Kepler-138 d
Astrophysical Synthesis & Habitability Characterization
Kepler-138 d is a confirmed super-earth exoplanet orbiting the star Kepler-138, located approximately 218.1 light-years (66.86 parsecs) from the Solar System. First announced in 2014 by Kepler using the Transit detection technique (producing an estimated primary transit depth of 0.067% across the stellar disk), this world measures 1.51 Earth radii and has a measured mass of 2.1 Earth masses. Combining its mass and radius yields a mean bulk density of 3.36 g/cm³ (an intermediate density indicative of a deep water-ice mantle or dense volatile envelope), a surface gravity of 0.92 g⊕ (9.02 m/s²), and an escape velocity of 13.2 km/s.
In its orbital architecture around Kepler-138, Kepler-138 d completes one full revolution every 23.092 Earth days at a semi-major axis of 0.1288 AU (orbiting interior to Mercury's 0.387 AU benchmark in the Solar System). Its mean orbital velocity is approximately 60.7 km/s (compared to Earth's 29.8 km/s). With a near-circular eccentricity of e = 0.01 (ranging from 0.1275 AU at periapsis to 0.1301 AU at apoapsis), the planet experiences minimal orbital distance variation.
ℹ️ Thermal & Atmospheric Regime: Current photometric and spectroscopic constraints for Kepler-138 d place it outside the conservative habitable zone definition pending further atmospheric characterization.
The host star Kepler-138 is classified as a low-mass red dwarf of spectral type M1 V with an effective photospheric temperature of 3,841 K, a radius of 0.535 R☉, a mass of 0.535 M☉, an estimated bolometric luminosity of 0.056 L☉. Stellar evolution modeling indicates an estimated stellar age of 2.21 Gyr and a metallicity of [Fe/H] = -0.18 dex (metal-poor relative to the Sun). From Earth, Kepler-138 shines at apparent V-band magnitude 13.04 (faint and requiring research-grade or CCD telescopic photometry) in the Northern celestial hemisphere. The Kepler-138 system is a multi-planet architecture hosting 4 confirmed worlds, with Kepler-138 d sharing its stellar neighborhood alongside Kepler-138 b, Kepler-138 c, Kepler-138 e.
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: Kepler · Catalogue Source: NASA Exoplanet Archive (IPAC / Caltech).
☀️ Host Star Telemetry
| Host Star Name | Kepler-138 |
| Spectral Type | M1 V |
| Effective Temp (Teff) | 3,841 K |
| Stellar Radius | 0.5350 R☉ |
| Stellar Mass | 0.5350 M☉ |
| Bolometric Luminosity | 0.056 L☉ |
| Metallicity [Fe/H] | -0.180 dex |
| Stellar Age | 2.210 Gyr |
| Apparent Magnitude | V: 13.040 · G: 12.472 |
| System Architecture | 1 Star, 4 Planets |
🪐 Orbital & Physical Dynamics
| Semi-Major Axis (a) | 0.128800 AU |
| Orbital Period | 23.092 days |
| Mean Orbital Velocity | 60.7 km/s |
| Orbital Eccentricity (e) | 0.01000 |
| Orbital Inclination (i) | 89.0400° |
| Mean Bulk Density | 3.36 g/cm³ (13.2 km/s vesc) |
| Insolation Flux | 3.4000 S⊕ (~4,627 W/m²) |
| Habitable Zone Status | Exterior / Non-HZ |
| Celestial Distance | 218.100 LY (66.862 pc) |