K2-138 d
Astrophysical Synthesis & Habitability Characterization
K2-138 d is a confirmed sub-neptune exoplanet orbiting the star K2-138, located approximately 660.7 light-years (202.59 parsecs) from the Solar System. First announced in 2017 by K2 using the Transit detection technique (producing an estimated primary transit depth of 0.0645% across the stellar disk), this world measures 2.39 Earth radii and has a measured mass of 7.92 Earth masses. Combining its mass and radius yields a mean bulk density of 3.2 g/cm³ (an intermediate density indicative of a deep water-ice mantle or dense volatile envelope), a surface gravity of 1.39 g⊕ (13.63 m/s²), and an escape velocity of 20.4 km/s.
In its orbital architecture around K2-138, K2-138 d completes one full revolution every 5.405 Earth days at a semi-major axis of 0.05893 AU (orbiting interior to Mercury's 0.387 AU benchmark in the Solar System). Its mean orbital velocity is approximately 118.6 km/s (compared to Earth's 29.8 km/s). With a near-circular eccentricity of e = 0.043 (ranging from 0.0564 AU at periapsis to 0.0615 AU at apoapsis), the planet experiences minimal orbital distance variation.
⚠️ Thermal Regime (Irradiated / Hot Zone): At an equilibrium temperature of 888 K (+615°C / +1139°F) and receiving 160.00 S⊕ (~217,760 W/m²), K2-138 d orbits interior to the runaway greenhouse boundary where surface liquid water cannot persist.
The host star K2-138 is classified as a solar-type yellow star of spectral type G8 V with an effective photospheric temperature of 5,356 K, a radius of 0.863 R☉, a mass of 0.935 M☉, an estimated bolometric luminosity of 0.55 L☉. Stellar evolution modeling indicates an estimated stellar age of 2.8 Gyr and a metallicity of [Fe/H] = 0.14 dex (metal-rich relative to the Sun). From Earth, K2-138 shines at apparent V-band magnitude 12.246 (faint and requiring research-grade or CCD telescopic photometry) in the Southern celestial hemisphere. The K2-138 system is a multi-planet architecture hosting 6 confirmed worlds, with K2-138 d sharing its stellar neighborhood alongside K2-138 b, K2-138 c, K2-138 e, K2-138 f, K2-138 g.
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 | K2-138 |
| Spectral Type | G8 V |
| Effective Temp (Teff) | 5,356 K |
| Stellar Radius | 0.8630 R☉ |
| Stellar Mass | 0.9350 M☉ |
| Bolometric Luminosity | 0.55 L☉ |
| Metallicity [Fe/H] | 0.140 dex |
| Stellar Age | 2.800 Gyr |
| Apparent Magnitude | V: 12.246 · G: 12.025 |
| System Architecture | 1 Star, 6 Planets |
🪐 Orbital & Physical Dynamics
| Semi-Major Axis (a) | 0.058930 AU |
| Orbital Period | 5.405 days |
| Mean Orbital Velocity | 118.6 km/s |
| Orbital Eccentricity (e) | 0.04300 |
| Orbital Inclination (i) | 89.0000° |
| Mean Bulk Density | 3.2 g/cm³ (20.4 km/s vesc) |
| Insolation Flux | 160.0000 S⊕ (~217,760 W/m²) |
| Habitable Zone Status | Exterior / Non-HZ |
| Celestial Distance | 660.700 LY (202.585 pc) |