K2-125 b
Astrophysical Synthesis & Habitability Characterization
K2-125 b is a confirmed sub-neptune exoplanet orbiting the star K2-125, located approximately 404.9 light-years (124.16 parsecs) from the Solar System. First announced in 2017 by K2 using the Transit detection technique (producing an estimated primary transit depth of 0.2472% across the stellar disk), this world measures 2.18 Earth radii and has a measured mass of 5.39 Earth masses. Combining its mass and radius yields a mean bulk density of 2.87 g/cm³ (an intermediate density indicative of a deep water-ice mantle or dense volatile envelope), a surface gravity of 1.13 g⊕ (11.08 m/s²), and an escape velocity of 17.6 km/s.
In its orbital architecture around K2-125, K2-125 b completes one full revolution every 21.75 Earth days at a semi-major axis of 0.121 AU (orbiting interior to Mercury's 0.387 AU benchmark in the Solar System). Its mean orbital velocity is approximately 60.5 km/s (compared to Earth's 29.8 km/s). Its moderate orbital eccentricity of e = 0.08 carries the world between 0.1113 AU at periapsis and 0.1307 AU at apoapsis, driving measurable seasonal insolation cycles.
✓ Classified as a Temperate Habitable Zone Candidate.
With an estimated equilibrium temperature of 321 K (+48°C / +118°F) and an incident stellar irradiance of 2.00 S⊕ (~2,722 W/m²), K2-125 b orbits within the empirical circumstellar habitable zone where a rocky surface under sufficient atmospheric pressure can maintain liquid surface water.
The host star K2-125 is classified as a stellar host with an effective photospheric temperature of 3,654 K, a radius of 0.402 R☉, a mass of 0.4947 M☉, an estimated bolometric luminosity of 0.026 L☉. From Earth, K2-125 shines at apparent V-band magnitude 16.4 (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 | K2-125 |
| Spectral Type | Unclassified |
| Effective Temp (Teff) | 3,654 K |
| Stellar Radius | 0.4020 R☉ |
| Stellar Mass | 0.4947 M☉ |
| Bolometric Luminosity | 0.026 L☉ |
| Metallicity [Fe/H] | — |
| Stellar Age | — |
| Apparent Magnitude | V: 16.400 · G: 15.151 |
| System Architecture | 1 Star, 1 Planet |
🪐 Orbital & Physical Dynamics
| Semi-Major Axis (a) | 0.121000 AU |
| Orbital Period | 21.75 days |
| Mean Orbital Velocity | 60.5 km/s |
| Orbital Eccentricity (e) | 0.08000 |
| Orbital Inclination (i) | 89.7200° |
| Mean Bulk Density | 2.87 g/cm³ (17.6 km/s vesc) |
| Insolation Flux | 2.0000 S⊕ (~2,722 W/m²) |
| Habitable Zone Status | ★ Temperate Zone |
| Celestial Distance | 404.900 LY (124.156 pc) |