Kepler-821 b
Astrophysical Synthesis & Habitability Characterization
Kepler-821 b is a confirmed super-earth exoplanet orbiting the star Kepler-821, located approximately 2004.5 light-years (614.57 parsecs) from the Solar System. First announced in 2016 by Kepler using the Transit detection technique (producing an estimated primary transit depth of 0.0225% across the stellar disk), this world measures 1.31 Earth radii and has a measured mass of 2.27 Earth masses. Combining its mass and radius yields a mean bulk density of 5.57 g/cm³ (a terrestrial silicate-iron composition comparable to Earth (5.51 g/cm³)), a surface gravity of 1.32 g⊕ (12.94 m/s²), and an escape velocity of 14.7 km/s.
In its orbital architecture around Kepler-821, Kepler-821 b completes one full revolution every 1.923 Earth days at a semi-major axis of 0.029 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 164.1 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 1215 K (+942°C / +1728°F) and receiving 496.82 S⊕ (~676,172 W/m²), Kepler-821 b orbits interior to the runaway greenhouse boundary where surface liquid water cannot persist.
The host star Kepler-821 is classified as a stellar host with an effective photospheric temperature of 5,191 K, a radius of 0.8 R☉, a mass of 0.84 M☉, an estimated bolometric luminosity of 0.417 L☉. Stellar evolution modeling indicates an estimated stellar age of 4.79 Gyr and a metallicity of [Fe/H] = -0.03 dex (near-solar heavy-element abundance). From Earth, Kepler-821 shines at apparent V-band magnitude 15.153 (faint and requiring research-grade or CCD telescopic photometry) in the Northern celestial hemisphere.
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-821 |
| Spectral Type | Unclassified |
| Effective Temp (Teff) | 5,191 K |
| Stellar Radius | 0.8000 R☉ |
| Stellar Mass | 0.8400 M☉ |
| Bolometric Luminosity | 0.417 L☉ |
| Metallicity [Fe/H] | -0.030 dex |
| Stellar Age | 4.790 Gyr |
| Apparent Magnitude | V: 15.153 · G: 14.818 |
| System Architecture | 1 Star, 1 Planet |
🪐 Orbital & Physical Dynamics
| Semi-Major Axis (a) | 0.029000 AU |
| Orbital Period | 1.923 days |
| Mean Orbital Velocity | 164.1 km/s |
| Orbital Eccentricity (e) | 0.00000 |
| Orbital Inclination (i) | 89.7400° |
| Mean Bulk Density | 5.57 g/cm³ (14.7 km/s vesc) |
| Insolation Flux | 496.8200 S⊕ (~676,172 W/m²) |
| Habitable Zone Status | Exterior / Non-HZ |
| Celestial Distance | 2004.500 LY (614.571 pc) |