K2-18 b (EPIC 201912552 b)
Coordinates, Gaia DR3 distance (38.07 pc), 32.94-day habitable-zone orbit, 8.63 M⊕ mass, 2.61 R⊕ radius, and JWST CH4/CO2 spectrum of K2-18 b.
Provenance-Backed Scientific Measurements
Every measurement below preserves its unit, uncertainty interval, observational method, reference epoch, quality status, and primary catalog source. See Uncertainty & Provenance for how competing measurements are handled.
| Parameter | Value | Uncertainty | Method & Epoch | Quality Status | Source Archive |
|---|---|---|---|---|---|
| Orbital period | 32.940045 d | ± 0.00001 d | Kepler K2, Spitzer, HST & JWST transit timing | definitive | NASA Exoplanet Archive |
| Planetary radius | 2.61 R⊕ | ± 0.087 R⊕ | Transit photometry around 0.444 R☉ M2.5 V host star | definitive | NASA Exoplanet Archive |
| Planetary mass | 8.63 M⊕ | ± 1.35 M⊕ | HARPS & CARMENES radial velocity (Benneke et al. 2019 / Cloutier et al. 2019) | high-confidence | NASA Exoplanet Archive |
| Equilibrium temperature (Bond albedo 0.3) | 255 K | ± 9 K | Derived from 1.005 ± 0.08 S⊕ Earth-equivalent insolation | model-dependent | NASA Exoplanet Archive |
| Host star distance (Gaia DR3) | 38.068 pc | ± 0.079 pc | Gaia DR3 astrometric parallax (26.27 ± 0.05 mas, 124.2 ly) (J2016.0) | definitive | ESA Gaia Archive — Data Release 3 |
J2000 Equatorial Sky Chart & Culmination Altitude
The chart below plots K2-18 b (Temperate Sub-Neptune in Leo) at RA 11h 30m 14.52s, Dec +07° 35′ 18.2″ within Leo, alongside peak meridian transit altitudes for 40°N and 30°S observers. Use the RA/Dec Coordinate Converter to compute exact culmination altitude for your latitude.
Astrophysical Context & Structure
K2-18 b (EPIC 201912552 b) is a temperate sub-Neptune exoplanet orbiting the M2.5 V red dwarf star K2-18, located 38.07 ± 0.08 parsecs (124.2 light-years) away in Leo. Discovered in 2015 during Campaign 1 of NASA's extended Kepler K2 mission, the planet completes one orbit every 32.94 days at a semi-major axis of 0.1591 AU, receiving almost exactly the same total stellar irradiance from its 3,457 K red-dwarf host as Earth receives from the Sun (1.005 ± 0.08 S⊕).
With a mass of 8.63 ± 1.35 Earth masses and a radius of 2.61 ± 0.09 Earth radii, K2-18 b belongs to the most abundant class of planets in the galaxy—sub-Neptunes between the sizes of Earth and Neptune—a planetary class completely absent from our own Solar System. When Hubble Space Telescope WFC3 spectra in 2019 detected a strong 1.4-micron absorption feature initially attributed to water vapor, K2-18 b became a flagship target for the James Webb Space Telescope.
In 2023 JWST's NIRISS and NIRSpec spectrographs observed K2-18 b across 0.9 to 5.2 microns and revealed that the 1.4-micron feature is actually dominated by abundant methane (CH4, ~1%) alongside carbon dioxide (CO2, ~1%) in a hydrogen-rich atmosphere, with very little ammonia (NH3). Whether this chemical signature reflects a liquid water ocean beneath a thin hydrogen atmosphere ('Hycean world') or thermochemical equilibrium inside a deep mini-Neptune gas envelope without a solid or liquid surface remains a major frontier of planetary physics.
Practical Visual Observing Guide
- The host star K2-18 (BD+08 2518) shines at visual magnitude V = 13.50 (and bright near-infrared magnitude J = 9.76) in southern Leo, about 8° south-southwest of Denebola.
- Visual observers with a 200–250 mm (8–10 inch) telescope under dark skies can spot the 13.5-magnitude orange-red M-dwarf host star at 150×–200×.
Catalog Cross-Identifications & Aliases
Common Aliases: EPIC 201912552 b · BD+08 2518 b
| Catalog Authority | Identifier |
|---|---|
| NASAEXOPLANET | K2-18 b |
| SIMBAD | K2-18 b |
| WIKIDATA | Q22906035 |
Related Objects, Comparisons & Guides
TRAPPIST-1 e (Temperate Terrestrial World)
Coordinates, Gaia distance (12.47 pc), 6.10-day orbital period, mass 0.692 M⊕, radius 0.920 R⊕, and JWST transit spectroscopy for TRAPPIST-1 e.
WASP-39 b (JWST Benchmark Saturn Exoplanet)
Coordinates, Gaia DR3 distance (213 pc), 4.055-day period, 0.28 M♃ mass, 1.27 R♃ radius, and JWST CO2/SO2 photochemical spectrum of WASP-39 b.
HD 209458 b (Osiris First Transiting Planet)
Coordinates, Gaia DR3 distance (48.3 pc), 3.5247-day transit period, 1.359 R♃ inflated radius, and atmospheric escape data for HD 209458 b.
Proxima Centauri b (Nearest Exoplanet)
Coordinates, ESPRESSO minimum mass (1.07 M⊕), 11.186-day habitable-zone orbit at 0.04856 AU, and stellar flare environment of Proxima Centauri b.
Measurement Uncertainty & Optics in Astronomy
How random vs systematic errors, asymmetric error bars, model dependencies, and optical aperture/exit-pupil limits shape astronomical measurements.
Trigonometric Parallax, Parsecs & Gaia DR3
How trigonometric parallax (d_pc = 1 / p_arcsec), the distance modulus equation, Hipparcos, and Gaia DR3 microarcsecond astrometry measure cosmic distances.
Confirmed Exoplanets & Host Systems Catalog
Curated NASA Exoplanet Archive reference catalog of benchmark exoplanets: Proxima Centauri b, TRAPPIST-1 e, 51 Pegasi b, HD 209458 b, K2-18 b, and WASP-39 b.
Scientific Sources & Dataset Provenance
- NASA Exoplanet Archive (2026-10-01 snapshot): Akeson, R. L., et al. 2013, PASP, 125, 989; NASA Exoplanet Archive (IPAC/Caltech) — NASA Exoplanet Science Institute (NExScI / Caltech / IPAC)
- JWST / MAST Science Archive & Mission Documentation (Cycle 1–4 Public Releases (2022–2026)): Rigby, J., et al. 2023, PASP, 135, 048001 (The Science Performance of JWST as Characterized in Commissioning) — NASA / ESA / CSA / STScI
- ESA Gaia Archive — Data Release 3 (DR3 (2022-06-13)): Gaia Collaboration, Vallenari, A., et al. 2023, A&A, 674, A1 (Gaia Data Release 3) — European Space Agency / Gaia DPAC