Kepler-257 d
Astrophysical Synthesis & Habitability Characterization
Kepler-257 d is a confirmed sub-neptune exoplanet orbiting the star Kepler-257, located approximately 2544.9 light-years (780.26 parsecs) from the Solar System. First announced in 2014 by Kepler using the Transit detection technique (producing an estimated primary transit depth of 0.1915% across the stellar disk), this world measures 4.95 Earth radii (0.44 Jupiter radii) and has a measured mass of 21.7 Earth masses. Combining its mass and radius yields a mean bulk density of 0.99 g/cm³ (a low bulk density dominated by an extended hydrogen-helium gaseous envelope), a surface gravity of 0.89 g⊕ (8.73 m/s²), and an escape velocity of 23.4 km/s.
In its orbital architecture around Kepler-257, Kepler-257 d completes one full revolution every 24.665 Earth days at a semi-major axis of 0.16 AU (orbiting interior to Mercury's 0.387 AU benchmark in the Solar System). Its mean orbital velocity is approximately 70.6 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 567 K (+294°C / +561°F) and receiving 15.20 S⊕ (~20,687 W/m²), Kepler-257 d orbits interior to the runaway greenhouse boundary where surface liquid water cannot persist.
The host star Kepler-257 is classified as a stellar host with an effective photospheric temperature of 5,180 K, a radius of 1.037 R☉, a mass of 0.916 M☉, an estimated bolometric luminosity of 0.695 L☉. Stellar evolution modeling indicates an estimated stellar age of 1.48 Gyr and a metallicity of [Fe/H] = 0.4 dex (metal-rich relative to the Sun). From Earth, Kepler-257 shines at apparent V-band magnitude 15.652 (faint and requiring research-grade or CCD telescopic photometry) in the Northern celestial hemisphere. The Kepler-257 system is a multi-planet architecture hosting 3 confirmed worlds, with Kepler-257 d sharing its stellar neighborhood alongside Kepler-257 b, Kepler-257 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: Kepler · Catalogue Source: NASA Exoplanet Archive (IPAC / Caltech).
☀️ Host Star Telemetry
| Host Star Name | Kepler-257 |
| Spectral Type | Unclassified |
| Effective Temp (Teff) | 5,180 K |
| Stellar Radius | 1.0370 R☉ |
| Stellar Mass | 0.9160 M☉ |
| Bolometric Luminosity | 0.695 L☉ |
| Metallicity [Fe/H] | 0.400 dex |
| Stellar Age | 1.480 Gyr |
| Apparent Magnitude | V: 15.652 · G: 15.422 |
| System Architecture | 1 Star, 3 Planets |
🪐 Orbital & Physical Dynamics
| Semi-Major Axis (a) | 0.160000 AU |
| Orbital Period | 24.665 days |
| Mean Orbital Velocity | 70.6 km/s |
| Orbital Eccentricity (e) | 0.00000 |
| Orbital Inclination (i) | 88.9800° |
| Mean Bulk Density | 0.99 g/cm³ (23.4 km/s vesc) |
| Insolation Flux | 15.2000 S⊕ (~20,687 W/m²) |
| Habitable Zone Status | Exterior / Non-HZ |
| Celestial Distance | 2544.900 LY (780.256 pc) |