K2-251 b
Astrophysical Synthesis & Habitability Characterization
K2-251 b is a confirmed sub-neptune exoplanet orbiting the star K2-251, located approximately 479.5 light-years (147.03 parsecs) from the Solar System. First announced in 2018 by K2 using the Transit detection technique (producing an estimated primary transit depth of 0.1933% across the stellar disk), this world measures 2.35 Earth radii and has a measured mass of 6.12 Earth masses. Combining its mass and radius yields a mean bulk density of 2.6 g/cm³ (an intermediate density indicative of a deep water-ice mantle or dense volatile envelope), a surface gravity of 1.11 g⊕ (10.89 m/s²), and an escape velocity of 18.1 km/s.
In its orbital architecture around K2-251, K2-251 b completes one full revolution every 9.301 Earth days at a semi-major axis of 0.0694 AU (orbiting interior to Mercury's 0.387 AU benchmark in the Solar System). Its mean orbital velocity is approximately 81.2 km/s (compared to Earth's 29.8 km/s).
⚠️ Thermal Regime (Irradiated / Hot Zone): At an equilibrium temperature of 437 K (+164°C / +327°F) and receiving 9.44 S⊕ (~12,854 W/m²), K2-251 b orbits interior to the runaway greenhouse boundary where surface liquid water cannot persist.
The host star K2-251 is classified as a stellar host with an effective photospheric temperature of 3,717 K, a radius of 0.49 R☉, a mass of 0.52 M☉, an estimated bolometric luminosity of 0.041 L☉. Stellar evolution modeling indicates a metallicity of [Fe/H] = -0.04 dex (near-solar heavy-element abundance). From Earth, K2-251 shines at apparent V-band magnitude 15.66 (faint and requiring research-grade or CCD telescopic photometry) in the Southern celestial hemisphere.
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 | K2-251 |
| Spectral Type | Unclassified |
| Effective Temp (Teff) | 3,717 K |
| Stellar Radius | 0.4900 R☉ |
| Stellar Mass | 0.5200 M☉ |
| Bolometric Luminosity | 0.041 L☉ |
| Metallicity [Fe/H] | -0.040 dex |
| Stellar Age | — |
| Apparent Magnitude | V: 15.660 · G: 14.770 |
| System Architecture | 1 Star, 1 Planet |
🪐 Orbital & Physical Dynamics
| Semi-Major Axis (a) | 0.069400 AU |
| Orbital Period | 9.301 days |
| Mean Orbital Velocity | 81.2 km/s |
| Orbital Eccentricity (e) | Circular (0.00) |
| Orbital Inclination (i) | — |
| Mean Bulk Density | 2.6 g/cm³ (18.1 km/s vesc) |
| Insolation Flux | 9.4444 S⊕ (~12,854 W/m²) |
| Habitable Zone Status | Exterior / Non-HZ |
| Celestial Distance | 479.500 LY (147.030 pc) |