Kepler-391 b
Astrophysical Synthesis & Habitability Characterization
Kepler-391 b is a confirmed sub-neptune exoplanet orbiting the star Kepler-391, located approximately 2835.1 light-years (869.24 parsecs) from the Solar System. First announced in 2014 by Kepler using the Transit detection technique (producing an estimated primary transit depth of 0.0063% across the stellar disk), this world measures 2.5 Earth radii and has a measured mass of 1100 Earth masses (3.46 Jupiter masses). Combining its mass and radius yields a mean bulk density of 388.19 g/cm³ (an iron-enriched, high-compression rocky core), a surface gravity of 176 g⊕ (1725.97 m/s²), and an escape velocity of 234.6 km/s.
In its orbital architecture around Kepler-391, Kepler-391 b completes one full revolution every 7.418 Earth days at a semi-major axis of 0.083 AU (orbiting interior to Mercury's 0.387 AU benchmark in the Solar System). Its mean orbital velocity is approximately 121.7 km/s (compared to Earth's 29.8 km/s). The planet follows a highly eccentric trajectory (e = 0.46, swinging from 0.0448 AU at periapsis out to 0.1212 AU at apoapsis), pointing to past dynamical scattering or secular gravitational perturbations.
⚠️ Thermal Regime (Irradiated / Hot Zone): At an equilibrium temperature of 930 K (+657°C / +1215°F) and receiving 690.00 S⊕ (~939,090 W/m²), Kepler-391 b orbits interior to the runaway greenhouse boundary where surface liquid water cannot persist.
The host star Kepler-391 is classified as an orange K-type star of spectral type K2 III-IV with an effective photospheric temperature of 5,038 K, a radius of 2.879 R☉, a mass of 1.27 M☉, an estimated bolometric luminosity of 4.791 L☉. Stellar evolution modeling indicates an estimated stellar age of 4.365 Gyr and a metallicity of [Fe/H] = 0.04 dex (near-solar heavy-element abundance). From Earth, Kepler-391 shines at apparent V-band magnitude 13.177 (faint and requiring research-grade or CCD telescopic photometry) in the Northern celestial hemisphere. The Kepler-391 system is a multi-planet architecture hosting 2 confirmed worlds, with Kepler-391 b sharing its stellar neighborhood alongside Kepler-391 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: Kepler · Catalogue Source: NASA Exoplanet Archive (IPAC / Caltech).
☀️ Host Star Telemetry
| Host Star Name | Kepler-391 |
| Spectral Type | K2 III-IV |
| Effective Temp (Teff) | 5,038 K |
| Stellar Radius | 2.8790 R☉ |
| Stellar Mass | 1.2700 M☉ |
| Bolometric Luminosity | 4.791 L☉ |
| Metallicity [Fe/H] | 0.040 dex |
| Stellar Age | 4.365 Gyr |
| Apparent Magnitude | V: 13.177 · G: 12.984 |
| System Architecture | 1 Star, 2 Planets |
🪐 Orbital & Physical Dynamics
| Semi-Major Axis (a) | 0.083000 AU |
| Orbital Period | 7.418 days |
| Mean Orbital Velocity | 121.7 km/s |
| Orbital Eccentricity (e) | 0.46000 |
| Orbital Inclination (i) | 85.5000° |
| Mean Bulk Density | 388.19 g/cm³ (234.6 km/s vesc) |
| Insolation Flux | 690.0000 S⊕ (~939,090 W/m²) |
| Habitable Zone Status | Exterior / Non-HZ |
| Celestial Distance | 2835.100 LY (869.236 pc) |