K2-129 b
Astrophysical Synthesis & Habitability Characterization
K2-129 b is a confirmed terrestrial exoplanet orbiting the star K2-129, located approximately 90.7 light-years (27.8 parsecs) from the Solar System. First announced in 2017 by K2 using the Transit detection technique (producing an estimated primary transit depth of 0.0701% across the stellar disk), this world measures 1.04 Earth radii and has a measured mass of 1.12 Earth masses. Combining its mass and radius yields a mean bulk density of 5.49 g/cm³ (a terrestrial silicate-iron composition comparable to Earth (5.51 g/cm³)), a surface gravity of 1.04 g⊕ (10.2 m/s²), and an escape velocity of 11.6 km/s.
In its orbital architecture around K2-129, K2-129 b completes one full revolution every 8.239 Earth days at a semi-major axis of 0.057 AU (orbiting interior to Mercury's 0.387 AU benchmark in the Solar System). Its mean orbital velocity is approximately 75.3 km/s (compared to Earth's 29.8 km/s). Its moderate orbital eccentricity of e = 0.13 carries the world between 0.0496 AU at periapsis and 0.0644 AU at apoapsis, driving measurable seasonal insolation cycles.
⚠️ Thermal Regime (Irradiated / Hot Zone): At an equilibrium temperature of 419 K (+146°C / +295°F) and receiving 4.50 S⊕ (~6,125 W/m²), K2-129 b orbits interior to the runaway greenhouse boundary where surface liquid water cannot persist.
The host star K2-129 is classified as a stellar host with an effective photospheric temperature of 3,459 K, a radius of 0.36 R☉, a mass of 0.36 M☉, an estimated bolometric luminosity of 0.017 L☉. From Earth, K2-129 shines at apparent V-band magnitude 13.639 (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 | K2-129 |
| Spectral Type | Unclassified |
| Effective Temp (Teff) | 3,459 K |
| Stellar Radius | 0.3600 R☉ |
| Stellar Mass | 0.3600 M☉ |
| Bolometric Luminosity | 0.017 L☉ |
| Metallicity [Fe/H] | — |
| Stellar Age | — |
| Apparent Magnitude | V: 13.639 · G: 12.462 |
| System Architecture | 1 Star, 1 Planet |
🪐 Orbital & Physical Dynamics
| Semi-Major Axis (a) | 0.057000 AU |
| Orbital Period | 8.239 days |
| Mean Orbital Velocity | 75.3 km/s |
| Orbital Eccentricity (e) | 0.13000 |
| Orbital Inclination (i) | 89.2100° |
| Mean Bulk Density | 5.49 g/cm³ (11.6 km/s vesc) |
| Insolation Flux | 4.5000 S⊕ (~6,125 W/m²) |
| Habitable Zone Status | Exterior / Non-HZ |
| Celestial Distance | 90.700 LY (27.796 pc) |