Super-Earth · K2-18

K2-18 b

ExoplanetsHabitable-zone candidateDiscovered 2015

Scientific overview

K2-18 b is a super-earth orbiting K2-18, completing an orbit every 33 days, about 124 light-years from Earth. It was discovered in 2015 using the transit method.

Host star

K2-18

Spectral type

M2.5 V

Temperature

3,457 K

Mass

0.359 M☉

Planetary system — K2-18 system

View the K2-18 system (2 known planets).

Planet	Class	Radius (R⊕)	Period (d)	Distance (ly)
K2-18 cRadial Velocity · 2017	Mini-Neptune	2.65	8.962	124
K2-18 bTransit · 2015	Super-Earth	2.37	32.9396	124

Discovery and detection

Detection method

Transit Method

Discovery year

2015

Discovered at

Kepler Space Telescope

Habitability notes

K2-18 b lies in the habitable zone of its star, the range of orbits where liquid water could in principle exist on a planet's surface. This makes it a candidate of interest for habitability studies — but lying in the habitable zone is not evidence that the planet is habitable or inhabited; its atmosphere and surface conditions remain unknown.

Knowledge connections

OrbitsK2-18
Member of systemK2-18 system
Discovered by methodTransit Method
Discovered by missionKepler Space Telescope
Catalogued inNASA Exoplanet Archive
Habitable-zone candidateHabitable zone

Quick facts

Class: Super-Earth
Host star: K2-18
Discovery method: Transit Method
Discovery year: 2015
Orbital period: 32.9396 d
Semi-major axis: 0.1429 AU
Eccentricity: 0.2
Radius: 2.37 R⊕
Mass: 8.92 M⊕
Density: 3.69 g/cm³ · derived
Equilibrium temp.: 284 K
Insolation: 1 S⊕
Distance: 124 ly

Density is derived from the archive mass and radius via ρ = M / (4⁄3·π·r³); all other values are catalogued measurements.

Derived values

Computed from source-backed inputs via standard formulae — not observations. Every input and constant is traced below.

Bulk density3.69 g/cm³derived
Formula & inputs
Formula: ρ = M / (4⁄3·π·r³) (v1.0.0, impl v1.0.0)
Inputs:
- massEarth = 8.92 M⊕ · NASA
- radiusEarth = 2.37 R⊕ · NASA
Assumptions: M⊕ = 5.972e+24 kg, R⊕ = 6371000 m (IAU 2015 B3 nominal); Assumes a uniform sphere.
Limitations: Withheld above 30 g/cm³ (inconsistent mass/radius inputs).; Archive mass/radius carry no propagated uncertainty here.
Uncertainty: not propagated (inputs carry no catalogued uncertainty)
Computed as of 2026-06-29.

Provenance & sources

review 2026.1

Asteria Scientific Review Process · verification: sourced · accurate

Moderate evidencelikely
K2-18 b is a sub-Neptune exoplanet orbiting in the habitable zone of a red dwarf. Its confirmed status and orbit are well established; molecules have been reported in its atmosphere, but claimed biosignatures remain debated and unconfirmed.
Source: NASA Exoplanet Science Institute (Caltech/IPAC) — NASA Exoplanet Archive · Freely available; acknowledgement requested
- NASA Exoplanet Archive, NASA Exoplanet Archive
Limitation: Existence and orbit are confirmed; biosignature interpretations are contested and are not asserted here.

Reviewed by the internal Asteria Scientific Review Process — not an external institutional review. See the evidence framework and authority dashboard.

Scientific citations

Real, source-backed references — primary papers first, then datasets and institutional sources. Formatted through the citation engine; nothing is fabricated.

Archive pageK2-18 b — NASA Exoplanet Archive

NASA Exoplanet Science Institute (Caltech/IPAC)

Cite (APA / BibTeX)

NASA Exoplanet Science Institute (Caltech/IPAC) (n.d.). K2-18 b — NASA Exoplanet Archive. NASA Exoplanet Science Institute (Caltech/IPAC). https://exoplanetarchive.ipac.caltech.edu/

@misc{cite:exoarchive-exoplanet-k2-18-b,
  title        = {K2-18 b — NASA Exoplanet Archive},
  organization = {NASA Exoplanet Science Institute (Caltech/IPAC)},
  year         = {n.d.},
  url          = {https://exoplanetarchive.ipac.caltech.edu/},
  note         = {Confirmed parameters and discovery references for K2-18 b.}
}

Press releaseNASA — Webb studies of K2-18 b

NASA

Cite (APA / BibTeX)

NASA (n.d.). NASA — Webb studies of K2-18 b. NASA. https://science.nasa.gov/

@misc{cite:nasa-k2-18b,
  title        = {NASA — Webb studies of K2-18 b},
  organization = {NASA},
  year         = {n.d.},
  url          = {https://science.nasa.gov/},
  note         = {Atmospheric studies; biosignature claims remain contested and are not asserted.}
}

Quality & authority

Substantial

Reviewed

Description: Complete
Source coverage: Partial
Relationship coverage: Complete
Review coverage: Complete
Citation coverage: Complete
Image coverage: Not applicable

Data from the NASA Exoplanet Archive. See source quality.