K2-18b (2023): Methane and CO2 on a Possible Ocean World

Nikku Madhusudhan’s team reported in 2023 that the James Webb Space Telescope detected methane and carbon dioxide on K2-18 b, a planet in its star’s habitable zone. They proposed an ocean world under a hydrogen-rich sky and a tentative hint of dimethyl sulfide, claims that remain disputed.

The question

K2-18 b is a sub-Neptune: bigger than Earth, smaller than Neptune, a type of planet our Solar System lacks. It orbits within its star’s habitable zone, where liquid water is possible in principle. Earlier observations had left its atmosphere ambiguous. Webb could show what the air is made of, and whether that chemistry fits an ocean world or something else.

What the paper found

• Methane, clearly. Methane was detected at 5 sigma, a level physicists usually treat as a firm detection. Sigma measures how unlikely a signal is to be noise.
• Carbon dioxide, more weakly. CO2 was detected at 3 sigma.
• About 1% each. Both gases make up roughly 1% of an atmosphere dominated by hydrogen.
• No ammonia. Ammonia was not detected, which the authors argue is consistent with a Hycean world, a temperate planet with a liquid water ocean under a hydrogen-rich atmosphere.
• A faint, tentative hint of DMS. The spectrum also suggested possible signs of dimethyl sulfide (DMS), a molecule that on Earth is made by life. The authors described it as low in statistical significance.

How they did it

The team used transmission spectroscopy with two of Webb’s instruments, NIRISS and NIRSpec, covering wavelengths from 0.9 to 5.2 micrometers. When the planet passes in front of its star, some starlight filters through its atmosphere. Each gas absorbs specific colors, so comparing starlight during and outside the transit reveals which gases are present. The same method had given Webb’s 2022 detection of carbon dioxide on WASP-39b.

Why it mattered

The methane detection was a real advance: it resolved a long-standing puzzle of “missing methane” in temperate exoplanets and removed ambiguity left by earlier data on K2-18 b. The interpretation is where the dispute began. The Hycean picture and the DMS hint are contested. Other teams argue the data are also consistent with a gas-rich mini-Neptune without a habitable surface.

K2-18 b became the most publicized test case for biosignature searches with the James Webb Space Telescope, and the debate is shaping the standards the field will use before claiming signs of life on an exoplanet. It also shows the limits of the habitable zone idea of Kasting and colleagues: being in the zone says nothing, by itself, about whether a planet has a surface or an ocean.

What we know now

In April 2025 the same group reported mid-infrared data from Webb’s MIRI instrument and claimed new evidence for DMS and/or a related molecule, dimethyl disulfide, at 3 sigma, while stating that more observations are needed. Independent reanalyses published in 2025 by Taylor, by Welbanks and colleagues, and by Luque and colleagues found no strong or statistically significant evidence for either molecule. As of October 2026 the biosignature claims remain disputed and unconfirmed. Our explainer on K2-18b follows the debate in more depth, within the wider field of astrobiology.

Who wrote it

The six authors were led by Nikku Madhusudhan. The paper argues that Hycean worlds, larger than rocky planets of the same mass and wrapped in more extended atmospheres, are far easier for Webb to read. That is why a sub-Neptune, not an Earth twin, ended up at the center of this debate.

Read the original

Madhusudhan, N., Sarkar, S., Constantinou, S., Holmberg, M., Piette, A. A. A. and Moses, J. I. (2023). “Carbon-bearing Molecules in a Possible Hycean Atmosphere.” The Astrophysical Journal Letters 956, L13. doi:10.3847/2041-8213/acf577

Free to read: arXiv preprint. The 2025 follow-ups are at doi:10.3847/2041-8213/adc1c8 (MIRI), doi:10.3847/2515-5172/add881 (Taylor), doi:10.1038/s41550-025-02730-4 (Welbanks et al.) and doi:10.1051/0004-6361/202555580 (Luque et al.).