Kepler-251 d
Astrophysical Synthesis & Habitability Characterization
Kepler-251 d is a confirmed sub-neptune exoplanet orbiting the star Kepler-251, located approximately 3015.7 light-years (924.63 parsecs) from the Solar System. First announced in 2014 by Kepler using the Transit detection technique (producing an estimated primary transit depth of 0.0814% across the stellar disk), this world measures 2.77 Earth radii and has a measured mass of 8.1 Earth masses. Combining its mass and radius yields a mean bulk density of 2.1 g/cm³ (an intermediate density indicative of a deep water-ice mantle or dense volatile envelope), a surface gravity of 1.06 g⊕ (10.4 m/s²), and an escape velocity of 19.1 km/s.
In its orbital architecture around Kepler-251, Kepler-251 d completes one full revolution every 30.133 Earth days at a semi-major axis of 0.182 AU (orbiting interior to Mercury's 0.387 AU benchmark in the Solar System). Its mean orbital velocity is approximately 65.7 km/s (compared to Earth's 29.8 km/s). The orbit is modeled as circular (e = 0.00), maintaining a constant stellar distance throughout its year.
⚠️ Thermal Regime (Irradiated / Hot Zone): At an equilibrium temperature of 536 K (+263°C / +505°F) and receiving 17.48 S⊕ (~23,786 W/m²), Kepler-251 d orbits interior to the runaway greenhouse boundary where surface liquid water cannot persist.
The host star Kepler-251 is classified as a stellar host with an effective photospheric temperature of 5,526 K, a radius of 0.89 R☉, a mass of 0.891 M☉, an estimated bolometric luminosity of 0.663 L☉. Stellar evolution modeling indicates an estimated stellar age of 2 Gyr and a metallicity of [Fe/H] = -0.06 dex (near-solar heavy-element abundance). From Earth, Kepler-251 shines at apparent V-band magnitude 15.541 (faint and requiring research-grade or CCD telescopic photometry) in the Northern celestial hemisphere. The Kepler-251 system is a multi-planet architecture hosting 4 confirmed worlds, with Kepler-251 d sharing its stellar neighborhood alongside Kepler-251 b, Kepler-251 c, Kepler-251 e.
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: Kepler · Catalogue Source: NASA Exoplanet Archive (IPAC / Caltech).
☀️ Host Star Telemetry
| Host Star Name | Kepler-251 |
| Spectral Type | Unclassified |
| Effective Temp (Teff) | 5,526 K |
| Stellar Radius | 0.8900 R☉ |
| Stellar Mass | 0.8910 M☉ |
| Bolometric Luminosity | 0.663 L☉ |
| Metallicity [Fe/H] | -0.060 dex |
| Stellar Age | 2.000 Gyr |
| Apparent Magnitude | V: 15.541 · G: 15.216 |
| System Architecture | 1 Star, 4 Planets |
🪐 Orbital & Physical Dynamics
| Semi-Major Axis (a) | 0.182000 AU |
| Orbital Period | 30.133 days |
| Mean Orbital Velocity | 65.7 km/s |
| Orbital Eccentricity (e) | 0.00000 |
| Orbital Inclination (i) | 88.2100° |
| Mean Bulk Density | 2.1 g/cm³ (19.1 km/s vesc) |
| Insolation Flux | 17.4770 S⊕ (~23,786 W/m²) |
| Habitable Zone Status | Exterior / Non-HZ |
| Celestial Distance | 3015.700 LY (924.625 pc) |