HIP 81991 b

Astrophysical Synthesis & Habitability Characterization

HIP 81991 b is a confirmed gas giant exoplanet orbiting the star HIP 81991, located approximately 143.2 light-years (43.9 parsecs) from the Solar System. First announced in 2026 by Multiple Facilities using the Radial Velocity detection technique (measuring periodic Doppler reflex motion in the host star's spectrum), this world measures 12.5 Earth radii (1.12 Jupiter radii) and has a measured mass of 3019.37 Earth masses (9.5 Jupiter masses). Combining its mass and radius yields a mean bulk density of 8.52 g/cm³ (an iron-enriched, high-compression rocky core), a surface gravity of 19.32 g⊕ (189.46 m/s²), and an escape velocity of 173.9 km/s.

In its orbital architecture around HIP 81991, HIP 81991 b completes one full revolution every 5860 Earth days (16.04Julian years) at a semi-major axis of 6.4 AU (a wide outer-system orbit beyond Jupiter's 5.20 AU distance in our Solar System). Its mean orbital velocity is approximately 11.9 km/s (compared to Earth's 29.8 km/s).

❄️ Thermal Regime (Cryogenic Zone): With an equilibrium temperature of 163 K (-110°C / -166°F) and an incident flux of 0.12 S⊕ (~161 W/m²), surface water and volatile species on HIP 81991 b would remain locked in cryogenic ice phases unless warmed by subsurface tidal or radiogenic heating.

The host star HIP 81991 is classified as a solar-type yellow star of spectral type G8 V with an effective photospheric temperature of 5,100 K, a radius of 2.8211 R☉, a mass of 1.09 M☉, an estimated bolometric luminosity of 4.831 L☉. Stellar evolution modeling indicates a metallicity of [Fe/H] = -0.27 dex (metal-poor relative to the Sun). From Earth, HIP 81991 shines at apparent V-band magnitude 6.562 (accessible to binoculars and small amateur telescopes) in the Northern celestial hemisphere.

Discovery Facility: Multiple Facilities · Catalogue Source: NASA Exoplanet Archive (IPAC / Caltech).