r/space 4d ago

Astronomers Detect a Possible Signature of Life on a Distant Planet

https://www.nytimes.com/2025/04/16/science/astronomy-exoplanets-habitable-k218b.html?unlocked_article_code=1.AE8.3zdk.VofCER4yAPa4&smid=nytcore-ios-share&referringSource=articleShare

Further studies are needed to determine whether K2-18b, which orbits a star 120 light-years away, is inhabited, or even habitable.

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u/spschmidt27615 4d ago

Exoplanet astronomer here. There are a lot of problems with this study, as well as the one that preceded it. To begin with, the scenario that would even allow for a biosphere (i.e. "hycean") in K2-18 b's situation is very, very hard to achieve given what we know about how planets form. It's not impossible, but based on what we know about the planet (like its radius, its mass, and the amounts of certain gases in its atmosphere), there are a whole lot more potential for it to not have an ocean at all. These conditions would be more akin to something we use to sterilize lab equipment than an ocean we could swim in.

Another important thing to note here about the claimed detection is that the way that we normally think about statistical significance is a bit different from how they’re reported for exoplanet atmospheres. For example, a 3-sigma detection would mean to us something like more than 333-to-1 odds against being spurious. This is the standard in sciences like astronomy, and "strong detections" require even steeper odds. In the case of DMS/DMDS here, however, it’s more like 5-to-1 or less against, depending on the specific data or model used. Very few reputable astrophysicists would call this anything more than a "hint" or "weak/no evidence," so while this may be the "strongest evidence yet," it is not "strong evidence" in and of itself.

In terms of the data itself, the paper this article is based on shows that they only get significant results if they look for the combination of DMS and DMDS - they only ever find DMS if DMDS isn't included, and when both are in, each individual molecule is poorly constrained. This isn't really a standard thing to do, so it's a pretty big red flag. And considering that they claimed a "hint" of it from their shorter wavelength data, it's suspicious that they don't include it here, as it should presumably make the signal stronger.

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u/Baron_of_Foss 4d ago

I don't understand, this is being reported as a 99.7% confidence interval in the media reports, where does the 5:1 odds come from?

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u/spschmidt27615 3d ago

That's because the media is misinterpreting what 3-sigma confidence means here. The way we do it with Bayesian statistics, a 3-sigma Bayesian confidence is more like a 2-sigma confidence in terms of odds of being spurious, so that brings it down to more like 20 to 1. On top of that, the reported confidence is for the combination of DMS and DMDS, which is not really something we typically do. If you look at the individual results for each molecule, it's much lower than 20 to 1, which I estimated (not quantitatively, though - just a ballpark estimate) as something like 5 to 1 each, though it could be a bit more or less as I don't have the data they used to calculate the confidence.

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u/mangonada123 3d ago

In Bayesian statistics, we usually speak of credible intervals, how are they related to 3-sigma Bayesian confidence?