Scientists have found tentative evidence  for life in another solar system.  

There's been headlines like this for several years  now. The most recent one comes from data from the  

James Webb Space Telescope and a planet called  K28b. Today I want to have a look at what we know  

about life on this and other exoplanets and why I  think astrophysicists have pretty much found life  

on other planets. It's just that they're not yet  allowed to say it. When astrophysicists talk about  

life on other planets, they mean biosignatures  that some sort of evidence that life in a form  

we know of might be present. They look for this  by scanning the atmosphere of these planets for  

certain molecules or combinations of molecules  that at least on Earth signal the presence of  

life. Bio signatures will be evidence for  life but not necessarily for complex life.  

To tell the two apart, astrophysicists also look  for technosignatures. That would be signs of any  

sort of technology indicating intelligent beings.  But it's a very crude distinction in that there is  

a lot that can happen between microbes and air  pollution so severe you can literally spot it  

from light years away. The key bio signatures I  look for are molecular combinations that should  

not exist in chemical equilibrium because  there are no geological processes that could  

produce them. At least none that we know of.  That said, let's look at the headline cases.  

The most intensively discussed potential detection  of life on an exoplanet is K218b. This planet is a  

little larger than Earth in a solar system about  124 light years away. It sits in the habitable  

zone of a red dwarf. Already in 2019, the Hubble  telescope detected water vapor in its atmosphere.  

That's not a biosignature on its own, but it's  suggestive. In 2023, an analysis of preliminary  

data from the James Webb Space Telescope revealed  methane and carbon dioxide. And then last year,  

the same team returned with better data and  reported a three sigma detection of dimethyl  

sulfide at concentrations that seems to be  thousands of times higher than on Earth. On Earth,  

this gas is produced almost exclusively by marine  microbes. They called it the strongest evidence  

yet of possible bio signature activity outside  the solar system. Alas, reanalyses by independent  

groups found that the data was consistent with  many other possible chemical compositions and  

basically inconclusive. There have been other  planets of interest. For example, TOI270D.  

This planet too is a little larger than Earth and  orbits around a dwarf star about 73 light years  

away from us. However, it's rather close to the  star which puts it at the edge of the habitable  

zone. For this planet, the data is much better.  Two independent groups have found reasonably  

strong signatures of methane, carbon dioxide, as  well as carbon dulfide and ethane. Again on Earth  

these are produced by microorganisms. However, in  this case one can't rule out that there are other  

nonbiological sources. So the data is also  inconclusive though for a different reason.  

There are many more planets that have attracted  interest in the past years. But where the data has  

remained inconclusive for one reason or another.  For example, the Trappist system that's only about  

40 light years away from us. Its sun is a red  dwarf, only slightly bigger than planet Jupiter,  

but it has at least seven rocky planets, three of  which seem to be in the habitable zone. So, if the  

system is that much closer to us, and that much  more promising, then why don't astrophysicists  

just look at those? It's because the red dwarf  is so small and yet very active. That it's so  

small means that the habitable zone is very close  to the star and that it's so active makes it very  

difficult to find out whether the planets there  even have an atmosphere. Astrophysicists have  

looked at this over and over but not been able to  get any good data. They've now scheduled more than  

a dozen additional observation runs with the James  Webb Space Telescope. So maybe we'll soon know  

more. There are more than these example planets  that astrophysicists have found in habitable  

zones. Some look promising, some not so much. The  sense I get is that it's only a matter of time  

until they'll find a candidate that checks all  boxes and the significance for the bio signatures  

is high enough. You can basically see it coming.  I have reason to believe I'm not the only one who  

thinks this way. The other day I came across  this comment on X Twitter was speaking to an  

academic astrophysicist in spectroscopy who told  me that they have so many plausible exoplanetary  

atmospheric bio signatures at this point that  there's no longer any real doubt. But the  

standard of proof for official acceptance is just  incredibly high. This is exactly my impression as  

well. Then again, some in the field have recently  claimed that with the James Webb telescope,  

we'll never be able to reach the required  significance to declare we have evidence for life  

on an exoplanet. Maybe they're right, but I think  it's just an excuse to ask for a bigger telescope,  

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