Kepler-138 e
Astrophysical Synthesis & Habitability Characterization
Kepler-138 e 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 2022 by Multiple Observatories using the Transit Timing Variations detection technique (producing an estimated primary transit depth of 0.0187% across the stellar disk), this world measures 0.797 Earth radii and has a measured mass of 0.43 Earth masses. Combining its mass and radius yields a mean bulk density of 4.68 g/cm³ (a terrestrial silicate-iron composition comparable to Earth (5.51 g/cm³)), a surface gravity of 0.68 g⊕ (6.67 m/s²), and an escape velocity of 8.2 km/s.
In its orbital architecture around Kepler-138, Kepler-138 e completes one full revolution every 38.23 Earth days at a semi-major axis of 0.1803 AU (orbiting interior to Mercury's 0.387 AU benchmark in the Solar System). Its mean orbital velocity is approximately 51.3 km/s (compared to Earth's 29.8 km/s). Its moderate orbital eccentricity of e = 0.112 carries the world between 0.1601 AU at periapsis and 0.2005 AU at apoapsis, driving measurable seasonal insolation cycles.
✓ Classified as a Temperate Habitable Zone Candidate.
With an estimated equilibrium temperature of 292 K (+19°C / +66°F) and an incident stellar irradiance of 1.73 S⊕ (~2,355 W/m²), Kepler-138 e orbits within the empirical circumstellar habitable zone where a rocky surface under sufficient atmospheric pressure can maintain liquid surface water.
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 e sharing its stellar neighborhood alongside Kepler-138 b, Kepler-138 c, Kepler-138 d.
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: Multiple Observatories · 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) | 0.180300 AU |
| Orbital Period | 38.23 days |
| Mean Orbital Velocity | 51.3 km/s |
| Orbital Eccentricity (e) | 0.11200 |
| Orbital Inclination (i) | 88.5300° |
| Mean Bulk Density | 4.68 g/cm³ (8.2 km/s vesc) |
| Insolation Flux | 1.7300 S⊕ (~2,355 W/m²) |
| Habitable Zone Status | ★ Temperate Zone |
| Celestial Distance | 218.100 LY (66.862 pc) |