K2-18 b
Astrophysical Synthesis & Habitability Characterization
K2-18 b is a confirmed sub-neptune exoplanet orbiting the star K2-18, located approximately 124 light-years (38.03 parsecs) from the Solar System. First announced in 2015 by K2 using the Transit detection technique (producing an estimated primary transit depth of 0.2795% across the stellar disk), this world measures 2.37 Earth radii and has a measured mass of 8.92 Earth masses. Combining its mass and radius yields a mean bulk density of 3.69 g/cm³ (an intermediate density indicative of a deep water-ice mantle or dense volatile envelope), a surface gravity of 1.59 g⊕ (15.59 m/s²), and an escape velocity of 21.7 km/s.
In its orbital architecture around K2-18, K2-18 b completes one full revolution every 32.94 Earth days at a semi-major axis of 0.1429 AU (orbiting interior to Mercury's 0.387 AU benchmark in the Solar System). Its mean orbital velocity is approximately 47.2 km/s (compared to Earth's 29.8 km/s). Its moderate orbital eccentricity of e = 0.2 carries the world between 0.1143 AU at periapsis and 0.1715 AU at apoapsis, driving measurable seasonal insolation cycles.
☁️ Atmospheric Regime (Volatile / Gas Envelope): With a radius of 2.370 R⊕, K2-18 b exceeds the empirical ~1.8–2.2 R⊕ Fulton radius valley threshold, indicating a substantial hydrogen-helium or supercritical volatile envelope without a conventional solid terrestrial surface.
The host star K2-18 is classified as a low-mass red dwarf of spectral type M2.5 V with an effective photospheric temperature of 3,457 K, a radius of 0.411 R☉, a mass of 0.359 M☉, an estimated bolometric luminosity of 0.022 L☉. Stellar evolution modeling indicates a metallicity of [Fe/H] = 0.12 dex (metal-rich relative to the Sun). From Earth, K2-18 shines at apparent V-band magnitude 13.477 (faint and requiring research-grade or CCD telescopic photometry) in the Northern celestial hemisphere. The K2-18 system is a multi-planet architecture hosting 2 confirmed worlds, with K2-18 b sharing its stellar neighborhood alongside K2-18 c.
Methodology: Computed using the Schulze-Makuch et al. (2011) weighted metric comparing physical interior (radius, estimated density) and surface thermal regime (escape velocity, equilibrium temperature).
Discovery Facility: K2 · Catalogue Source: NASA Exoplanet Archive (IPAC / Caltech).
☀️ Host Star Telemetry
| Host Star Name | K2-18 |
| Spectral Type | M2.5 V |
| Effective Temp (Teff) | 3,457 K |
| Stellar Radius | 0.4110 R☉ |
| Stellar Mass | 0.3590 M☉ |
| Bolometric Luminosity | 0.022 L☉ |
| Metallicity [Fe/H] | 0.120 dex |
| Stellar Age | — |
| Apparent Magnitude | V: 13.477 · G: 12.415 |
| System Architecture | 1 Star, 2 Planets |
🪐 Orbital & Physical Dynamics
| Semi-Major Axis (a) | 0.142900 AU |
| Orbital Period | 32.94 days |
| Mean Orbital Velocity | 47.2 km/s |
| Orbital Eccentricity (e) | 0.20000 |
| Orbital Inclination (i) | 89.5785° |
| Mean Bulk Density | 3.69 g/cm³ (21.7 km/s vesc) |
| Insolation Flux | 1.0050 S⊕ (~1,368 W/m²) |
| Habitable Zone Status | Exterior / Non-HZ |
| Celestial Distance | 124.000 LY (38.027 pc) |