Planet K2-18b may not be habitable after all – Sky and Telescopes

Exoplanet K2-18b made headlines when researchers reported that JWST's 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 supposed water world is instead a gas-rich planet with no habitable surface.

Everyone wants to govern and find a habitable world

The small, cool star K2-18 is home to two planets, one of which has attracted a lot of attention in the decade since its discovery. Recently, JWST data from K2-18b, an 8.6 Earth mass planet, revealed the presence of carbon dioxide and methane in the atmosphere. Some researchers have interpreted this data, along 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 things 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 viewers to the surprising conclusion that K2-18b is not only habitable but inhabited.

However, these intriguing interpretations are far from resolved. Is K2-18b really a habitable ocean world, or could alternative explanations fit the JWST data equally well?

World of water or planet of gas?

A team led by Nicholas Wogan (NASA Ames Research Center and University of Washington) addressed this question by applying two sets of models to the JWST data. The first set describes rocky planets with oceans on the surface, 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 would not contain enough methane in its atmosphere to produce the signal seen in JWST observations. Interestingly, an aquatic world with microbial life is more promising: acetotrophic methanotrophs (a twisted name for simple methane-producing organisms) may be able to produce the supply of methane seen in the planet's atmosphere.

Not so fast…

As exciting as it may sound, Wogan and his collaborators found that the gas-rich uninhabitable 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, but it is also difficult to reconcile the necessary cold surface temperature with the high probability that the planet will experience a runaway greenhouse effect.

This is not the last word on K2-18b: there are features in the planet's spectrum that do not fit well with an animated ocean world or a lifeless, gas-rich planet, and both models have their challenges. Future JWST data could uncover a detection of ammonia, which would point to a gaseous planet, or dimethyl sulfide, which would tip the balance considerably toward an inhabited aquatic world. Meanwhile, the search for habitable planets continues.

Citation

“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 Newfeaturing notable research from the journals of the American Astronomical Society.