Kepler-222 d
Astrophysical Synthesis & Habitability Characterization
Kepler-222 d is a confirmed sub-neptune exoplanet orbiting the star Kepler-222, located approximately 2475 light-years (758.85 parsecs) from the Solar System. First announced in 2014 by Kepler using the Transit detection technique (producing an estimated primary transit depth of 0.1515% across the stellar disk), this world measures 3.69 Earth radii (0.33 Jupiter radii) and has a measured mass of 13.2 Earth masses. Combining its mass and radius yields a mean bulk density of 1.45 g/cm³ (a low bulk density dominated by an extended hydrogen-helium gaseous envelope), a surface gravity of 0.97 g⊕ (9.51 m/s²), and an escape velocity of 21.2 km/s.
In its orbital architecture around Kepler-222, Kepler-222 d completes one full revolution every 28.082 Earth days at a semi-major axis of 0.18 AU (orbiting interior to Mercury's 0.387 AU benchmark in the Solar System). Its mean orbital velocity is approximately 69.7 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 522 K (+249°C / +480°F) and receiving 15.09 S⊕ (~20,533 W/m²), Kepler-222 d orbits interior to the runaway greenhouse boundary where surface liquid water cannot persist.
The host star Kepler-222 is classified as a stellar host with an effective photospheric temperature of 5,433 K, a radius of 0.869 R☉, a mass of 0.933 M☉, an estimated bolometric luminosity of 0.59 L☉. Stellar evolution modeling indicates an estimated stellar age of 1.17 Gyr and a metallicity of [Fe/H] = 0.33 dex (metal-rich relative to the Sun). From Earth, Kepler-222 shines at apparent V-band magnitude 15.407 (faint and requiring research-grade or CCD telescopic photometry) in the Northern celestial hemisphere. The Kepler-222 system is a multi-planet architecture hosting 3 confirmed worlds, with Kepler-222 d sharing its stellar neighborhood alongside Kepler-222 b, Kepler-222 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-222 |
| Spectral Type | Unclassified |
| Effective Temp (Teff) | 5,433 K |
| Stellar Radius | 0.8690 R☉ |
| Stellar Mass | 0.9330 M☉ |
| Bolometric Luminosity | 0.59 L☉ |
| Metallicity [Fe/H] | 0.330 dex |
| Stellar Age | 1.170 Gyr |
| Apparent Magnitude | V: 15.407 · G: 15.060 |
| System Architecture | 1 Star, 3 Planets |
🪐 Orbital & Physical Dynamics
| Semi-Major Axis (a) | 0.180000 AU |
| Orbital Period | 28.082 days |
| Mean Orbital Velocity | 69.7 km/s |
| Orbital Eccentricity (e) | 0.00000 |
| Orbital Inclination (i) | 89.4600° |
| Mean Bulk Density | 1.45 g/cm³ (21.2 km/s vesc) |
| Insolation Flux | 15.0870 S⊕ (~20,533 W/m²) |
| Habitable Zone Status | Exterior / Non-HZ |
| Celestial Distance | 2475.000 LY (758.854 pc) |