K2-18 b vs WASP-39 b JWST Exoplanets
Compare JWST transmission spectroscopy benchmarks K2-18 b (habitable-zone sub-Neptune) and WASP-39 b (warm puffed Saturn) in mass, radius, and gases.
Compared Celestial Objects
K2-18 b (Temperate Sub-Neptune in Leo)
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.
Constellation: Leo · Coords: 11h 30m 14.52s, +07° 35′ 18.2″
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.
Constellation: Virgo · Coords: 14h 29m 18.42s, -03° 26′ 40.2″
Normalized Shared Parameter Table
| Parameter | Unit / Scale | K2-18 b (Temperate Sub-Neptune in Leo) | WASP-39 b (JWST Benchmark Saturn Exoplanet) |
|---|---|---|---|
| Gaia DR3 Heliocentric Distance | pc | 38.07 ± 0.08 pc (124.2 ly) | 213.3 ± 1.5 pc (696 ly) |
| Planetary Mass | M⊕ / MJup | 8.63 ± 1.35 M⊕ (0.027 M_Jup) | 89.0 ± 9.5 M⊕ (0.280 M_Jup) |
| Planetary Radius | R⊕ / RJup | 2.610 ± 0.087 R⊕ (0.233 R_Jup) | 14.2 ± 0.4 R⊕ (1.27 R_Jup) |
| Orbital Period & Equilibrium Temp | days / K | 32.9396 d (Teq = 255 ± 15 K) | 4.05526 d (Teq = 1,166 ± 14 K) |
| Confirmed JWST Atmospheric Molecules | species | CH₄, CO₂ (H₂-rich envelope) | CO₂, SO₂, H₂O, CO, Na |
Astrophysical & Observational Synthesis
WASP-39 b is an extraordinarily puffed-up Saturn-mass planet (0.28 M_Jup, 89 M⊕) swollen to 1.27 Jupiter radii in a scorching 4.06-day, 1,166 K orbit around a G8 V star in Virgo. Because atmospheric scale height H = kT / (μg) is inversely proportional to surface gravity and mean molecular weight, WASP-39 b's low surface gravity (~4.1 m/s²) and high temperature inflate its hydrogen atmosphere across thousands of kilometers—making it the ideal target where JWST achieved the first unambiguous exoplanet detections of carbon dioxide (CO₂ at 4.3 μm) and photochemically produced sulfur dioxide (SO₂ at 4.05 μm).
K2-18 b, orbiting an M2.5 V red dwarf in Leo every 32.94 days at 38.07 parsecs, occupies the completely different sub-Neptune regime (8.63 M⊕, 2.61 R⊕, bulk density ~2.67 g/cm³)—a class of planet absent from our Solar System. Receiving almost the same stellar insolation as Earth (Teq ≈ 255 K), K2-18 b sits inside its star's habitable zone.
JWST NIRISS and NIRSpec transmission spectra of K2-18 b revealed strong absorption bands of methane (CH₄) and carbon dioxide (CO₂) with a scarcity of ammonia (NH₃), fuels ongoing debate between a 'Hycean' world (a hydrogen-rich atmosphere overlying a global liquid water ocean) and a deep supercritical mini-Neptune gas envelope.
Related Reference Guides
Exoplanet Transit & Radial Velocity Methods
How astronomers detect and characterize exoplanets using photometric transit depth ((Rp/R*)²), Doppler radial velocity (M sin i), and JWST transmission spectra.
Stellar Spectral Classification (OBAFGKM)
Reference guide to Morgan-Keenan (MK) stellar classification (O B A F G K M), luminosity classes (I–V, WD), color index (B-V), and the H-R diagram.
Primary Catalogs & Scientific Sources
- 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
- SIMBAD Astronomical Database (CDS) (2026-10 release): Wenger, M., et al. 2000, A&AS, 143, 9 (The SIMBAD astronomical database) — Centre de Données astronomiques de Strasbourg
- Revised New General Catalogue and Index Catalogue (RNGC/IC) (2024/2026 edition): Steinicke, W. 2010, Observing and Cataloguing Nebulae and Star Clusters (Cambridge Univ. Press); Verga, M. OpenNGC (CC-BY-SA 4.0 / data tables) — Wolfgang Steinicke / OpenNGC