Kepler-391 c
Astrophysical Synthesis & Habitability Characterization
Kepler-391 c 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 0.0084% across the stellar disk), this world measures 2.88 Earth radii and has a measured mass of 450 Earth masses (1.42 Jupiter masses). Combining its mass and radius yields a mean bulk density of 103.87 g/cm³ (an iron-enriched, high-compression rocky core), a surface gravity of 54.25 g⊕ (532.01 m/s²), and an escape velocity of 139.8 km/s.
In its orbital architecture around Kepler-391, Kepler-391 c completes one full revolution every 20.501 Earth days at a semi-major axis of 0.164 AU (orbiting interior to Mercury's 0.387 AU benchmark in the Solar System). Its mean orbital velocity is approximately 87 km/s (compared to Earth's 29.8 km/s). The planet follows a highly eccentric trajectory (e = 0.27, swinging from 0.1197 AU at periapsis out to 0.2083 AU at apoapsis), pointing to past dynamical scattering or secular gravitational perturbations.
⚠️ Thermal Regime (Irradiated / Hot Zone): At an equilibrium temperature of 662 K (+389°C / +732°F) and receiving 179.00 S⊕ (~243,619 W/m²), Kepler-391 c 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 c sharing its stellar neighborhood alongside Kepler-391 b.
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) | 0.164000 AU |
| Orbital Period | 20.501 days |
| Mean Orbital Velocity | 87 km/s |
| Orbital Eccentricity (e) | 0.27000 |
| Orbital Inclination (i) | 87.2700° |
| Mean Bulk Density | 103.87 g/cm³ (139.8 km/s vesc) |
| Insolation Flux | 179.0000 S⊕ (~243,619 W/m²) |
| Habitable Zone Status | Exterior / Non-HZ |
| Celestial Distance | 2835.100 LY (869.236 pc) |