K2-258 b
Astrophysical Synthesis & Habitability Characterization
K2-258 b is a confirmed sub-neptune exoplanet orbiting the star K2-258, located approximately 2158.7 light-years (661.87 parsecs) from the Solar System. First announced in 2018 by K2 using the Transit detection technique (producing an estimated primary transit depth of 0.0441% across the stellar disk), this world measures 3 Earth radii (0.27 Jupiter radii) and has a measured mass of 9.27 Earth masses. Combining its mass and radius yields a mean bulk density of 1.89 g/cm³ (a low bulk density dominated by an extended hydrogen-helium gaseous envelope), a surface gravity of 1.03 g⊕ (10.1 m/s²), and an escape velocity of 19.7 km/s.
In its orbital architecture around K2-258, K2-258 b completes one full revolution every 19.092 Earth days at a semi-major axis of 0.149 AU (orbiting interior to Mercury's 0.387 AU benchmark in the Solar System). Its mean orbital velocity is approximately 84.9 km/s (compared to Earth's 29.8 km/s).
⚠️ Thermal Regime (Irradiated / Hot Zone): At an equilibrium temperature of 831 K (+558°C / +1036°F) and receiving 84.30 S⊕ (~114,731 W/m²), K2-258 b orbits interior to the runaway greenhouse boundary where surface liquid water cannot persist.
The host star K2-258 is classified as a stellar host with an effective photospheric temperature of 6,351 K, a radius of 1.31 R☉, a mass of 1.21 M☉, an estimated bolometric luminosity of 2.505 L☉. Stellar evolution modeling indicates a metallicity of [Fe/H] = 0.01 dex (near-solar heavy-element abundance). From Earth, K2-258 shines at apparent V-band magnitude 13.361 (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 | K2-258 |
| Spectral Type | Unclassified |
| Effective Temp (Teff) | 6,351 K |
| Stellar Radius | 1.3100 R☉ |
| Stellar Mass | 1.2100 M☉ |
| Bolometric Luminosity | 2.505 L☉ |
| Metallicity [Fe/H] | 0.010 dex |
| Stellar Age | — |
| Apparent Magnitude | V: 13.361 · G: 13.139 |
| System Architecture | 1 Star, 1 Planet |
🪐 Orbital & Physical Dynamics
| Semi-Major Axis (a) | 0.149000 AU |
| Orbital Period | 19.092 days |
| Mean Orbital Velocity | 84.9 km/s |
| Orbital Eccentricity (e) | Circular (0.00) |
| Orbital Inclination (i) | — |
| Mean Bulk Density | 1.89 g/cm³ (19.7 km/s vesc) |
| Insolation Flux | 84.2989 S⊕ (~114,731 W/m²) |
| Habitable Zone Status | Exterior / Non-HZ |
| Celestial Distance | 2158.700 LY (661.873 pc) |