K2-239 d
Astrophysical Synthesis & Habitability Characterization
K2-239 d is a confirmed terrestrial exoplanet orbiting the star K2-239, located approximately 101.4 light-years (31.09 parsecs) from the Solar System. First announced in 2018 by K2 using the Transit detection technique (producing an estimated primary transit depth of 0.0785% across the stellar disk), this world measures 1.1 Earth radii and has a measured mass of 1.37 Earth masses. Combining its mass and radius yields a mean bulk density of 5.68 g/cm³ (a terrestrial silicate-iron composition comparable to Earth (5.51 g/cm³)), a surface gravity of 1.13 g⊕ (11.08 m/s²), and an escape velocity of 12.5 km/s.
In its orbital architecture around K2-239, K2-239 d completes one full revolution every 10.115 Earth days at a semi-major axis of 0.0685 AU (orbiting interior to Mercury's 0.387 AU benchmark in the Solar System). Its mean orbital velocity is approximately 73.7 km/s (compared to Earth's 29.8 km/s).
⚠️ Thermal Regime (Irradiated / Hot Zone): At an equilibrium temperature of 399 K (+126°C / +259°F) and receiving 3.41 S⊕ (~4,641 W/m²), K2-239 d orbits interior to the runaway greenhouse boundary where surface liquid water cannot persist.
The host star K2-239 is classified as a low-mass red dwarf of spectral type M3.0 V with an effective photospheric temperature of 3,420 K, a radius of 0.36 R☉, a mass of 0.4 M☉, an estimated bolometric luminosity of 0.016 L☉. Stellar evolution modeling indicates a metallicity of [Fe/H] = -0.1 dex (near-solar heavy-element abundance). From Earth, K2-239 shines at apparent V-band magnitude 14.632 (faint and requiring research-grade or CCD telescopic photometry) in the Northern celestial hemisphere. The K2-239 system is a multi-planet architecture hosting 3 confirmed worlds, with K2-239 d sharing its stellar neighborhood alongside K2-239 b, K2-239 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 | K2-239 |
| Spectral Type | M3.0 V |
| Effective Temp (Teff) | 3,420 K |
| Stellar Radius | 0.3600 R☉ |
| Stellar Mass | 0.4000 M☉ |
| Bolometric Luminosity | 0.016 L☉ |
| Metallicity [Fe/H] | -0.100 dex |
| Stellar Age | — |
| Apparent Magnitude | V: 14.632 · G: 13.437 |
| System Architecture | 1 Star, 3 Planets |
🪐 Orbital & Physical Dynamics
| Semi-Major Axis (a) | 0.068500 AU |
| Orbital Period | 10.115 days |
| Mean Orbital Velocity | 73.7 km/s |
| Orbital Eccentricity (e) | Circular (0.00) |
| Orbital Inclination (i) | 89.4300° |
| Mean Bulk Density | 5.68 g/cm³ (12.5 km/s vesc) |
| Insolation Flux | 3.4099 S⊕ (~4,641 W/m²) |
| Habitable Zone Status | Exterior / Non-HZ |
| Celestial Distance | 101.400 LY (31.085 pc) |