K2-25 b
Astrophysical Synthesis & Habitability Characterization
K2-25 b is a confirmed sub-neptune exoplanet orbiting the star K2-25, located approximately 146.6 light-years (44.96 parsecs) from the Solar System. First announced in 2015 by K2 using the Transit detection technique (producing an estimated primary transit depth of 1.157% across the stellar disk), this world measures 3.44 Earth radii (0.31 Jupiter radii) and has a measured mass of 24.5 Earth masses. Combining its mass and radius yields a mean bulk density of 3.32 g/cm³ (an intermediate density indicative of a deep water-ice mantle or dense volatile envelope), a surface gravity of 2.07 g⊕ (20.3 m/s²), and an escape velocity of 29.9 km/s.
In its orbital architecture around K2-25, K2-25 b completes one full revolution every 3.485 Earth days at a semi-major axis of 0.0287 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 89.6 km/s (compared to Earth's 29.8 km/s). The planet follows a highly eccentric trajectory (e = 0.428, swinging from 0.0164 AU at periapsis out to 0.041 AU at apoapsis), pointing to past dynamical scattering or secular gravitational perturbations.
⚠️ Thermal Regime (Irradiated / Hot Zone): At an equilibrium temperature of 494 K (+221°C / +430°F) and receiving 9.91 S⊕ (~13,488 W/m²), K2-25 b orbits interior to the runaway greenhouse boundary where surface liquid water cannot persist.
The host star K2-25 is classified as a low-mass red dwarf of spectral type M4.5 V with an effective photospheric temperature of 3,207 K, a radius of 0.2932 R☉, a mass of 0.2634 M☉, an estimated bolometric luminosity of 0.00816 L☉. Stellar evolution modeling indicates an estimated stellar age of 0.73 Gyr and a metallicity of [Fe/H] = 0.15 dex (metal-rich relative to the Sun). From Earth, K2-25 shines at apparent V-band magnitude 16.21 (faint and requiring research-grade or CCD telescopic photometry) in the Northern 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 | K2-25 |
| Spectral Type | M4.5 V |
| Effective Temp (Teff) | 3,207 K |
| Stellar Radius | 0.2932 R☉ |
| Stellar Mass | 0.2634 M☉ |
| Bolometric Luminosity | 0.00816 L☉ |
| Metallicity [Fe/H] | 0.150 dex |
| Stellar Age | 0.730 Gyr |
| Apparent Magnitude | V: 16.210 · G: 14.382 |
| System Architecture | 1 Star, 1 Planet |
🪐 Orbital & Physical Dynamics
| Semi-Major Axis (a) | 0.028700 AU |
| Orbital Period | 3.485 days |
| Mean Orbital Velocity | 89.6 km/s |
| Orbital Eccentricity (e) | 0.42800 |
| Orbital Inclination (i) | 87.1600° |
| Mean Bulk Density | 3.32 g/cm³ (29.9 km/s vesc) |
| Insolation Flux | 9.9100 S⊕ (~13,488 W/m²) |
| Habitable Zone Status | Exterior / Non-HZ |
| Celestial Distance | 146.600 LY (44.957 pc) |