K2-139 b
Astrophysical Synthesis & Habitability Characterization
K2-139 b is a confirmed gas giant exoplanet orbiting the star K2-139, located approximately 501.8 light-years (153.86 parsecs) from the Solar System. First announced in 2017 by K2 using the Transit detection technique (producing an estimated primary transit depth of 0.9008% across the stellar disk), this world measures 9.11 Earth radii (0.81 Jupiter radii) and has a measured mass of 123 Earth masses (0.39 Jupiter masses). Combining its mass and radius yields a mean bulk density of 0.9 g/cm³ (a low bulk density dominated by an extended hydrogen-helium gaseous envelope), a surface gravity of 1.48 g⊕ (14.51 m/s²), and an escape velocity of 41.1 km/s.
In its orbital architecture around K2-139, K2-139 b completes one full revolution every 28.381 Earth days at a semi-major axis of 0.1796 AU (orbiting interior to Mercury's 0.387 AU benchmark in the Solar System). Its mean orbital velocity is approximately 68.8 km/s (compared to Earth's 29.8 km/s). Its moderate orbital eccentricity of e = 0.12 carries the world between 0.158 AU at periapsis and 0.2012 AU at apoapsis, driving measurable seasonal insolation cycles.
⚠️ Thermal Regime (Irradiated / Hot Zone): At an equilibrium temperature of 524 K (+251°C / +484°F) and receiving 15.56 S⊕ (~21,172 W/m²), K2-139 b orbits interior to the runaway greenhouse boundary where surface liquid water cannot persist.
The host star K2-139 is classified as an orange K-type star of spectral type K0 V with an effective photospheric temperature of 5,370 K, a radius of 0.88 R☉, a mass of 0.96 M☉, an estimated bolometric luminosity of 0.578 L☉. Stellar evolution modeling indicates an estimated stellar age of 1.8 Gyr and a metallicity of [Fe/H] = 0.24 dex (metal-rich relative to the Sun). From Earth, K2-139 shines at apparent V-band magnitude 11.652 (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 | K2-139 |
| Spectral Type | K0 V |
| Effective Temp (Teff) | 5,370 K |
| Stellar Radius | 0.8800 R☉ |
| Stellar Mass | 0.9600 M☉ |
| Bolometric Luminosity | 0.578 L☉ |
| Metallicity [Fe/H] | 0.240 dex |
| Stellar Age | 1.800 Gyr |
| Apparent Magnitude | V: 11.652 · G: 11.463 |
| System Architecture | 1 Star, 1 Planet |
🪐 Orbital & Physical Dynamics
| Semi-Major Axis (a) | 0.179600 AU |
| Orbital Period | 28.381 days |
| Mean Orbital Velocity | 68.8 km/s |
| Orbital Eccentricity (e) | 0.12000 |
| Orbital Inclination (i) | 89.5567° |
| Mean Bulk Density | 0.9 g/cm³ (41.1 km/s vesc) |
| Insolation Flux | 15.5562 S⊕ (~21,172 W/m²) |
| Habitable Zone Status | Exterior / Non-HZ |
| Celestial Distance | 501.800 LY (153.856 pc) |