Kepler-309 c
Astrophysical Synthesis & Habitability Characterization
Kepler-309 c is a confirmed sub-neptune exoplanet orbiting the star Kepler-309, located approximately 1776.1 light-years (544.57 parsecs) from the Solar System. First announced in 2014 by Kepler using the Transit detection technique (producing an estimated primary transit depth of 0.1019% across the stellar disk), this world measures 2.51 Earth radii and has a measured mass of 6.85 Earth masses. Combining its mass and radius yields a mean bulk density of 2.39 g/cm³ (an intermediate density indicative of a deep water-ice mantle or dense volatile envelope), a surface gravity of 1.09 g⊕ (10.69 m/s²), and an escape velocity of 18.5 km/s.
In its orbital architecture around Kepler-309, Kepler-309 c completes one full revolution every 105.356 Earth days at a semi-major axis of 0.401 AU (situated at an orbital separation between Mercury's 0.39 AU and Venus's 0.72 AU). Its mean orbital velocity is approximately 41.4 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.
☁️ Atmospheric Regime (Volatile / Gas Envelope): With a radius of 2.510 R⊕, Kepler-309 c 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-309 is classified as a low-mass red dwarf of spectral type M0 V with an effective photospheric temperature of 4,713 K, a radius of 0.721 R☉, a mass of 0.816 M☉, an estimated bolometric luminosity of 0.23 L☉. Stellar evolution modeling indicates an estimated stellar age of 3.02 Gyr and a metallicity of [Fe/H] = 0.27 dex (metal-rich relative to the Sun). From Earth, Kepler-309 shines at apparent V-band magnitude 15.418 (faint and requiring research-grade or CCD telescopic photometry) in the Northern celestial hemisphere. The Kepler-309 system is a multi-planet architecture hosting 2 confirmed worlds, with Kepler-309 c sharing its stellar neighborhood alongside Kepler-309 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-309 |
| Spectral Type | M0 V |
| Effective Temp (Teff) | 4,713 K |
| Stellar Radius | 0.7210 R☉ |
| Stellar Mass | 0.8160 M☉ |
| Bolometric Luminosity | 0.23 L☉ |
| Metallicity [Fe/H] | 0.270 dex |
| Stellar Age | 3.020 Gyr |
| Apparent Magnitude | V: 15.418 · G: 15.126 |
| System Architecture | 1 Star, 2 Planets |
🪐 Orbital & Physical Dynamics
| Semi-Major Axis (a) | 0.401000 AU |
| Orbital Period | 105.356 days |
| Mean Orbital Velocity | 41.4 km/s |
| Orbital Eccentricity (e) | 0.00000 |
| Orbital Inclination (i) | 89.7100° |
| Mean Bulk Density | 2.39 g/cm³ (18.5 km/s vesc) |
| Insolation Flux | 2.1420 S⊕ (~2,915 W/m²) |
| Habitable Zone Status | Exterior / Non-HZ |
| Celestial Distance | 1776.100 LY (544.568 pc) |