K2-8 b
Astrophysical Synthesis & Habitability Characterization
K2-8 b is a confirmed sub-neptune exoplanet orbiting the star K2-8, located approximately 1316.1 light-years (403.51 parsecs) from the Solar System. First announced in 2015 by K2 using the Transit detection technique (producing an estimated primary transit depth of 0.1967% across the stellar disk), this world measures 3.58 Earth radii (0.32 Jupiter radii) and has a measured mass of 12.5 Earth masses. Combining its mass and radius yields a mean bulk density of 1.5 g/cm³ (a low bulk density dominated by an extended hydrogen-helium gaseous envelope), a surface gravity of 0.98 g⊕ (9.61 m/s²), and an escape velocity of 20.9 km/s.
In its orbital architecture around K2-8, K2-8 b completes one full revolution every 10.352 Earth days at a semi-major axis of 0.0856 AU (orbiting interior to Mercury's 0.387 AU benchmark in the Solar System). Its mean orbital velocity is approximately 90 km/s (compared to Earth's 29.8 km/s).
⚠️ Thermal Regime (Irradiated / Hot Zone): At an equilibrium temperature of 631 K (+358°C / +676°F) and receiving 37.70 S⊕ (~51,310 W/m²), K2-8 b orbits interior to the runaway greenhouse boundary where surface liquid water cannot persist.
The host star K2-8 is classified as an orange K-type star of spectral type K3 V with an effective photospheric temperature of 4,870 K, a radius of 0.74 R☉, a mass of 0.78 M☉, an estimated bolometric luminosity of 0.276 L☉. Stellar evolution modeling indicates a metallicity of [Fe/H] = -0.02 dex (near-solar heavy-element abundance). From Earth, K2-8 shines at apparent V-band magnitude 14.801 (faint and requiring research-grade or CCD telescopic photometry) in the Southern celestial hemisphere. The K2-8 system is a multi-planet architecture hosting 2 confirmed worlds, with K2-8 b sharing its stellar neighborhood alongside K2-8 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: K2 · Catalogue Source: NASA Exoplanet Archive (IPAC / Caltech).
☀️ Host Star Telemetry
| Host Star Name | K2-8 |
| Spectral Type | K3 V |
| Effective Temp (Teff) | 4,870 K |
| Stellar Radius | 0.7400 R☉ |
| Stellar Mass | 0.7800 M☉ |
| Bolometric Luminosity | 0.276 L☉ |
| Metallicity [Fe/H] | -0.020 dex |
| Stellar Age | — |
| Apparent Magnitude | V: 14.801 · G: 14.336 |
| System Architecture | 1 Star, 2 Planets |
🪐 Orbital & Physical Dynamics
| Semi-Major Axis (a) | 0.085600 AU |
| Orbital Period | 10.352 days |
| Mean Orbital Velocity | 90 km/s |
| Orbital Eccentricity (e) | Circular (0.00) |
| Orbital Inclination (i) | 86.4200° |
| Mean Bulk Density | 1.5 g/cm³ (20.9 km/s vesc) |
| Insolation Flux | 37.7000 S⊕ (~51,310 W/m²) |
| Habitable Zone Status | Exterior / Non-HZ |
| Celestial Distance | 1316.100 LY (403.513 pc) |