Kepler-20 d
Astrophysical Synthesis & Habitability Characterization
Kepler-20 d is a confirmed sub-neptune exoplanet orbiting the star Kepler-20, located approximately 921.6 light-years (282.56 parsecs) from the Solar System. First announced in 2011 by Kepler using the Transit detection technique (producing an estimated primary transit depth of 0.068% across the stellar disk), this world measures 2.606 Earth radii and has a measured mass of 13.4 Earth masses. Combining its mass and radius yields a mean bulk density of 4.17 g/cm³ (a terrestrial silicate-iron composition comparable to Earth (5.51 g/cm³)), a surface gravity of 1.97 g⊕ (19.32 m/s²), and an escape velocity of 25.4 km/s.
In its orbital architecture around Kepler-20, Kepler-20 d completes one full revolution every 77.611 Earth days at a semi-major axis of 0.3474 AU (orbiting interior to Mercury's 0.387 AU benchmark in the Solar System). Its mean orbital velocity is approximately 48.7 km/s (compared to Earth's 29.8 km/s). Its moderate orbital eccentricity of e = 0.082 carries the world between 0.3189 AU at periapsis and 0.3759 AU at apoapsis, driving measurable seasonal insolation cycles.
⚠️ Thermal Regime (Irradiated / Hot Zone): At an equilibrium temperature of 430 K (+157°C / +315°F) and receiving 5.71 S⊕ (~7,771 W/m²), Kepler-20 d orbits interior to the runaway greenhouse boundary where surface liquid water cannot persist.
The host star Kepler-20 is classified as a stellar host with an effective photospheric temperature of 5,495 K, a radius of 0.9164 R☉, a mass of 0.929 M☉, an estimated bolometric luminosity of 0.687 L☉. Stellar evolution modeling indicates an estimated stellar age of 5.6 Gyr and a metallicity of [Fe/H] = 0.07 dex (metal-rich relative to the Sun). From Earth, Kepler-20 shines at apparent V-band magnitude 12.61 (faint and requiring research-grade or CCD telescopic photometry) in the Northern celestial hemisphere. The Kepler-20 system is a multi-planet architecture hosting 6 confirmed worlds, with Kepler-20 d sharing its stellar neighborhood alongside Kepler-20 b, Kepler-20 e, Kepler-20 c, Kepler-20 f, Kepler-20 g.
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-20 |
| Spectral Type | Unclassified |
| Effective Temp (Teff) | 5,495 K |
| Stellar Radius | 0.9164 R☉ |
| Stellar Mass | 0.9290 M☉ |
| Bolometric Luminosity | 0.687 L☉ |
| Metallicity [Fe/H] | 0.070 dex |
| Stellar Age | 5.600 Gyr |
| Apparent Magnitude | V: 12.610 · G: 12.454 |
| System Architecture | 2 Stars, 6 Planets |
🪐 Orbital & Physical Dynamics
| Semi-Major Axis (a) | 0.347400 AU |
| Orbital Period | 77.611 days |
| Mean Orbital Velocity | 48.7 km/s |
| Orbital Eccentricity (e) | 0.08200 |
| Orbital Inclination (i) | 89.7080° |
| Mean Bulk Density | 4.17 g/cm³ (25.4 km/s vesc) |
| Insolation Flux | 5.7100 S⊕ (~7,771 W/m²) |
| Habitable Zone Status | Exterior / Non-HZ |
| Celestial Distance | 921.600 LY (282.563 pc) |