Kepler-21 b
Astrophysical Synthesis & Habitability Characterization
Kepler-21 b is a confirmed super-earth exoplanet orbiting the star Kepler-21, located approximately 354 light-years (108.52 parsecs) from the Solar System. First announced in 2012 by Kepler using the Transit detection technique (producing an estimated primary transit depth of 0.0062% across the stellar disk), this world measures 1.639 Earth radii and has a measured mass of 7.5 Earth masses. Combining its mass and radius yields a mean bulk density of 9.39 g/cm³ (an iron-enriched, high-compression rocky core), a surface gravity of 2.79 g⊕ (27.36 m/s²), and an escape velocity of 23.9 km/s.
In its orbital architecture around Kepler-21, Kepler-21 b completes one full revolution every 2.786 Earth days at a semi-major axis of 0.0434 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 169.5 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 2015 K (+1742°C / +3168°F) and receiving 2,749.00 S⊕ (~3,741,389 W/m²), Kepler-21 b orbits interior to the runaway greenhouse boundary where surface liquid water cannot persist.
The host star Kepler-21 is classified as a warm F-type star of spectral type F6 IV with an effective photospheric temperature of 6,305 K, a radius of 1.902 R☉, a mass of 1.408 M☉, an estimated bolometric luminosity of 5.129 L☉. Stellar evolution modeling indicates an estimated stellar age of 2.84 Gyr and a metallicity of [Fe/H] = -0.03 dex (near-solar heavy-element abundance). From Earth, Kepler-21 shines at apparent V-band magnitude 8.25 (accessible to binoculars and small amateur telescopes) 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-21 |
| Spectral Type | F6 IV |
| Effective Temp (Teff) | 6,305 K |
| Stellar Radius | 1.9020 R☉ |
| Stellar Mass | 1.4080 M☉ |
| Bolometric Luminosity | 5.129 L☉ |
| Metallicity [Fe/H] | -0.030 dex |
| Stellar Age | 2.840 Gyr |
| Apparent Magnitude | V: 8.250 · G: 8.128 |
| System Architecture | 2 Stars, 1 Planet |
🪐 Orbital & Physical Dynamics
| Semi-Major Axis (a) | 0.043400 AU |
| Orbital Period | 2.786 days |
| Mean Orbital Velocity | 169.5 km/s |
| Orbital Eccentricity (e) | 0.00000 |
| Orbital Inclination (i) | 83.2000° |
| Mean Bulk Density | 9.39 g/cm³ (23.9 km/s vesc) |
| Insolation Flux | 2749.0000 S⊕ (~3,741,389 W/m²) |
| Habitable Zone Status | Exterior / Non-HZ |
| Celestial Distance | 354.000 LY (108.522 pc) |