K2-18 b

K2-18 b

Planet· orbits the Sun

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
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
Surface gravity
12.4 m/s² · 1.27× Earth's
Escape velocity
20.3 km/s · 1.82× Earth's

Orbital

Semi-major axis
0.143 AU
Eccentricity
0
Inclination
0.00 °
Orbital period
32.9 days
Perihelion
0.143 AU
Aphelion
0.143 AU
Mean orbital velocity
47.2 km/s

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 has detected methane and CO₂; a hint of dimethyl sulphide is contested. 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 Vision mode; Realism shows a plain spectral colour.

Radiation & temperature

Insolation
1.06 S⊕ · 1.06× the sunlight Earth receives
Equilibrium temperature
284 K (11 °C)

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 (2023–) reported methane and CO₂ plus a contested, much-debated hint of dimethyl sulphide. 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