New research suggests Exoplanet K2-18b may actually be a gas-rich planet with no habitable surface instead of a habitable water world.

cartoon showing different types of exoplanets
Cartoon showing a variety of exoplanet types. Figuring out whether a planet is rocky or gaseous can be a challenge, as is the case for K2-18b.
NASA / JPL-Caltech / Lizbeth B. De La Torre

Exoplanet K2-18b made headlines when researchers reported that JWST observations of the planet were consistent with a habitable ocean world. Now, another team has published a different interpretation of the data, suggesting that the purported water world is instead a gas-rich planet with no habitable surface.

Everybody Wants to Rule the Find a Habitable World

An artist's impression of K2-18b as an ocean world
An artist’s impression of K2-18b as an ocean world.
NASA / ESA, CSA / Joseph Olmsted (STScI)

The small, cool star K2-18 is home to two planets, one of which has garnered plenty of attention in the decade since its discovery. Recently, JWST data of K2-18b, an 8.6-Earth-mass planet, revealed the presence of atmospheric carbon dioxide and methane. Some researchers have interpreted these data, coupled with the non-detection of ammonia, water, and carbon monoxide, to mean that K2-18b is a Hycean world: a rocky planet covered in oceans.

To make matters more interesting, the same research team reported weak evidence for dimethyl sulfide, a compound that on Earth forms almost exclusively due to life. This led many onlookers to the eyebrow-raising conclusion that K2-18b is not just habitable but inhabited.

These intriguing interpretations, however, are far from settled. Is K2-18b truly a habitable ocean world, or could alternative explanations fit the JWST data equally well?

Water World or Gas Planet?

simulation of the atmosphere of K2-18b as a gas-rich planet without a habitable surface
Example simulation output for K2-18b as a gas-rich planet without a habitable surface. Click to enlarge.
Wogan et al. 2024

A team led by Nicholas Wogan (NASA Ames Research Center and University of Washington) tackled this question by applying two sets of models to the JWST data. The first set describes rocky planets with surface oceans, with and without life, and the second set describes gaseous planets without a surface and without life. The models predict the planet’s photochemistry — chemical reactions in the atmosphere driven by photons from the host star — and climate.

Wogan’s team found that K2-18b is unlikely to be a lifeless water world, since this type of planet wouldn’t contain enough methane in its atmosphere to produce the signal seen in the JWST observations. Intriguingly, a water world with microbial life is more promising: acetotrophic methanotrophs — a tongue-twisting name for simple methane-producing organisms — may be able to produce the supply of methane seen in the planet’s atmosphere.

Not So Fast…

simulated spectra of K2-18b as an ocean world with and without life and as a gas-rich planet
JWST transmission spectra (black and gray points with error bars) and modeled spectra for K2-18b as a lifeless ocean world (top left), an ocean world with life (bottom left), and a lifeless gas-rich planet (bottom right). Click to enlarge.
Wogan et al. 2024

As exciting as this sounds, Wogan and collaborators found that the uninhabitable gas-rich exoplanet model fits the JWST data equally well, and this model may pose fewer problems. Not only does the ocean-world model require life to explain its atmospheric composition, it’s also hard to reconcile the necessary cool surface temperature with the high likelihood of the planet experiencing a runaway greenhouse effect.

This isn’t the last word on K2-18b — there are features in the planet’s spectrum that aren’t well fit by a lively ocean world or a lifeless gas-rich planet, and both models have their challenges. Future data from JWST might dredge up a detection of ammonia, which would point to a gaseous planet, or dimethyl sulfide, which would tilt the scales considerably toward an inhabited water world. In the meantime, the hunt for habitable planets goes on.


“JWST Observations of K2-18b Can Be Explained by a Gas-Rich Mini-Neptune with No Habitable Surface,” Nicholas F. Wogan et al 2024 ApJL 963 L7. doi:10.3847/2041-8213/ad2616

This post originally appeared on AAS Nova, which features research highlights from the journals of the American Astronomical Society.


