K2-257 b
Astrophysical Synthesis & Habitability Characterization
K2-257 b is a confirmed terrestrial exoplanet orbiting the star K2-257, located approximately 208.8 light-years (64.01 parsecs) from the Solar System. First announced in 2018 by K2 using the Transit detection technique (producing an estimated primary transit depth of 0.0232% across the stellar disk), this world measures 0.83 Earth radii and has a measured mass of 0.498 Earth masses. Combining its mass and radius yields a mean bulk density of 4.8 g/cm³ (a terrestrial silicate-iron composition comparable to Earth (5.51 g/cm³)), a surface gravity of 0.72 g⊕ (7.06 m/s²), and an escape velocity of 8.7 km/s.
In its orbital architecture around K2-257, K2-257 b completes one full revolution every 1.606 Earth days at a semi-major axis of 0.0216 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 146.3 km/s (compared to Earth's 29.8 km/s).
⚠️ Thermal Regime (Irradiated / Hot Zone): At an equilibrium temperature of 789 K (+516°C / +961°F) and receiving 89.15 S⊕ (~121,328 W/m²), K2-257 b orbits interior to the runaway greenhouse boundary where surface liquid water cannot persist.
The host star K2-257 is classified as a stellar host with an effective photospheric temperature of 3,725 K, a radius of 0.5 R☉, a mass of 0.52 M☉, an estimated bolometric luminosity of 0.043 L☉. Stellar evolution modeling indicates a metallicity of [Fe/H] = -0.03 dex (near-solar heavy-element abundance). From Earth, K2-257 shines at apparent V-band magnitude 13.716 (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 | K2-257 |
| Spectral Type | Unclassified |
| Effective Temp (Teff) | 3,725 K |
| Stellar Radius | 0.5000 R☉ |
| Stellar Mass | 0.5200 M☉ |
| Bolometric Luminosity | 0.043 L☉ |
| Metallicity [Fe/H] | -0.030 dex |
| Stellar Age | — |
| Apparent Magnitude | V: 13.716 · G: 12.860 |
| System Architecture | 1 Star, 1 Planet |
🪐 Orbital & Physical Dynamics
| Semi-Major Axis (a) | 0.021600 AU |
| Orbital Period | 1.606 days |
| Mean Orbital Velocity | 146.3 km/s |
| Orbital Eccentricity (e) | Circular (0.00) |
| Orbital Inclination (i) | — |
| Mean Bulk Density | 4.8 g/cm³ (8.7 km/s vesc) |
| Insolation Flux | 89.1463 S⊕ (~121,328 W/m²) |
| Habitable Zone Status | Exterior / Non-HZ |
| Celestial Distance | 208.800 LY (64.007 pc) |