Kepler-209 c
Astrophysical Synthesis & Habitability Characterization
Kepler-209 c is a confirmed sub-neptune exoplanet orbiting the star Kepler-209, located approximately 1881.7 light-years (576.95 parsecs) from the Solar System. First announced in 2014 by Kepler using the Transit detection technique (producing an estimated primary transit depth of 0.0924% across the stellar disk), this world measures 3.1 Earth radii (0.28 Jupiter radii) and has a measured mass of 9.8 Earth masses. Combining its mass and radius yields a mean bulk density of 1.81 g/cm³ (a low bulk density dominated by an extended hydrogen-helium gaseous envelope), a surface gravity of 1.02 g⊕ (10 m/s²), and an escape velocity of 19.9 km/s.
In its orbital architecture around Kepler-209, Kepler-209 c completes one full revolution every 41.75 Earth days at a semi-major axis of 0.231 AU (orbiting interior to Mercury's 0.387 AU benchmark in the Solar System). Its mean orbital velocity is approximately 60.2 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 486 K (+213°C / +415°F) and receiving 13.64 S⊕ (~18,560 W/m²), Kepler-209 c orbits interior to the runaway greenhouse boundary where surface liquid water cannot persist.
The host star Kepler-209 is classified as a stellar host with an effective photospheric temperature of 5,513 K, a radius of 0.935 R☉, a mass of 0.971 M☉, an estimated bolometric luminosity of 0.725 L☉. Stellar evolution modeling indicates an estimated stellar age of 1.95 Gyr and a metallicity of [Fe/H] = 0.11 dex (metal-rich relative to the Sun). From Earth, Kepler-209 shines at apparent V-band magnitude 14.132 (faint and requiring research-grade or CCD telescopic photometry) in the Northern celestial hemisphere. The Kepler-209 system is a multi-planet architecture hosting 2 confirmed worlds, with Kepler-209 c sharing its stellar neighborhood alongside Kepler-209 b.
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-209 |
| Spectral Type | Unclassified |
| Effective Temp (Teff) | 5,513 K |
| Stellar Radius | 0.9350 R☉ |
| Stellar Mass | 0.9710 M☉ |
| Bolometric Luminosity | 0.725 L☉ |
| Metallicity [Fe/H] | 0.110 dex |
| Stellar Age | 1.950 Gyr |
| Apparent Magnitude | V: 14.132 · G: 13.952 |
| System Architecture | 1 Star, 2 Planets |
🪐 Orbital & Physical Dynamics
| Semi-Major Axis (a) | 0.231000 AU |
| Orbital Period | 41.75 days |
| Mean Orbital Velocity | 60.2 km/s |
| Orbital Eccentricity (e) | 0.00000 |
| Orbital Inclination (i) | 89.7700° |
| Mean Bulk Density | 1.81 g/cm³ (19.9 km/s vesc) |
| Insolation Flux | 13.6370 S⊕ (~18,560 W/m²) |
| Habitable Zone Status | Exterior / Non-HZ |
| Celestial Distance | 1881.700 LY (576.946 pc) |