K2-18b

K2-18b is a sub-Neptune exoplanet about 124 light-years away, and there is no confirmed evidence of life on it. In April 2025 a Cambridge-led team reported a possible signal of dimethyl sulfide, a gas linked to life on Earth, at about 3 sigma. Independent reanalyses since then have found the signal is not statistically significant. As of October 2026 the claim is unconfirmed.

K2-18b is the most publicized test case for the search for biosignatures, the chemical traces of life described in our astrobiology guide. How the claim rose and stalled is a lesson in how science handles an extraordinary result.

K2-18b at a glance

Property	Value
Distance	About 124 light-years, in Leo
Host star	K2-18, a cool M-type red dwarf
Orbit	About 33 days, in the habitable zone
Mass	About 8.6 Earth masses (Cloutier et al. 2019)
Radius	About 2.6 Earth radii (NASA’s catalog lists 2.37)
Type	Sub-Neptune, with a hydrogen-rich atmosphere
Discovered	2015, by NASA’s K2 mission (Montet et al.)

A planet two and a half times Earth’s width with almost nine times its mass cannot be a rocky world like ours with a thin atmosphere. It is either a water-rich planet under hydrogen, a mini-Neptune with a deep gas envelope, or something in between. That ambiguity is the root of the whole debate.

The timeline, 2015 to 2026

2015 to 2019: a temperate sub-Neptune with water vapor

• August 2015. Benjamin Montet and colleagues report K2-18b from data taken by Kepler’s extended K2 mission. It receives about as much starlight as Earth (Montet et al. 2015).
• September 2019. Two teams using Hubble report water vapor in the atmosphere: Angelos Tsiaras and colleagues in Nature Astronomy, and Björn Benneke and colleagues, who also find clouds and a hydrogen envelope. Headlines call it the first habitable-zone planet with water vapor. Later JWST data show no detectable water at those altitudes, likely because it condenses deeper down in a cold trap.

2023: methane, carbon dioxide and the “hycean” idea

• September 2023. Nikku Madhusudhan and colleagues publish the first JWST spectrum, from the NIRISS and NIRSpec instruments (Madhusudhan et al. 2023). They detect methane at 5 sigma and carbon dioxide at 3 sigma, each about 1% of the atmosphere, and no ammonia. They argue this fits a hycean world, an ocean planet under hydrogen, and mention a weak, tentative hint of dimethyl sulfide (DMS).
• Early 2024. Rival models appear. Nicholas Wogan and colleagues show a gas-rich mini-Neptune about 100 times richer in heavy elements than the Sun, with no habitable surface, fits the data (Wogan et al. 2024). Oliver Shorttle and colleagues show a magma ocean could also explain the missing ammonia (Shorttle et al. 2024).

April 2025: the DMS and DMDS claim

• 16 to 17 April 2025. Madhusudhan’s team publishes new data from JWST’s mid-infrared instrument, MIRI, in The Astrophysical Journal Letters (Madhusudhan et al. 2025). They report features consistent with DMS and/or dimethyl disulfide (DMDS) at 3 sigma, with at least one of them at 10 parts per million or more, which they describe as thousands of times Earth’s levels. The University of Cambridge release calls it the strongest hints yet of biological activity beyond the solar system and suggests 16 to 24 more hours of JWST time could reach the 5 sigma threshold for a discovery.

A 3 sigma result means roughly a 1 in 370 chance that pure noise would produce a signal at least that strong, if the model is right. Physicists usually demand 5 sigma, about 1 in 3.5 million, before claiming a discovery.

2025 to 2026: the rebuttals

Within weeks, other teams reanalyzed the same public data. In order of first appearance:

