A Cool Ocean World Beyond Earth? JWST Reveals K2-18 b’s Watery Interior

Since the discovery of planets outside our Solar System, astronomers have wondered which of them might have oceans and potentially support life. While Earth-sized planets are often the focus of this search, astronomers have also found many larger, volatile-rich worlds, called sub-Neptunes, which may harbor water in ways very different from Earth. The planet K2-18 b is one such world: about 8.6 times Earth’s mass and 2.6 times its radius, orbiting a cool star. Earlier observations suggested K2-18 b might have an atmosphere dominated by hydrogen (H₂) with water vapor, methane (CH₄), and perhaps signs of biological activity. However, those findings were uncertain. In this paper, Renyu Hu and collaborators present new, high-precision observations from the James Webb Space Telescope (JWST), to explore whether K2-18 b has a water-rich interior and whether its atmosphere could support a liquid-water ocean.

Observations and Data Reduction

To investigate K2-18 b’s atmosphere, the team used JWST to observe the planet as it passed in front of its star, a method called transit spectroscopy. They observed four new transits with JWST’s NIRSpec instrument, covering wavelengths from 1.7 to 5.2 micrometers, and combined these with earlier JWST and Hubble data. The observations spanned more than a year and used different instrument modes to collect the most complete spectrum possible. The data were carefully processed to remove noise, correct for telescope and detector effects, and isolate the tiny dip in starlight caused by the planet’s atmosphere absorbing light at certain wavelengths.

Atmospheric Models

With these data in hand, the researchers used several sophisticated models to figure out which gases are present in K2-18 b’s atmosphere. They compared the observed spectrum to models that included different gases, like methane, carbon dioxide (CO₂), water vapor (H₂O), ammonia (NH₃), and more, and calculated how likely each model was to explain the data. They also ran “self-consistent” models that accounted for the physics and chemistry of the atmosphere and how it relates to the planet’s interior structure.

Results: What’s in the Atmosphere?

The team found strong evidence for methane and carbon dioxide in K2-18 b’s atmosphere. Surprisingly, they detected no water vapor, ammonia, or carbon monoxide (CO). The absence of water vapor suggests that water might be trapped lower in the atmosphere, a phenomenon called a cold trap, or hidden beneath a thin hydrogen-rich layer. The detection of CH₄ and CO₂, alongside the lack of NH₃ and CO, supports the idea that K2-18 b has a relatively small hydrogen atmosphere overlying a liquid water reservoir, rather than a thick, gas-dominated envelope. Signals of potential biological gases, dimethyl sulfide (DMS), methyl mercaptan (CH₃SH), and nitrous oxide (N₂O), were weak and statistically insignificant.

Discussion: Oceans and Habitability?

The findings point to a water-rich interior, but the nature of the water remains uncertain. One possibility is a massive envelope with at least 10% water mixed in, while another is a thin hydrogen atmosphere above a liquid-water ocean. The researchers note that maintaining such an ocean would require fine-tuned conditions, such as a high reflectivity (albedo) and a delicate balance of atmospheric thickness and composition. They also caution that alternative explanations, like a steam-rich envelope or high-metallicity atmosphere, are possible.

Roadmap for Future Observations

The authors outline how future observations can test these scenarios. For example, detecting certain ratios of CO₂ to CO, or looking for other gases predicted by different interior models, could distinguish between a deep ocean and a thick steam envelope. More data, especially at longer wavelengths and with higher sensitivity, will be essential to confirm the presence of water in the interior and to better assess the planet’s habitability.

Conclusion

Renyu Hu and colleagues conclude that K2-18 b is one of the most promising sub-Neptunes for studying water-rich environments beyond Earth. The planet’s atmosphere and interior appear to be consistent with a world that could host liquid water, though not necessarily in a way familiar to us. This work highlights both the power of JWST to probe distant worlds and the complexity of interpreting the clues we find.

Source: Hu