K2-18 c
Astrophysical Synthesis & Habitability Characterization
K2-18 c 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 2017 by La Silla Observatory using the Radial Velocity detection technique (measuring periodic Doppler reflex motion in the host star's spectrum), this world measures 2.65 Earth radii and has a measured mass of 7.51 Earth masses. Combining its mass and radius yields a mean bulk density of 2.23 g/cm³ (an intermediate density indicative of a deep water-ice mantle or dense volatile envelope), a surface gravity of 1.07 g⊕ (10.49 m/s²), and an escape velocity of 18.8 km/s.
In its orbital architecture around K2-18, K2-18 c completes one full revolution every 8.962 Earth days at a semi-major axis of 0.06 AU (orbiting interior to Mercury's 0.387 AU benchmark in the Solar System). Its mean orbital velocity is approximately 72.8 km/s (compared to Earth's 29.8 km/s). The planet follows a highly eccentric trajectory (e = 0.47, swinging from 0.0318 AU at periapsis out to 0.0882 AU at apoapsis), pointing to past dynamical scattering or secular gravitational perturbations.
⚠️ Thermal Regime (Irradiated / Hot Zone): At an equilibrium temperature of 363 K (+90°C / +194°F) and receiving 7.03 S⊕ (~9,565 W/m²), K2-18 c orbits interior to the runaway greenhouse boundary where surface liquid water cannot persist.
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] = 0.09 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 c sharing its stellar neighborhood alongside K2-18 b.
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: La Silla Observatory · 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] | 0.090 dex |
| Stellar Age | — |
| Apparent Magnitude | V: 13.477 · G: 12.415 |
| System Architecture | 1 Star, 2 Planets |
🪐 Orbital & Physical Dynamics
| Semi-Major Axis (a) | 0.060000 AU |
| Orbital Period | 8.962 days |
| Mean Orbital Velocity | 72.8 km/s |
| Orbital Eccentricity (e) | 0.47000 |
| Orbital Inclination (i) | — |
| Mean Bulk Density | 2.23 g/cm³ (18.8 km/s vesc) |
| Insolation Flux | 7.0278 S⊕ (~9,565 W/m²) |
| Habitable Zone Status | Exterior / Non-HZ |
| Celestial Distance | 124.000 LY (38.027 pc) |