HD 128311 b
Astrophysical Synthesis & Habitability Characterization
HD 128311 b 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 2002 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 13.5 Earth radii (1.2 Jupiter radii) and has a measured mass of 562.241 Earth masses (1.77 Jupiter masses). Combining its mass and radius yields a mean bulk density of 1.26 g/cm³ (a low bulk density dominated by an extended hydrogen-helium gaseous envelope), a surface gravity of 3.08 g⊕ (30.2 m/s²), and an escape velocity of 72.2 km/s.
In its orbital architecture around HD 128311, HD 128311 b completes one full revolution every 453.019 Earth days (1.24Julian years) at a semi-major axis of 1.084 AU (comparable to the inner terrestrial zone between Venus, Earth at 1.00 AU, and Mars at 1.52 AU). Its mean orbital velocity is approximately 26 km/s (compared to Earth's 29.8 km/s). The planet follows a highly eccentric trajectory (e = 0.303, swinging from 0.7555 AU at periapsis out to 1.4125 AU at apoapsis), pointing to past dynamical scattering or secular gravitational perturbations.
☁️ Atmospheric Regime (Volatile / Gas Envelope): With a radius of 13.500 R⊕, HD 128311 b 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 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 b sharing its stellar neighborhood alongside HD 128311 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: 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) | 1.084000 AU |
| Orbital Period | 453.019 days |
| Mean Orbital Velocity | 26 km/s |
| Orbital Eccentricity (e) | 0.30300 |
| Orbital Inclination (i) | 30.0000° |
| Mean Bulk Density | 1.26 g/cm³ (72.2 km/s vesc) |
| Insolation Flux | 0.2620 S⊕ (~357 W/m²) |
| Habitable Zone Status | Exterior / Non-HZ |
| Celestial Distance | 53.300 LY (16.329 pc) |