Kepler-399 d
Astrophysical Synthesis & Habitability Characterization
Kepler-399 d is a confirmed super-earth exoplanet orbiting the star Kepler-399, located approximately 2415.3 light-years (740.54 parsecs) from the Solar System. First announced in 2014 by Kepler using the Transit detection technique (producing an estimated primary transit depth of 0.0649% across the stellar disk), this world measures 1.89 Earth radii and has a measured mass of 4.23 Earth masses. Combining its mass and radius yields a mean bulk density of 3.45 g/cm³ (an intermediate density indicative of a deep water-ice mantle or dense volatile envelope), a surface gravity of 1.18 g⊕ (11.57 m/s²), and an escape velocity of 16.7 km/s.
In its orbital architecture around Kepler-399, Kepler-399 d completes one full revolution every 58.035 Earth days at a semi-major axis of 0.261 AU (orbiting interior to Mercury's 0.387 AU benchmark in the Solar System). Its mean orbital velocity is approximately 48.9 km/s (compared to Earth's 29.8 km/s). The orbit is modeled as circular (e = 0.00), maintaining a constant stellar distance throughout its year.
⚠️ Thermal Regime (Irradiated / Hot Zone): At an equilibrium temperature of 486 K (+213°C / +415°F) and receiving 5.57 S⊕ (~7,581 W/m²), Kepler-399 d orbits interior to the runaway greenhouse boundary where surface liquid water cannot persist.
The host star Kepler-399 is classified as a stellar host with an effective photospheric temperature of 5,502 K, a radius of 0.68 R☉, a mass of 0.902 M☉, an estimated bolometric luminosity of 0.38 L☉. Stellar evolution modeling indicates an estimated stellar age of 3.98 Gyr and a metallicity of [Fe/H] = 0.14 dex (metal-rich relative to the Sun). From Earth, Kepler-399 shines at apparent V-band magnitude 14.686 (faint and requiring research-grade or CCD telescopic photometry) in the Northern celestial hemisphere. The Kepler-399 system is a multi-planet architecture hosting 3 confirmed worlds, with Kepler-399 d sharing its stellar neighborhood alongside Kepler-399 b, Kepler-399 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-399 |
| Spectral Type | Unclassified |
| Effective Temp (Teff) | 5,502 K |
| Stellar Radius | 0.6800 R☉ |
| Stellar Mass | 0.9020 M☉ |
| Bolometric Luminosity | 0.38 L☉ |
| Metallicity [Fe/H] | 0.140 dex |
| Stellar Age | 3.980 Gyr |
| Apparent Magnitude | V: 14.686 · G: 14.528 |
| System Architecture | 1 Star, 3 Planets |
🪐 Orbital & Physical Dynamics
| Semi-Major Axis (a) | 0.261000 AU |
| Orbital Period | 58.035 days |
| Mean Orbital Velocity | 48.9 km/s |
| Orbital Eccentricity (e) | 0.00000 |
| Orbital Inclination (i) | 89.9800° |
| Mean Bulk Density | 3.45 g/cm³ (16.7 km/s vesc) |
| Insolation Flux | 5.5700 S⊕ (~7,581 W/m²) |
| Habitable Zone Status | Exterior / Non-HZ |
| Celestial Distance | 2415.300 LY (740.537 pc) |