K2-18b is a sub-Neptune exoplanet approximately 124 light-years from Earth, orbiting the red dwarf K2-18 in the star’s habitable zone. It is approximately 8.6 Earth masses and 2.6 Earth diameters, with a 33-day orbit.
K2-18b is the leading candidate for the proposed hycean world class — a rocky or mini-Neptune-like planet with a thick hydrogen atmosphere overlying a liquid ocean — theorized by nikku-madhusudhan.
Observational status
- 2023-2025 (JWST, biosignature claim) — jwst transmission spectroscopy detected methane, CO₂, and the controversial dimethyl sulfide (DMS) / dimethyl disulfide (DMDS) signal, interpreted by Madhusudhan’s team as a potential biosignature (src-jwst-k2-18b-biosignature-2025). The interpretation is heavily disputed within the astrobiology community.
- 2023-2026 (VLA + MeerKAT, technosignature null) — A 33-day multi-telescope radio search across 544 MHz to 9.8 GHz found no technosignatures (src-k2-18b-technosignature-null-2026).
The pairing of disputed biosignature and technosignature null makes K2-18b the most methodologically important single exoplanet in the current astrobiology discourse. sara-seager et al. (2025) treat K2-18b as the canonical case study for the transmission-spectrum “parallel interpretations” problem — the same data supports multiple atmospheric models, and JWST cannot arbitrate (src-jwst-biosignature-prospects-2025).
Comparative Context: TOI-1130b (2026)
A contemporaneous jwst mini-Neptune characterization — toi-1130b (Barat et al., 2026) — provides methodological context for the K2-18b “parallel interpretations” problem. TOI-1130b’s high-mean-molecular-weight-atmosphere (μ = 5.5 amu) and high metallicity argue for ex-situ formation beyond the water-ice-line, illustrating one of the non-hycean retrievals in tension with the K2-18b hycean reading (src-jwst-toi-1130b-atmosphere-2026-05).