HD 191939 g
Astrophysical Synthesis & Habitability Characterization
HD 191939 g is a confirmed sub-neptune exoplanet orbiting the star HD 191939, located approximately 174.8 light-years (53.61 parsecs) from the Solar System. First announced in 2022 by Multiple Observatories using the Radial Velocity detection technique (measuring periodic Doppler reflex motion in the host star's spectrum), this world measures 3.74 Earth radii (0.33 Jupiter radii) and has a measured mass of 13.5 Earth masses. Combining its mass and radius yields a mean bulk density of 1.42 g/cm³ (a low bulk density dominated by an extended hydrogen-helium gaseous envelope), a surface gravity of 0.97 g⊕ (9.51 m/s²), and an escape velocity of 21.3 km/s.
In its orbital architecture around HD 191939, HD 191939 g completes one full revolution every 284 Earth days at a semi-major axis of 0.812 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 31.1 km/s (compared to Earth's 29.8 km/s). With a near-circular eccentricity of e = 0.03 (ranging from 0.7876 AU at periapsis to 0.8364 AU at apoapsis), the planet experiences minimal orbital distance variation.
☁️ Atmospheric Regime (Volatile / Gas Envelope): With a radius of 3.740 R⊕, HD 191939 g 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 191939 is classified as a solar-type yellow star of spectral type G9 V with an effective photospheric temperature of 5,348 K, a radius of 0.94 R☉, a mass of 0.81 M☉, an estimated bolometric luminosity of 0.649 L☉. Stellar evolution modeling indicates an estimated stellar age of 8.7 Gyr and a metallicity of [Fe/H] = -0.15 dex (metal-poor relative to the Sun). From Earth, HD 191939 shines at apparent V-band magnitude 8.97 (accessible to binoculars and small amateur telescopes) in the Northern celestial hemisphere. The HD 191939 system is a multi-planet architecture hosting 6 confirmed worlds, with HD 191939 g sharing its stellar neighborhood alongside HD 191939 b, HD 191939 c, HD 191939 d, HD 191939 e, HD 191939 f.
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 191939 |
| Spectral Type | G9 V |
| Effective Temp (Teff) | 5,348 K |
| Stellar Radius | 0.9400 R☉ |
| Stellar Mass | 0.8100 M☉ |
| Bolometric Luminosity | 0.649 L☉ |
| Metallicity [Fe/H] | -0.150 dex |
| Stellar Age | 8.700 Gyr |
| Apparent Magnitude | V: 8.970 · G: 8.775 |
| System Architecture | 1 Star, 6 Planets |
🪐 Orbital & Physical Dynamics
| Semi-Major Axis (a) | 0.812000 AU |
| Orbital Period | 284 days |
| Mean Orbital Velocity | 31.1 km/s |
| Orbital Eccentricity (e) | 0.03000 |
| Orbital Inclination (i) | — |
| Mean Bulk Density | 1.42 g/cm³ (21.3 km/s vesc) |
| Insolation Flux | 0.9900 S⊕ (~1,347 W/m²) |
| Habitable Zone Status | Exterior / Non-HZ |
| Celestial Distance | 174.800 LY (53.609 pc) |