Kepler-1921 b
Astrophysical Synthesis & Habitability Characterization
Kepler-1921 b is a confirmed sub-neptune exoplanet orbiting the star Kepler-1921, located approximately 3030 light-years (928.99 parsecs) from the Solar System. First announced in 2021 by Kepler using the Transit detection technique (producing an estimated primary transit depth of 0.0176% across the stellar disk), this world measures 2.895 Earth radii and has a measured mass of 8.72 Earth masses. Combining its mass and radius yields a mean bulk density of 1.98 g/cm³ (a low bulk density dominated by an extended hydrogen-helium gaseous envelope), a surface gravity of 1.04 g⊕ (10.2 m/s²), and an escape velocity of 19.4 km/s.
In its orbital architecture around Kepler-1921, Kepler-1921 b completes one full revolution every 20.035 Earth days at a semi-major axis of 0.15865 AU (orbiting interior to Mercury's 0.387 AU benchmark in the Solar System). Its mean orbital velocity is approximately 86.1 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 967 K (+694°C / +1281°F) and receiving 216.20 S⊕ (~294,248 W/m²), Kepler-1921 b orbits interior to the runaway greenhouse boundary where surface liquid water cannot persist.
The host star Kepler-1921 is classified as a stellar host with an effective photospheric temperature of 6,120 K, a radius of 2 R☉, a mass of 1.251 M☉, an estimated bolometric luminosity of 5.034 L☉. Stellar evolution modeling indicates an estimated stellar age of 3.39 Gyr and a metallicity of [Fe/H] = 0.02 dex (near-solar heavy-element abundance). From Earth, Kepler-1921 shines at apparent V-band magnitude 13.103 (faint and requiring research-grade or CCD telescopic photometry) in the Northern celestial hemisphere. The Kepler-1921 system is a multi-planet architecture hosting 2 confirmed worlds, with Kepler-1921 b sharing its stellar neighborhood alongside Kepler-1921 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-1921 |
| Spectral Type | Unclassified |
| Effective Temp (Teff) | 6,120 K |
| Stellar Radius | 2.0000 R☉ |
| Stellar Mass | 1.2510 M☉ |
| Bolometric Luminosity | 5.034 L☉ |
| Metallicity [Fe/H] | 0.020 dex |
| Stellar Age | 3.390 Gyr |
| Apparent Magnitude | V: 13.103 · G: 12.984 |
| System Architecture | 1 Star, 2 Planets |
🪐 Orbital & Physical Dynamics
| Semi-Major Axis (a) | 0.158650 AU |
| Orbital Period | 20.035 days |
| Mean Orbital Velocity | 86.1 km/s |
| Orbital Eccentricity (e) | 0.00000 |
| Orbital Inclination (i) | 89.5300° |
| Mean Bulk Density | 1.98 g/cm³ (19.4 km/s vesc) |
| Insolation Flux | 216.2000 S⊕ (~294,248 W/m²) |
| Habitable Zone Status | Exterior / Non-HZ |
| Celestial Distance | 3030.000 LY (928.989 pc) |