Kepler-446 d
Astrophysical Synthesis & Habitability Characterization
Kepler-446 d is a confirmed super-earth exoplanet orbiting the star Kepler-446, located approximately 314.2 light-years (96.34 parsecs) from the Solar System. First announced in 2015 by Kepler using the Transit detection technique (producing an estimated primary transit depth of 0.2659% across the stellar disk), this world measures 1.35 Earth radii and has a measured mass of 2.39 Earth masses. Combining its mass and radius yields a mean bulk density of 5.36 g/cm³ (a terrestrial silicate-iron composition comparable to Earth (5.51 g/cm³)), a surface gravity of 1.31 g⊕ (12.85 m/s²), and an escape velocity of 14.9 km/s.
In its orbital architecture around Kepler-446, Kepler-446 d completes one full revolution every 5.149 Earth days at a semi-major axis of 0.0403 AU (an ultra-close orbit less than one-seventh of Mercury's 0.387 AU distance from the Sun, where strong stellar tidal forces typically induce synchronous rotation). Its mean orbital velocity is approximately 85.1 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 436 K (+163°C / +325°F) and receiving 3.31 S⊕ (~4,501 W/m²), Kepler-446 d orbits interior to the runaway greenhouse boundary where surface liquid water cannot persist.
The host star Kepler-446 is classified as a low-mass red dwarf of spectral type M4 with an effective photospheric temperature of 3,359 K, a radius of 0.24 R☉, a mass of 0.22 M☉, an estimated bolometric luminosity of 0.00658 L☉. Stellar evolution modeling indicates an estimated stellar age of 3.72 Gyr and a metallicity of [Fe/H] = -0.3 dex (metal-poor relative to the Sun). From Earth, Kepler-446 shines at apparent V-band magnitude 17.504 (faint and requiring research-grade or CCD telescopic photometry) in the Northern celestial hemisphere. The Kepler-446 system is a multi-planet architecture hosting 3 confirmed worlds, with Kepler-446 d sharing its stellar neighborhood alongside Kepler-446 b, Kepler-446 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-446 |
| Spectral Type | M4 |
| Effective Temp (Teff) | 3,359 K |
| Stellar Radius | 0.2400 R☉ |
| Stellar Mass | 0.2200 M☉ |
| Bolometric Luminosity | 0.00658 L☉ |
| Metallicity [Fe/H] | -0.300 dex |
| Stellar Age | 3.720 Gyr |
| Apparent Magnitude | V: 17.504 · G: 16.310 |
| System Architecture | 1 Star, 3 Planets |
🪐 Orbital & Physical Dynamics
| Semi-Major Axis (a) | 0.040300 AU |
| Orbital Period | 5.149 days |
| Mean Orbital Velocity | 85.1 km/s |
| Orbital Eccentricity (e) | 0.00000 |
| Orbital Inclination (i) | 88.7200° |
| Mean Bulk Density | 5.36 g/cm³ (14.9 km/s vesc) |
| Insolation Flux | 3.3070 S⊕ (~4,501 W/m²) |
| Habitable Zone Status | Exterior / Non-HZ |
| Celestial Distance | 314.200 LY (96.339 pc) |