HD 128311 c
Astrophysical Synthesis & Habitability Characterization
HD 128311 c is a confirmed gas giant exoplanet orbiting the star HD 128311, located approximately 53.3 light-years (16.33 parsecs) from the Solar System. First announced in 2005 by W. M. Keck Observatory using the Radial Velocity detection technique (measuring periodic Doppler reflex motion in the host star's spectrum), this world measures 13 Earth radii (1.16 Jupiter radii) and has a measured mass of 1204.258 Earth masses (3.79 Jupiter masses). Combining its mass and radius yields a mean bulk density of 3.02 g/cm³ (an intermediate density indicative of a deep water-ice mantle or dense volatile envelope), a surface gravity of 7.13 g⊕ (69.92 m/s²), and an escape velocity of 107.7 km/s.
In its orbital architecture around HD 128311, HD 128311 c completes one full revolution every 921.538 Earth days (2.52Julian years) at a semi-major axis of 1.74 AU (corresponding to the middle-system regime between Mars at 1.52 AU and Jupiter at 5.20 AU). Its mean orbital velocity is approximately 20.5 km/s (compared to Earth's 29.8 km/s). Its moderate orbital eccentricity of e = 0.159 carries the world between 1.4633 AU at periapsis and 2.0167 AU at apoapsis, driving measurable seasonal insolation cycles.
❄️ Thermal Regime (Cryogenic Zone): With an equilibrium temperature of 158 K (-115°C / -175°F) and an incident flux of 0.10 S⊕ (~138 W/m²), surface water and volatile species on HD 128311 c would remain locked in cryogenic ice phases unless warmed by subsurface tidal or radiogenic heating.
The host star HD 128311 is classified as an orange K-type star of spectral type K0 with an effective photospheric temperature of 4,965 K, a radius of 0.76 R☉, a mass of 0.828 M☉, an estimated bolometric luminosity of 0.315 L☉. Stellar evolution modeling indicates an estimated stellar age of 0.96 Gyr and a metallicity of [Fe/H] = 0.08 dex (metal-rich relative to the Sun). From Earth, HD 128311 shines at apparent V-band magnitude 7.48 (accessible to binoculars and small amateur telescopes) in the Northern celestial hemisphere. The HD 128311 system is a multi-planet architecture hosting 2 confirmed worlds, with HD 128311 c sharing its stellar neighborhood alongside HD 128311 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: W. M. Keck Observatory · Catalogue Source: NASA Exoplanet Archive (IPAC / Caltech).
☀️ Host Star Telemetry
| Host Star Name | HD 128311 |
| Spectral Type | K0 |
| Effective Temp (Teff) | 4,965 K |
| Stellar Radius | 0.7600 R☉ |
| Stellar Mass | 0.8280 M☉ |
| Bolometric Luminosity | 0.315 L☉ |
| Metallicity [Fe/H] | 0.080 dex |
| Stellar Age | 0.960 Gyr |
| Apparent Magnitude | V: 7.480 · G: 7.167 |
| System Architecture | 1 Star, 2 Planets |
🪐 Orbital & Physical Dynamics
| Semi-Major Axis (a) | 1.740000 AU |
| Orbital Period | 921.538 days |
| Mean Orbital Velocity | 20.5 km/s |
| Orbital Eccentricity (e) | 0.15900 |
| Orbital Inclination (i) | 55.9500° |
| Mean Bulk Density | 3.02 g/cm³ (107.7 km/s vesc) |
| Insolation Flux | 0.1017 S⊕ (~138 W/m²) |
| Habitable Zone Status | Exterior / Non-HZ |
| Celestial Distance | 53.300 LY (16.329 pc) |