Thanks for the shout-out!

I agree with you that saying K2-18 b “can’t” have an ocean or “isn’t” an ocean world is a stretch - we can’t totally rule it out with the present data, but it does appear that Neptune-like or gas dwarf models are consistent with what we know about the planet and require much less fine-tuning

It’s tricky to maintain a thin (~a few bar) H2 atmosphere over Gyr timescales near the intense XUV environment of an M star. This aspect is under-explored as far as I’m aware but is mentioned by e.g. Wogan et al. 2024. You definitely need a lot of model fine-tuning to maintain a thin H2 atmosphere

The planet would also need a high albedo (>~0.6) to prevent a runaway greenhouse scenario where a liquid ocean is unsustainable. Recent analysis of the JWST NIR spectrum suggests that the albedo of K2-18 b is lower than this (arxiv.org/abs/2504.12030, good paper kinda lost in the DMS noise I think)

There are a number of factors which challenge the ocean world hypothesis for K2-18 b. Due to its bulk density, the planet would need to be ~90% H2O by mass to sustain a liquid ocean. Based on analysis of the outer solar system, it’s tricky to see how a planet would form with >>50% H2O

Definitely some kind of space needed, otherwise you’d end up with 51 Pegasib and 55 Cancrie

Congratulations!!! I enjoyed (virtually) attending your thesis defence btw, great talk!

Thanks so much for this great summary! Also, happy birthday for yesterday!!! Sorry I didn't realise until today 🎂

If you're interested in the gory details of all 90 hydrocarbons we tested, or the subtleties of Bayesian statistics, check out the full paper which we have submitted for peer review: arxiv.org/abs/2504.21788

But in the mean time, we need to take a careful approach to interpreting the spectra that we have, and keep in mind the limitations of our models as well as the impact of what possibilities we do and don’t consider. That way, we can make sure we’re building towards truly understanding these worlds.

So, if we can't confirm the presence of propyne, or indeed anything, in the atmosphere of K2-18 b from this spectrum alone, what do we do? Of course, more observations would help, both to make sure there's a real signal from the planet, and to disentangle the different gases that could explain it.

The gas that came closest to explaining the observations out of everything we tested was propyne (C3H4), a hydrocarbon found on Neptune that hadn’t been tested before for K2-18 b. But it still fell far short of what we would consider a detection.

In fact, by those same Bayesian metrics mentioned earlier, we found that we could “detect” a whole range of gases, as long as you only compared them against a model with most other gases excluded. But if you test all of them together, those supposed detections vanish, even for DMS and DMDS.

We found that, due to the small signal size and high measurement uncertainty, anything from a collection of gases to random noise can explain the data. There just isn’t enough information in the spectrum to reach strong conclusions. But that only becomes clear if you test a large set of models.

However, that initial analysis only considered a small set of chemical species in their atmospheric model. So we tried a range of alternative models, and considered dozens of chemical species that weren’t explored in previous work, some of which we might expect to see on this planet, others less so.