K2-18 b

Planet· orbits K2-18

K2-18 b — a transiting sub-Neptune (~8.6 Earth masses, 2.6 Earth radii) in its star's habitable zone, orbiting every 33 days. Because it transits AND its mass is measured, its density is real — consistent with a small, volatile-rich world, possibly with a deep water layer under a hydrogen sky. It is the focus of a live debate over whether its air carries a fingerprint of life.

Open in 3D viewSee K2-18 b in the live, interactive 3D map

Key facts

Mass

5.15 × 10²⁵ kg

8.63 M⊕

Radius

16,628 km

2.61× wider than Earth

Surface gravity

12.4 m/s²

1.27× Earth's gravity

Year

32.94 Earth days

Distance from K2-18

0.143 AU

Science

Physical

Mass: 5.15 × 10²⁵ kg · 8.63 M⊕
Radius: 16,628 km · 2.61× wider than Earth
Density: 2,676 kg/m³ · 2.7× water; Computed from mass and radius
Surface gravity: 12.4 m/s² · 1.27× Earth's gravity; Computed from mass and radius
Escape velocity: 20.3 km/s · 1.82× Earth's escape velocity; Computed from mass and radius

Orbital

Semi-major axis: 0.143 AU; From the NASA Exoplanet Archive (transit + orbit fit)
Eccentricity: 0; From the NASA Exoplanet Archive (transit + orbit fit)
Inclination: 0.00 °; From the NASA Exoplanet Archive (transit + orbit fit)
Orbital period: 32.9 days; From the NASA Exoplanet Archive (transit + orbit fit)
Perihelion: 0.143 AU; Computed from the orbital elements
Aphelion: 0.143 AU; Computed from the orbital elements
Mean orbital velocity: 47.2 km/s; Computed from the orbital elements

Composition

Bulk composition: A sub-Neptune of ~8.6 Earth masses and 2.6 Earth radii; because it transits AND its mass is measured, its density is real — a small, volatile-rich world, possibly with a deep water layer under a hydrogen sky (a 'hycean' candidate). JWST detected methane and CO₂ in 2023. A later claim of dimethyl sulphide — a possible biosignature — remains disputed: the discovery team reports it at about 3σ, but several independent re-analyses of the same JWST data have not confirmed it, and non-biological chemistry can also produce DMS. Never imaged.
Cloud appearance: Appearance based on NASA/JPL-Caltech's artist's concept. No spacecraft has imaged this world, so its real appearance is unknown — The banded clouds shown are a representation of its class, shown only in Visual mode; Realism shows a plain spectral colour.

Radiation & temperature

Insolation: 1.06 S⊕ · 1.06× the sunlight Earth receives; Computed from K2-18's luminosity and the orbital distance (inverse-square; Earth's sunlight = 1).
Equilibrium temperature: 284 K (11 °C); Computed model value for its 33-day habitable-zone orbit — not measured.

Discovery & history

Method: Transit detection
Name: Designated 'b' under the IAU convention that letters a star's planets 'b', 'c', 'd'... in the order they are found (the star is the unwritten 'a'); no IAU proper name has been assigned.

Found transiting by K2 in 2015. Hubble detected water vapour in 2019 — a first for a habitable-zone planet — and JWST reported methane and CO₂ in 2023. A later claim of dimethyl sulphide — a possible biosignature — remains disputed: the discovery team reports it at about 3σ, but several independent re-analyses of the same JWST data have not confirmed it. Nothing about life is confirmed.

Images

Artist's impression of the habitable-zone sub-Neptune K2-18 b, its star and a sibling planet — an artist's concept, not a photograph. — NASA, ESA, CSA, and Joseph Olmsted (STScI) / M. Kornmesser (public domain) · Wikimedia Commons

Illustration of the sub-Neptune K2-18 b built from the science data — an artist's concept, not a photograph. — NASA, CSA, ESA, J. Olmsted (STScI) (public domain) · Wikimedia Commons

Images from open archives · credit shown per image