How can you as a person who is not a scientist know if a given claim of having found alien life is valid?  In 2014 I used the planet Kepler 186f as an object for discussing this very question.  First of all, at best we can know the odds that a given planet has life.  We can't be 100% certain until we get a sample and if that planet is around another star, then it'll take a while.   What we can do is roughly estimate the odds based on three primary lines of evidence.  These three types of evidence are things anyone can understand and assess based on the output of professional scientists.  You need not know how to analyze the data, but any scientist claiming to have found alien life on planet X needs to be able to claim to have made a few crucial observations. 

  1.  A Earth mass planet in orbit in the habitable zone of a star. 
  2. Spectral analysis of the atmosphere of this planet indicating that it would support life. This life can be in an ocean or air breathing. 
  3. At least marginal detection of non-random, non- natural, radio or optical signals that indicate technology is in use.  A techno-signature of intelligent life.

If all three of these things are present, then we can be 99% sure that there is intelligent life on the planet under study.   IF the life is not intelligent or not technological, at least not at the time when their light is reaching us, then we can't ever be that certain.  Remember the farther we look in space, the farther back we look in time.   To an alien civilization looking at us from 500 light years away, they see us as we were in 1523.   To put that in perspective Jamestown Virginia wasn't founded until 1607, and Istanbul was still Constantinople 25-30 years before then.

K2-18 b 

So, what of the latest possible find by JWST K2-18 b?   K2-18 is a red dwarf 124 light years away.  If there were technological life there it would observe us as we were in 1899 or there about.   Likewise, we'd be seeing light from them that traveled just that long. 

K2-18 b is not Earth mass, but it is in the habitable zone of its star. It is a sub Neptune/ Super Earth.   So it meets some aspects of the first criterion, but not all of them.  The reason to insist on an Earth mass planet to have greatest confidence the habitability criterion has been met is that we know of just one habitable planet, Earth. The more different a planet is from Earth the less we can know from interstellar distances.   I'd give the planet 1/2 credit for this one.  

According to "Carbon-bearing Molecules in a Possible Hycean Atmosphere" Madhusudhan, Sarkar,  Constantinou, et al arXiv:2309.05566 K2-18 b.  (The hallmark of a good scientific paper announcing a ground breaking result is a humble title.)  

 Here we report a transmission spectrum of the candidate Hycean world, K2-18 b, observed with the JWST NIRISS and NIRSpec instruments in the 0.9-5.2 µm range. The spectrum reveals strong detections of methane (CH4) and carbon dioxide (CO2) at 5σ and 3σ confidence, respectively, with high volume mixing ratios of ∼1% each in a H2- rich atmosphere. The abundant CH4 and CO2 along with the non-detection of ammonia (NH3) are consistent with chemical predictions for an ocean under a temperate H2-rich atmosphere on K2-18 b. The spectrum also suggests potential signs of dimethyl sulfide (DMS), which has been predicted to be an observable biomarker in Hycean worlds, motivating considerations of possible biological activity on the planet. 

In simple terms, by observing light from the host star, which passed through the atmosphere of this planet, using JWST, this team was able to observe the chemical signature of molecules strongly associated with life.  A confounding factor is that these molecules can also be created by non-biological processes, and we have nearly been fooled before.   That is why this alone is not enough.  A strong case can be built on spectroscopy.  These observations fulfill the spectral criterion.  So full credit for this one. 

As for techno-signatures.  No observations I know of so far of any such signatures.   It is very unlikely that there is any technological life within the radius of about 100 light years.  We have had radio and would've picked up unquestionable signals or noise or something by now.   There are non-radio methods of communication like optical.  Data could be sent via laser light from one planet to another.  A big problem with searching for such signatures from a water world is, even if the life was very smart, it's hard to build a radio with a flipper.  

Does K2-18 b have life?

This kind of question can really only be answered in terms of probability.  There is no way to be 100% certain that there is life, and many ways to rule it out.  

 1/2*33% + 33% gives 49.5% chance of life by this simple reckoning.  This does NOT mean a high chance.  50/50 would mean a state of maximum ignoranceI'm leaning a bit towards no life.  Mainly because the planet is not Earth like enough.  I admit I am not a planetary scientist.  I am not a chemist or atmosphere expert.  I'm just a plain old run of the mill theoretical astrophysicist doing my small part.   Think of the three criteria, Earth mass habitable zone planet, spectral evidence of an atmosphere that can and does support life, and techno signatures such as radio, as being like the legs of a stool.  With all three legs a conclusion that there is life on a given planet can be strongly supported.  With one- and one-half legs... well... not so much. 

Compared to the majority of planets that are found which are way to hot or cold for life, as we know it K2-18b is a much better candidate for life.  Keep an eye on it.     

If you like my work you can read a bit more of it or in a different format on my Substack.  Science 2.0 is the place for breaking news that I come across or to hear a no- nonsense easy to understand breakdown of issues I write about.  

I want to reply to a good comment by John down in our Disqus powered comment section but can't because I'd have to verify the very old email I used for this.  I don't have access to that email anymore and can't change it because I don't have the password and they don't seem to offer support. 

Here is my reply.  We can only base an analysis on things which could be sensed at astronomical distances.  To be 100% certain we'd need to send a probe to an exoplanet.  That total certainty is impossible is  why in this simple framework 100% is impossible.   Claiming to not know if there is or is not life,  basically 50/50, is not a wild claim.  Claiming to know for sure either way is the extraordinary claim in the face of the evidence we have about this planet.  That is my hill and I stand on it. As for more complicated ways of thinking about these questions.  Those are for experts or for teaching in classrooms.  A blog should be first and foremost for the average person.