HD 99109 b
Astrophysical Synthesis & Habitability Characterization
HD 99109 b is a confirmed gas giant exoplanet orbiting the star HD 99109, located approximately 178.7 light-years (54.79 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 14.3 Earth radii (1.28 Jupiter radii) and has a measured mass of 139.845 Earth masses (0.44 Jupiter masses). Combining its mass and radius yields a mean bulk density of 0.26 g/cm³ (a low bulk density dominated by an extended hydrogen-helium gaseous envelope), a surface gravity of 0.68 g⊕ (6.67 m/s²), and an escape velocity of 35 km/s.
In its orbital architecture around HD 99109, HD 99109 b completes one full revolution every 439.3 Earth days (1.2Julian years) at a semi-major axis of 1.11 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 27.5 km/s (compared to Earth's 29.8 km/s). Its moderate orbital eccentricity of e = 0.09 carries the world between 1.0101 AU at periapsis and 1.2099 AU at apoapsis, driving measurable seasonal insolation cycles.
☁️ Atmospheric Regime (Volatile / Gas Envelope): With a radius of 14.300 R⊕, HD 99109 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 99109 is classified as a solar-type yellow star of spectral type G8/K0 IV with an effective photospheric temperature of 5,282 K, a radius of 0.98 R☉, a mass of 0.76 M☉, an estimated bolometric luminosity of 0.671 L☉. Stellar evolution modeling indicates an estimated stellar age of 1 Gyr and a metallicity of [Fe/H] = 0.35 dex (metal-rich relative to the Sun). From Earth, HD 99109 shines at apparent V-band magnitude 9.1 (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: W. M. Keck Observatory · Catalogue Source: NASA Exoplanet Archive (IPAC / Caltech).
☀️ Host Star Telemetry
| Host Star Name | HD 99109 |
| Spectral Type | G8/K0 IV |
| Effective Temp (Teff) | 5,282 K |
| Stellar Radius | 0.9800 R☉ |
| Stellar Mass | 0.7600 M☉ |
| Bolometric Luminosity | 0.671 L☉ |
| Metallicity [Fe/H] | 0.350 dex |
| Stellar Age | 1.000 Gyr |
| Apparent Magnitude | V: 9.100 · G: 8.868 |
| System Architecture | 1 Star, 1 Planet |
🪐 Orbital & Physical Dynamics
| Semi-Major Axis (a) | 1.110000 AU |
| Orbital Period | 439.3 days |
| Mean Orbital Velocity | 27.5 km/s |
| Orbital Eccentricity (e) | 0.09000 |
| Orbital Inclination (i) | — |
| Mean Bulk Density | 0.26 g/cm³ (35 km/s vesc) |
| Insolation Flux | 0.5465 S⊕ (~744 W/m²) |
| Habitable Zone Status | Exterior / Non-HZ |
| Celestial Distance | 178.700 LY (54.790 pc) |