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K2-18b

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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 study reached formal publication in July 2026, adding the COSMIC/BLUSE pipeline names and an Arecibo-comparable transmitter-power upper bound (src-k2-18b-technosignature-scan-2026-07).

  • 2025 (A&A, DMS/DMDS rebuttal) — rafael-luque et al. published the first panchromatic (0.6–12 µm) joint analysis combining NIRISS, NIRSpec, and MIRI using three independent pipelines. Found insufficient statistical evidence for DMS/DMDS; ethane (C₂H₆) fits equally well. Formally rebuts the Madhusudhan biosignature claim (src-k2-18b-dms-insufficient-evidence-2025).

The pairing of formally rebutted 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).

Photochemical modelling (Fujisawa et al., May 2026)

A 1D photochemical + radiative-convective equilibrium model (Fujisawa et al., Astrobiology, May 2026) shows that abiotic H₂-CH₄-H₂O atmospheres reproduce the JWST transmission spectrum of K2-18b without requiring DMS. CO/CO₂ constraints cannot yet discriminate between hycean and abiotic interpretations. The study adds weight to the non-biological reading while leaving the hycean world hypothesis alive (src-k2-18b-hycean-photochemistry-2026-05).

Lunine & Bahcall four-criterion evaluation (PNAS, 2025)

A PNAS special-feature survey by Lunine (JPL/Caltech) and Bahcall (Princeton) (src-pnas-jwst-exoplanet-characterization-era-2025) applies a rigorous four-criterion evidentiary framework to assess biosignature claims: (1) strong signal, (2) unequivocal molecular identification, (3) consistency with planetary conditions, (4) no plausible abiotic alternative. Applied to the K2-18b DMS claim, the survey finds it fails every test. The survey does accept K2-18b as a viable hycean world candidate, distinguishing between the atmospheric structure interpretation (credible) and the specific DMS biosignature claim (not credible). Lunine & Bahcall identify sub-Neptunes as “the most abundant exoplanets and the most enigmatic,” with no solar-system equivalent to compare against.

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