Kepler-421 b
Astrophysical Synthesis & Habitability Characterization
Kepler-421 b is a confirmed sub-neptune exoplanet orbiting the star Kepler-421, located approximately 1136.8 light-years (348.56 parsecs) from the Solar System. First announced in 2014 by Kepler using the Transit detection technique (producing an estimated primary transit depth of 0.2538% across the stellar disk), this world measures 4.16 Earth radii (0.37 Jupiter radii) and has a measured mass of 16.1 Earth masses. Combining its mass and radius yields a mean bulk density of 1.23 g/cm³ (a low bulk density dominated by an extended hydrogen-helium gaseous envelope), a surface gravity of 0.93 g⊕ (9.12 m/s²), and an escape velocity of 22 km/s.
In its orbital architecture around Kepler-421, Kepler-421 b completes one full revolution every 704.198 Earth days (1.93Julian years) at a semi-major axis of 1.219 AU (comparable to the inner terrestrial zone between Venus, Earth at 1.00 AU, and Mars at 1.52 AU). Its mean orbital velocity is approximately 18.8 km/s (compared to Earth's 29.8 km/s). With a near-circular eccentricity of e = 0.041 (ranging from 1.169 AU at periapsis to 1.269 AU at apoapsis), the planet experiences minimal orbital distance variation.
☁️ Atmospheric Regime (Volatile / Gas Envelope): With a radius of 4.160 R⊕, Kepler-421 b exceeds the empirical ~1.8–2.2 R⊕ Fulton radius valley threshold, indicating a substantial hydrogen-helium or supercritical volatile envelope without a conventional solid terrestrial surface.
The host star Kepler-421 is classified as a solar-type yellow star of spectral type G9/K0 with an effective photospheric temperature of 5,308 K, a radius of 0.757 R☉, a mass of 0.794 M☉, an estimated bolometric luminosity of 0.408 L☉. Stellar evolution modeling indicates an estimated stellar age of 4 Gyr and a metallicity of [Fe/H] = -0.25 dex (metal-poor relative to the Sun). From Earth, Kepler-421 shines at apparent V-band magnitude 13.611 (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-421 |
| Spectral Type | G9/K0 |
| Effective Temp (Teff) | 5,308 K |
| Stellar Radius | 0.7570 R☉ |
| Stellar Mass | 0.7940 M☉ |
| Bolometric Luminosity | 0.408 L☉ |
| Metallicity [Fe/H] | -0.250 dex |
| Stellar Age | 4.000 Gyr |
| Apparent Magnitude | V: 13.611 · G: 13.348 |
| System Architecture | 1 Star, 1 Planet |
🪐 Orbital & Physical Dynamics
| Semi-Major Axis (a) | 1.219000 AU |
| Orbital Period | 704.198 days |
| Mean Orbital Velocity | 18.8 km/s |
| Orbital Eccentricity (e) | 0.04100 |
| Orbital Inclination (i) | 89.9650° |
| Mean Bulk Density | 1.23 g/cm³ (22 km/s vesc) |
| Insolation Flux | 0.2000 S⊕ (~272 W/m²) |
| Habitable Zone Status | Exterior / Non-HZ |
| Celestial Distance | 1136.800 LY (348.558 pc) |