Kepler-18 c
Astrophysical Synthesis & Habitability Characterization
Kepler-18 c is a confirmed sub-neptune exoplanet orbiting the star Kepler-18, located approximately 1412.4 light-years (433.05 parsecs) from the Solar System. First announced in 2011 by Kepler using the Transit detection technique (producing an estimated primary transit depth of 0.2064% across the stellar disk), this world measures 5.49 Earth radii (0.49 Jupiter radii) and has a measured mass of 17.3 Earth masses. Combining its mass and radius yields a mean bulk density of 0.58 g/cm³ (a low bulk density dominated by an extended hydrogen-helium gaseous envelope), a surface gravity of 0.57 g⊕ (5.59 m/s²), and an escape velocity of 19.9 km/s.
In its orbital architecture around Kepler-18, Kepler-18 c completes one full revolution every 7.642 Earth days at a semi-major axis of 0.0752 AU (orbiting interior to Mercury's 0.387 AU benchmark in the Solar System). Its mean orbital velocity is approximately 107.1 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 899 K (+626°C / +1159°F) and receiving 126.08 S⊕ (~171,596 W/m²), Kepler-18 c orbits interior to the runaway greenhouse boundary where surface liquid water cannot persist.
The host star Kepler-18 is classified as a stellar host with an effective photospheric temperature of 5,345 K, a radius of 1.108 R☉, a mass of 0.972 M☉, an estimated bolometric luminosity of 0.899 L☉. Stellar evolution modeling indicates an estimated stellar age of 10 Gyr and a metallicity of [Fe/H] = 0.19 dex (metal-rich relative to the Sun). From Earth, Kepler-18 shines at apparent V-band magnitude 13.771 (faint and requiring research-grade or CCD telescopic photometry) in the Northern celestial hemisphere. The Kepler-18 system is a multi-planet architecture hosting 3 confirmed worlds, with Kepler-18 c sharing its stellar neighborhood alongside Kepler-18 b, Kepler-18 d.
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-18 |
| Spectral Type | Unclassified |
| Effective Temp (Teff) | 5,345 K |
| Stellar Radius | 1.1080 R☉ |
| Stellar Mass | 0.9720 M☉ |
| Bolometric Luminosity | 0.899 L☉ |
| Metallicity [Fe/H] | 0.190 dex |
| Stellar Age | 10.000 Gyr |
| Apparent Magnitude | V: 13.771 · G: 13.539 |
| System Architecture | 1 Star, 3 Planets |
🪐 Orbital & Physical Dynamics
| Semi-Major Axis (a) | 0.075200 AU |
| Orbital Period | 7.642 days |
| Mean Orbital Velocity | 107.1 km/s |
| Orbital Eccentricity (e) | 0.00000 |
| Orbital Inclination (i) | 87.6800° |
| Mean Bulk Density | 0.58 g/cm³ (19.9 km/s vesc) |
| Insolation Flux | 126.0810 S⊕ (~171,596 W/m²) |
| Habitable Zone Status | Exterior / Non-HZ |
| Celestial Distance | 1412.400 LY (433.047 pc) |