K2-138 c
Astrophysical Synthesis & Habitability Characterization
K2-138 c is a confirmed sub-neptune exoplanet orbiting the star K2-138, located approximately 660.7 light-years (202.59 parsecs) from the Solar System. First announced in 2017 by K2 using the Transit detection technique (producing an estimated primary transit depth of 0.0596% across the stellar disk), this world measures 2.299 Earth radii and has a measured mass of 6.31 Earth masses. Combining its mass and radius yields a mean bulk density of 2.86 g/cm³ (an intermediate density indicative of a deep water-ice mantle or dense volatile envelope), a surface gravity of 1.19 g⊕ (11.67 m/s²), and an escape velocity of 18.5 km/s.
In its orbital architecture around K2-138, K2-138 c completes one full revolution every 3.56 Earth days at a semi-major axis of 0.04461 AU (an ultra-close orbit less than one-seventh of Mercury's 0.387 AU distance from the Sun, where strong stellar tidal forces typically induce synchronous rotation). Its mean orbital velocity is approximately 136.3 km/s (compared to Earth's 29.8 km/s). With a near-circular eccentricity of e = 0.045 (ranging from 0.0426 AU at periapsis to 0.0466 AU at apoapsis), the planet experiences minimal orbital distance variation.
⚠️ Thermal Regime (Irradiated / Hot Zone): At an equilibrium temperature of 1012 K (+739°C / +1362°F) and receiving 279.00 S⊕ (~379,719 W/m²), K2-138 c orbits interior to the runaway greenhouse boundary where surface liquid water cannot persist.
The host star K2-138 is classified as a solar-type yellow star of spectral type G8 V with an effective photospheric temperature of 5,356 K, a radius of 0.863 R☉, a mass of 0.935 M☉, an estimated bolometric luminosity of 0.55 L☉. Stellar evolution modeling indicates an estimated stellar age of 2.8 Gyr and a metallicity of [Fe/H] = 0.14 dex (metal-rich relative to the Sun). From Earth, K2-138 shines at apparent V-band magnitude 12.246 (faint and requiring research-grade or CCD telescopic photometry) in the Southern celestial hemisphere. The K2-138 system is a multi-planet architecture hosting 6 confirmed worlds, with K2-138 c sharing its stellar neighborhood alongside K2-138 b, K2-138 d, K2-138 e, K2-138 f, K2-138 g.
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 | K2-138 |
| Spectral Type | G8 V |
| Effective Temp (Teff) | 5,356 K |
| Stellar Radius | 0.8630 R☉ |
| Stellar Mass | 0.9350 M☉ |
| Bolometric Luminosity | 0.55 L☉ |
| Metallicity [Fe/H] | 0.140 dex |
| Stellar Age | 2.800 Gyr |
| Apparent Magnitude | V: 12.246 · G: 12.025 |
| System Architecture | 1 Star, 6 Planets |
🪐 Orbital & Physical Dynamics
| Semi-Major Axis (a) | 0.044610 AU |
| Orbital Period | 3.56 days |
| Mean Orbital Velocity | 136.3 km/s |
| Orbital Eccentricity (e) | 0.04500 |
| Orbital Inclination (i) | 88.1000° |
| Mean Bulk Density | 2.86 g/cm³ (18.5 km/s vesc) |
| Insolation Flux | 279.0000 S⊕ (~379,719 W/m²) |
| Habitable Zone Status | Exterior / Non-HZ |
| Celestial Distance | 660.700 LY (202.585 pc) |