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April 12, 2024 at 6:48 pm

I hate to sound SO sceptical, but this is another example of NON-chemists trying to fit modelling using purely thermodynamical data and transport modelling without considering other factors.

We see this all the time these days - if climatic science is so good, and models worthy of consideration, we should see WAY better fits to our own models on this planet, where data is a-plenty!

There are several issues I see in these Hycean scenarios, and many models can fit the data as presented: First, in photochemical modelling, a deep red star like K2-18 has VERY low UV present - so this would have to be flare-event created, or cosmic ray/Xray driven, or why have these folks not even considered certain lightning events that COULD occur at times?

If the latter are ignored, then what OTHER moieties are detectable that could permit some shift in equilibria? We have no data other than upper atmospheric information - and VERY little of that yet. So why all the speculation... to justify a research grant? Science is supposed to be an information gathering process, not a pseudo-arbitrary computational modelling exercise. We are beginning to see this more and more, and it concerns me - not just in planetary science studies, but cosmology too!

A model is only as good as the data you put in - less data means a likely lower chance of fit to the observables.

So I think that we need more physical chemist specialists in these mixes and fewer climate and astrophysics types - I wish they'd stay in their lane! Playing chemist is bad science, IMO.

DH - Edmonton Canada
(and I AM a physical chemist BTW!)

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Andrew James

April 15, 2024 at 5:58 am

I did chemistry too. Just a few questions. Could more UV radiation have existed when the star was formed? Can lightning exist in a mostly hydrogen atmosphere? Theories and ideas exist so that they can be tested, and if necessary, destroyed. There are so many unknowns, and other variables, that the science is still decades ahead to resolving these issues. There is nothing wrong in the hypothesis, because the hypothesis drives further experimentation and evidence.
Astronomy is tough because the most of the data is so difficult to get. We are taking a picture of this star and exoplanet, yet the naysayers persistently say they want a movie. Chemistry deal in terms of 10^26 molecules or atoms. Astronomers have to find evidence from ridiculous distances, and determine the chemical behaviour from spectra gathered in minute amount of data. That's the point.

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April 12, 2024 at 9:59 pm

I re-read the paper by Wogen et al in more detail... His model assumes WAY too much - maybe a general case for Hygean and mini-Neptunian worlds, but totally inapplicable IMO, to this planet. He talk of water vapour undergoing photolysis at the Lyman alpha wavelength of 124nm - When I run a blackbody emulation of a 2950K star, this is at least 10^-12 lower flux than Sol at that wavelength! So, unless flaring events and such are taken into account, photolytic reactions are rare and likely not to occur - do these teams ever consider flux form stars in their model, because I see little of this mentioned in this paper!

So - I think a tea leaf reading would have as much predictive power as these models they are considering. Lots of talk on transport and such, but little on the material (photolysis side reactions, etc.).

I hate to say this, but if this were a Ph.D.paper, I'd have failed this fellow. sorry, Mr. Wogen!

Edmonton, AB Canada

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Andrew James

April 15, 2024 at 5:35 am

A comment. Saying:"...I think a tea leaf reading would have as much predictive power as these models they are considering." Tea leaf reading from 120 light years is a pretty amazing fact in itself.

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Andrew James

April 15, 2024 at 4:37 am

On the NASA website dated 11 September 2023, Savvas Constantinou of the University of Cambridge said about these Webb results: “These results are the product of just two observations of K2-18 b, with many more on the way,… this means our work here is but an early demonstration of what Webb can observe in habitable-zone exoplanets.”

Both points stated above I think are premature criticisms, because much of this as a science is in its infancy. I think it will take many observations of many systems before we can get a consensus about the detect ability of planets that can possibly have life. When you have a database of one, that is the Earth's atmosphere, of course the science is going to be not precise. The bigger question is: What kind of spectrum will positively identify a water based world and that life could possibly exist there? Do we need an even bigger telescope to achieve this?

This kind of analysis can be seen under this astronomer's link. You might like to read the papers here.

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