you might not notice it until you happen to put two photos next to each other. But in doing so, you see movement that is organic in nature.

Note that to leave the solar system and move onto another, they may have already committed to their new trajectory before their closest approach to the planet. So 'spotting it while near to the planet' and 'spotting it only after they've committed to leaving the system' aren't exactly mutually exclusive.

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    $\begingroup$ I don't expect visual cues from orbit will be irrefutable evidence. Variation in the appearance of the surface can be due to weather, geological, or seasonal effects. Humans are really, really good at pareidolia and will find seemingly meaningful patterns in completely random images. Mars, for example, gets seasonal dark patches, but biological explanations have been ruled out. $\endgroup$ Feb 2 at 16:29
  • $\begingroup$ @NuclearHoagie Yeah, this is another reason I'm looking more at chemical analysis since it's more "concrete" proof. The method of travel involves collapsing the star into a black hole in order to be able to move on to the next system, so this moment of the MC realizing they discovered the first alien life in the galaxy and almost certainly doomed it without even realizing is an important plot moment. $\endgroup$ Feb 2 at 16:43
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    $\begingroup$ There could be visual cues that are pretty irrefutable, like the en.wikipedia.org/wiki/Pufferfish_mating_ritual, especially when observed along a timeline. Could be a whale-sized creature doing something like that with foam, observable from orbit. Image is received immediately, but only on (automated) classification it stands out $\endgroup$
    – bukwyrm
    Feb 3 at 12:36
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    $\begingroup$ There's always another explanation, 'organic' chemicals in the atmosphere could conceivably be created by purely chemical means. That some sort of 'Reverse entropy' was caused by life is quite convincing but could of course be an emergent property of a 'non-life' system. You're then into a semantic argument about 'what is life' if not some sort of system that (locally) reverses entropy. I would put both in the story! DMS AND some visual evidence would be doubly compelling evidence. $\endgroup$
    – JeffUK
    Feb 4 at 13:00

Frame challenge:

Not a chemical compound, but since you specifically outrule automatic analysis tools you could just resort to human error. Your scientist have to go over the thousands of chemical compunds detected by the spectroscopy by hand (or rather "by eye"). And eyes get tired.

If you ever read a book until deep in the night, you might have experienced reading a line, that doesn't make sense, only to realize that you just skipped a line. With a list of chemical compounds you do not have the "error correction" due to context because each line is just another compound.

Since they obviously know about this risk, the scientists have introduced a rule that makes in mandatory to have not only one but two scientist go over every printed list. But like most such "saftey measurements" this rule is lived laxly and if more interesting stuff is available to do the "recheck" is postponed and postponed again.

However the one guy, that always wants to keep every little rule, nags he others regulary to do their "rechecks" at least before leaving the system.

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    $\begingroup$ This is the general idea I had in mind yeah, but what I'm looking for is more what that evidence (in this case, which compound(s)) would be the indicator of life. $\endgroup$ Feb 2 at 15:39
  • $\begingroup$ Note that the more common term for this sort of redundant verification is cross-checking. $\endgroup$
    – Kevin
    Feb 5 at 18:47

Oxygen is not a reliable indicator of life on it's own - but taken together with other facts about the planet it can be. Ideal as a late-realization tool for the plot.

NASA gives the photocatalytic activity of Titanium Oxide as an example of how even large ratios of oxygen might lifelessly exist in a planetary atmosphere - there is bound to be more such compounds. Also, water-vapor rich atmosphere + loads of UV equals oxygen, etc. .

So the spacecraft is nearing the planet, seeing lots of water vapor in the atmosphere with long range sensors, sun is UV-heavy, crew is primed to see non-life-indicating oxygen, sees it, no biggie. Spacecraft passes planet, oxygen content is confirmed. They do not pass through a veil of ultrathinned hydrogen though, which is a fact that rules out the UV-cleaving of water in the outer reaches of the atmosphere (hydrogen would need to escape into space in copious amounts to avoid recombination).

Taken together with not enough candidates for catalytic compounds detected on the planet's moon (=remainder of protoplanetary disc that has a very similar composition to the planet, but much more observable because no atmosphere) - e.g. via spectroscopy done during an observed impact ... life!


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