HD 128356 b
Astrophysical Synthesis & Habitability Characterization
HD 128356 b is a confirmed gas giant exoplanet orbiting the star HD 128356, located approximately 85.3 light-years (26.16 parsecs) from the Solar System. First announced in 2016 by Multiple Observatories using the Radial Velocity detection technique (measuring periodic Doppler reflex motion in the host star's spectrum), this world measures 13.9 Earth radii (1.24 Jupiter radii) and has a measured mass of 282.869 Earth masses (0.89 Jupiter 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 1.46 g⊕ (14.32 m/s²), and an escape velocity of 50.5 km/s.
In its orbital architecture around HD 128356, HD 128356 b completes one full revolution every 298.2 Earth days at a semi-major axis of 0.87 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 31.7 km/s (compared to Earth's 29.8 km/s). The planet follows a highly eccentric trajectory (e = 0.57, swinging from 0.3741 AU at periapsis out to 1.3659 AU at apoapsis), pointing to past dynamical scattering or secular gravitational perturbations.
☁️ Atmospheric Regime (Volatile / Gas Envelope): With a radius of 13.900 R⊕, HD 128356 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 128356 is classified as an orange K-type star of spectral type K3 V with an effective photospheric temperature of 4,875 K, a radius of 0.85 R☉, a mass of 0.65 M☉, an estimated bolometric luminosity of 0.366 L☉. Stellar evolution modeling indicates an estimated stellar age of 10 Gyr and a metallicity of [Fe/H] = 0.17 dex (metal-rich relative to the Sun). From Earth, HD 128356 shines at apparent V-band magnitude 8.241 (accessible to binoculars and small amateur telescopes) in the Southern celestial hemisphere.
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: Multiple Observatories · Catalogue Source: NASA Exoplanet Archive (IPAC / Caltech).
☀️ Host Star Telemetry
| Host Star Name | HD 128356 |
| Spectral Type | K3 V |
| Effective Temp (Teff) | 4,875 K |
| Stellar Radius | 0.8500 R☉ |
| Stellar Mass | 0.6500 M☉ |
| Bolometric Luminosity | 0.366 L☉ |
| Metallicity [Fe/H] | 0.170 dex |
| Stellar Age | 10.000 Gyr |
| Apparent Magnitude | V: 8.241 · G: 7.977 |
| System Architecture | 1 Star, 1 Planet |
🪐 Orbital & Physical Dynamics
| Semi-Major Axis (a) | 0.870000 AU |
| Orbital Period | 298.2 days |
| Mean Orbital Velocity | 31.7 km/s |
| Orbital Eccentricity (e) | 0.57000 |
| Orbital Inclination (i) | — |
| Mean Bulk Density | 0.58 g/cm³ (50.5 km/s vesc) |
| Insolation Flux | 0.4756 S⊕ (~647 W/m²) |
| Habitable Zone Status | Exterior / Non-HZ |
| Celestial Distance | 85.300 LY (26.155 pc) |