Kepler-138 b
Astrophysical Synthesis & Habitability Characterization
Kepler-138 b is a confirmed sub-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 0.012% across the stellar disk), this world measures 0.64 Earth radii and has a measured mass of 0.07 Earth masses. Combining its mass and radius yields a mean bulk density of 1.47 g/cm³ (a low bulk density dominated by an extended hydrogen-helium gaseous envelope), a surface gravity of 0.17 g⊕ (1.67 m/s²), and an escape velocity of 3.7 km/s.
In its orbital architecture around Kepler-138, Kepler-138 b completes one full revolution every 10.313 Earth days at a semi-major axis of 0.0753 AU (orbiting interior to Mercury's 0.387 AU benchmark in the Solar System). Its mean orbital velocity is approximately 79.4 km/s (compared to Earth's 29.8 km/s). With a near-circular eccentricity of e = 0.02 (ranging from 0.0738 AU at periapsis to 0.0768 AU at apoapsis), the planet experiences minimal orbital distance variation.
⚠️ Thermal Regime (Irradiated / Hot Zone): At an equilibrium temperature of 452 K (+179°C / +354°F) and receiving 9.90 S⊕ (~13,474 W/m²), Kepler-138 b orbits interior to the runaway greenhouse boundary where surface liquid water cannot persist.
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 b sharing its stellar neighborhood alongside Kepler-138 c, Kepler-138 d, 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) | 0.075300 AU |
| Orbital Period | 10.313 days |
| Mean Orbital Velocity | 79.4 km/s |
| Orbital Eccentricity (e) | 0.02000 |
| Orbital Inclination (i) | 88.6700° |
| Mean Bulk Density | 1.47 g/cm³ (3.7 km/s vesc) |
| Insolation Flux | 9.9000 S⊕ (~13,474 W/m²) |
| Habitable Zone Status | Exterior / Non-HZ |
| Celestial Distance | 218.100 LY (66.862 pc) |