81 Cet b
Astrophysical Synthesis & Habitability Characterization
81 Cet b is a confirmed gas giant exoplanet orbiting the star 81 Cet, located approximately 330.3 light-years (101.26 parsecs) from the Solar System. First announced in 2008 by Okayama Astrophysical Observatory using the Radial Velocity detection technique (measuring periodic Doppler reflex motion in the host star's spectrum), this world measures 13.1 Earth radii (1.17 Jupiter radii) and has a measured mass of 1051.059 Earth masses (3.31 Jupiter masses). Combining its mass and radius yields a mean bulk density of 2.58 g/cm³ (an intermediate density indicative of a deep water-ice mantle or dense volatile envelope), a surface gravity of 6.12 g⊕ (60.02 m/s²), and an escape velocity of 100.2 km/s.
In its orbital architecture around 81 Cet, 81 Cet b completes one full revolution every 1005.57 Earth days (2.75Julian years) at a semi-major axis of 2.104 AU (corresponding to the middle-system regime between Mars at 1.52 AU and Jupiter at 5.20 AU). Its mean orbital velocity is approximately 22.8 km/s (compared to Earth's 29.8 km/s). With a near-circular eccentricity of e = 0.037 (ranging from 2.0262 AU at periapsis to 2.1818 AU at apoapsis), the planet experiences minimal orbital distance variation.
⚠️ Thermal Regime (Irradiated / Hot Zone): At an equilibrium temperature of 527 K (+254°C / +489°F) and receiving 13.01 S⊕ (~17,703 W/m²), 81 Cet b orbits interior to the runaway greenhouse boundary where surface liquid water cannot persist.
The host star 81 Cet is classified as a solar-type yellow star of spectral type G5 III with an effective photospheric temperature of 4,734 K, a radius of 11.21 R☉, a mass of 1.23 M☉, an estimated bolometric luminosity of 56.626 L☉. Stellar evolution modeling indicates a metallicity of [Fe/H] = -0.11 dex (metal-poor relative to the Sun). From Earth, 81 Cet shines at apparent V-band magnitude 5.643 (visible to the unaided human eye under dark skies) 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: Okayama Astrophysical Observatory · Catalogue Source: NASA Exoplanet Archive (IPAC / Caltech).
☀️ Host Star Telemetry
| Host Star Name | 81 Cet |
| Spectral Type | G5 III |
| Effective Temp (Teff) | 4,734 K |
| Stellar Radius | 11.2100 R☉ |
| Stellar Mass | 1.2300 M☉ |
| Bolometric Luminosity | 56.626 L☉ |
| Metallicity [Fe/H] | -0.110 dex |
| Stellar Age | — |
| Apparent Magnitude | V: 5.643 · G: 5.376 |
| System Architecture | 1 Star, 1 Planet |
🪐 Orbital & Physical Dynamics
| Semi-Major Axis (a) | 2.104000 AU |
| Orbital Period | 1005.57 days |
| Mean Orbital Velocity | 22.8 km/s |
| Orbital Eccentricity (e) | 0.03700 |
| Orbital Inclination (i) | — |
| Mean Bulk Density | 2.58 g/cm³ (100.2 km/s vesc) |
| Insolation Flux | 13.0071 S⊕ (~17,703 W/m²) |
| Habitable Zone Status | Exterior / Non-HZ |
| Celestial Distance | 330.300 LY (101.260 pc) |