1. Schmidt et al. (posted January 2025, published November 2025 in The Astronomical Journal). A full reanalysis of the near-infrared data found methane but “no statistically significant or reliable evidence” for carbon dioxide or DMS, favoring an oxygen-poor mini-Neptune (arXiv 2501.18477).
2. Taylor (April 2025, Research Notes of the AAS, May 2025). A statistical test of the MIRI data found that a flat, featureless line was preferred in five of six tests; the evidence at the DMS wavelengths amounted to about 2 sigma (arXiv 2504.15916).
3. Welbanks et al. (April 2025, Nature Astronomy, December 2025). When the set of candidate molecules and models is widened, the biosignature evidence vanishes: many other gases fit as well (arXiv 2504.21788).
4. Luque et al. (May 2025, Astronomy & Astrophysics, August 2025). A joint analysis of all JWST data from 0.6 to 12 micrometers found insufficient evidence for DMS or DMDS; ethane fits equally well. About 25 more MIRI transits would be needed (arXiv 2505.13407).
5. Hu et al. (July 2025, preprint, not yet peer reviewed). Four new NIRSpec transits, with co-authors including Madhusudhan and Benneke, confirmed methane and carbon dioxide, found no water, ammonia or carbon monoxide, and put DMS below 3 sigma, noting it could form without life (arXiv 2507.12622).
6. Stevenson et al. (August 2025, The Astronomical Journal, October 2025). Found “no statistically significant evidence” for biosignatures and showed correlated noise affects the MIRI data (arXiv 2508.05961).
7. Fernández-Rodríguez et al. (October 2025, Astronomy & Astrophysics, June 2026). The DMS hint disappears in the most complete models; the best fit is a well-mixed mini-Neptune (arXiv 2510.18098).

The original team’s response

• May 2025 to May 2026. Madhusudhan’s group, led by Luis Pica-Ciamarra, tested 661 candidate molecules and reported that only DMS consistently reached their evidence threshold across datasets, while acknowledging other gases fit some subsets comparably. The paper was published in The Astrophysical Journal Letters on 8 May 2026 (arXiv 2505.10539).

Why the claim is so hard to confirm

Three problems stack up.

• The signal is small. The features sit near the noise level of a few transits, and different data-reduction choices change the answer.
• The model menu matters. If you only compare “DMS” against “nothing,” DMS can win. If you allow dozens of other plausible gases, several fit as well.
• DMS is not a unique fingerprint. In 2024 Nora Hänni and colleagues found evidence of DMS on comet 67P, where no life exists (Hänni et al. 2024), and laboratory experiments the same year produced DMS from ultraviolet light acting on simple gases. A confirmed detection would still need a non-biological explanation ruled out.

What we can say in October 2026

Claim	Status
K2-18b has methane	Robust, confirmed by several teams
K2-18b has carbon dioxide	Likely, but some analyses find it weak
K2-18b has a hydrogen-rich atmosphere	Robust
K2-18b is a hycean ocean world	One of several models; unproven
K2-18b has DMS or DMDS	Not statistically significant in independent analyses
K2-18b has life	No evidence

The planet itself is real and fascinating: a temperate sub-Neptune, a class with no counterpart in our solar system, and one of the best-studied small exoplanets. See how it compares with rocky candidates in the most Earth-like exoplanets, and how JWST reads atmospheres in the exoplanets guide. The James Webb Space Telescope has more K2-18b observations in its archive and queue, so this page will be updated as results appear.

For a careful video walkthrough of the 2025 claim, our video library includes Dr. Becky’s “Did JWST find a marker of life in an exoplanet atmosphere?”

Sources

1. Madhusudhan, N. et al. (2023). Carbon-bearing Molecules in a Possible Hycean Atmosphere. ApJL 956, L13. https://doi.org/10.3847/2041-8213/acf577 [primary]
2. Madhusudhan, N. et al. (2025). New Constraints on DMS and DMDS in the Atmosphere of K2-18 b from JWST MIRI. ApJL 983, L40. https://doi.org/10.3847/2041-8213/adc1c8 [primary]
3. Luque, R. et al. (2025). Insufficient evidence for DMS and DMDS in the atmosphere of K2-18 b. A&A 700, A284. https://doi.org/10.1051/0004-6361/202555580 [primary]
4. Welbanks, L. et al. (2025). Nature Astronomy 10, 234. https://doi.org/10.1038/s41550-025-02730-4 [primary]
5. Stevenson, K. B. et al. (2025). The Astronomical Journal 170, 257. https://doi.org/10.3847/1538-3881/ae0338 [primary]
6. Pica-Ciamarra, L. et al. (2026). ApJL 1002, L34. https://doi.org/10.3847/2041-8213/ae5dcc [primary]
7. University of Cambridge (17 April 2025). Strongest hints yet of biological activity outside the solar system. https://www.cam.ac.uk/stories/strongest-hints-of-biological-activity [press]