K2-18b life?

[Tom Mazanec]

What are your thoughts on the best hint of exolife yet? I realize it is not yet conclusive, but if it pans out, how would it affect our timeline and background?

[Drashner1]

There is now apparently some amount of doubt being raised about this, but assuming it were to pan out as real, I don't think it would make a huge difference to the setting. The star in question is over 120 ly away - so it's not like early OA tech could get there fast enough to impact the early timeline. And we may possibly have life bearing exoplanets discovered closer (I'm not sure off the top of my head). So it would be interesting, but probably not massively setting altering IMHO.

Todd

[stevebowers]

The data on this planet looks very doubtful, and the detection of la biosignature is controversial. Additionally it is not dense enough to be anything except a mini-neptune, and planets like that won't have any dry land (the rocky core is probably thousands of kilometres below the visible surface). If there is any life there, it can only live in a very restricted layer of the water mantle, so wouldn't be anything like Earthlike.

[MiyuwiAuthor]

The consensus on the discord is that the recent discovery is extremely overhyped, and likely isn't true or means nothing.

[AstroChara]

Speaking as someone who follows a lot of astronomers, addressing everyone.

This whole thing was a trainwreck. Grifters doing shoddy science, hacking statistics to get the results they want, and then overblowing their results to the media who of course proceed to inflate it even further. So far, the claim of DMS comes from one (1) group only. Some people said this situation reminded them of the Venusian phosphine detection, but that team was responsible about their findings. This team is anything but.

Here are a bunch of sources you can read to understand the situation:

https://arxiv.org/abs/2504.12267
This is the paper in question.

https://bsky.app/profile/chrislintott.bs...y5sdsv5s27
This is a "general" thread which gives sort of an overview of the situation, including:
- The past works, including this exact team's past claim of DMS back in 2023, and also another team's paper which fails to detect DMS (which failed to be cited)
- If there is DMS, then we know that it is not only produced biotically; we have found DMS in a comet and interstellar space, and synthesized it abiotically. The papers which demonstrated these appears to also fail to be cited.

https://bsky.app/profile/distantworlds.s...zihugafk2x
This is a more technical thread which goes into why the paper did not in fact detect DMS or DMDS (the other compound claimed to have been detected).
In short, the actual detection was ~2 sigma — no detection. The 3 sigma claim came from them removing every other gas other than DMS and DMDS, then try to fit that, therefore artificially raising the significance of the detection. In short, statistics hacking. Then the press release of Cambridge came out claiming a 0.3% chance of detection being false. The actual thing is closer to 30%, or 1 sigma.

https://girlandkat.com/research-blog/202...-detection
This blog post was written back in 2023 when the previous paper on K2-18 b DMS came out. Ever relevant.

https://www.nature.com/articles/d41586-025-01264-z

https://bsky.app/profile/climatebook.bsk...44s6jtds2j
Another thread. It also notes that even the presence of CO2, an easier molecule to work with using an instrument easier to work with, is debated. Notes that K2-18 b most likely have a hydrogen-helium atmosphere, and either doesn't have a habitable surface or is over a magma ocean.
Bonus: Another model of K2-18 b as a "hydrothermal world" with supercritical water.

https://bigthink.com/starts-with-a-bang/...8b-flimsy/
Another article.

https://arxiv.org/abs/2504.15916
https://www.npr.org/2025/04/25/g-s1-6261...anet-doubt
A paper commenting on the claim, and an article discussing it.

And here's a less direct rebuke, but offers an insight into the motives of the authors, something I daresay is as important to keep in mind: https://bsky.app/profile/chrislintott.bs...ymhir6xs27

TL;DR all rubbish. This paper https://arxiv.org/abs/2504.12030 coming out the same day on arXiv is frankly worth your time a lot more.

[stevebowers]

From the 'bonus' link above;

We find that the observed atmospheric CH4/CO2 ratio implies a minimum ocean temperature of ~710 K, whereas the corresponding CO/CO2 ratio allows ocean temperatures up to ~1070 K. These results indicate that a global supercritical water ocean on K2-18 b is plausible. While life cannot survive in such an ocean, this work represents the first step toward understanding how a global supercritical water ocean may influence observable atmospheric characteristics on volatile-rich sub-Neptunes.

This seems more likely than a magma ocean to me, but we should include examples of both in OA. Neither type seems very hospitable to colonisation, unless you can build a suprashell above the atmosphere.

[AstroChara]

From the 'bonus' link above;

We find that the observed atmospheric CH4/CO2 ratio implies a minimum ocean temperature of ~710 K, whereas the corresponding CO/CO2 ratio allows ocean temperatures up to ~1070 K. These results indicate that a global supercritical water ocean on K2-18 b is plausible. While life cannot survive in such an ocean, this work represents the first step toward understanding how a global supercritical water ocean may influence observable atmospheric characteristics on volatile-rich sub-Neptunes.

This seems more likely than a magma ocean to me, but we should include examples of both in OA. Neither type seems very hospitable to colonisation, unless you can build a suprashell above the atmosphere.

A lot of rocks wouldn't melt until higher temperatures, although there isn't any issue with supercritical water interacting with magma ocean. The thing that sets it apart from the magma ocean model is that there is the supercritical water layer, I believe? Not sure.

[stevebowers]

The magma ocean model is puzzling; magma is just a little bit less dense than rock, so it implies a deep, dry hydrogen rich atmosphere covering a semi-molten rocky core. If there is any available oxygen in the crust it will combine with hydrogen to form a layer of water, which would be supercritical at the top, but become more liquid with depth.

[MacGregor]

So I’ll confess having only read the news articles when this came out and some YouTube videos I thought the case for life was stronger. But from what y’all are saying this is most likely bunk?

[stevebowers]

Yeah, since the announcement comes from a particular group of astronomers who seem to be invested in the concept of a 'hycean' planet. These are superearths which are large enough to retain hydrogen; this would create an odd species of planet where any free oxygen is converted into water, so they could only support anaerobic life.

However most hydrogen-rich superearths would be mini-neptunes, and the oceans would probably be pyrothallassic or pyrohydrothallassic (to use OA terms), making life as we know it very unlikely.

The fraction of worlds that are 'hycean' strictu sensu, with shallow Earth-like oceans and a hydrogen/helium atmosphere, is likely to be small, and this isn't one (judging by the density). I note that (in some models) Earth had a hydrogen-rich atmosphere in the early stages of its existence, but it can't have lasted very long.