HD 20868 b
Astrophysical Synthesis & Habitability Characterization
HD 20868 b is a confirmed gas giant exoplanet orbiting the star HD 20868, located approximately 155.6 light-years (47.72 parsecs) from the Solar System. First announced in 2008 by La Silla Observatory using the Radial Velocity detection technique (measuring periodic Doppler reflex motion in the host star's spectrum), this world measures 13.7 Earth radii (1.22 Jupiter radii) and has a measured mass of 397.288 Earth masses (1.25 Jupiter masses). Combining its mass and radius yields a mean bulk density of 0.85 g/cm³ (a low bulk density dominated by an extended hydrogen-helium gaseous envelope), a surface gravity of 2.12 g⊕ (20.79 m/s²), and an escape velocity of 60.2 km/s.
In its orbital architecture around HD 20868, HD 20868 b completes one full revolution every 380.85 Earth days (1.04Julian years) at a semi-major axis of 0.95 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 27.1 km/s (compared to Earth's 29.8 km/s). The planet follows a highly eccentric trajectory (e = 0.75, swinging from 0.2375 AU at periapsis out to 1.6625 AU at apoapsis), pointing to past dynamical scattering or secular gravitational perturbations.
☁️ Atmospheric Regime (Volatile / Gas Envelope): With a radius of 13.700 R⊕, HD 20868 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 20868 is classified as an orange K-type star of spectral type K3/4 IV with an effective photospheric temperature of 4,802 K, a radius of 0.76 R☉, a mass of 0.39 M☉, an estimated bolometric luminosity of 0.276 L☉. Stellar evolution modeling indicates an estimated stellar age of 4.5 Gyr and a metallicity of [Fe/H] = 0.05 dex (near-solar heavy-element abundance). From Earth, HD 20868 shines at apparent V-band magnitude 9.92 (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: La Silla Observatory · Catalogue Source: NASA Exoplanet Archive (IPAC / Caltech).
☀️ Host Star Telemetry
| Host Star Name | HD 20868 |
| Spectral Type | K3/4 IV |
| Effective Temp (Teff) | 4,802 K |
| Stellar Radius | 0.7600 R☉ |
| Stellar Mass | 0.3900 M☉ |
| Bolometric Luminosity | 0.276 L☉ |
| Metallicity [Fe/H] | 0.050 dex |
| Stellar Age | 4.500 Gyr |
| Apparent Magnitude | V: 9.920 · G: 9.613 |
| System Architecture | 1 Star, 1 Planet |
🪐 Orbital & Physical Dynamics
| Semi-Major Axis (a) | 0.950000 AU |
| Orbital Period | 380.85 days |
| Mean Orbital Velocity | 27.1 km/s |
| Orbital Eccentricity (e) | 0.75000 |
| Orbital Inclination (i) | — |
| Mean Bulk Density | 0.85 g/cm³ (60.2 km/s vesc) |
| Insolation Flux | 0.3278 S⊕ (~446 W/m²) |
| Habitable Zone Status | Exterior / Non-HZ |
| Celestial Distance | 155.600 LY (47.722 pc) |