HD 81688 b
Astrophysical Synthesis & Habitability Characterization
HD 81688 b is a confirmed gas giant exoplanet orbiting the star HD 81688, located approximately 279.6 light-years (85.72 parsecs) from the Solar System. First announced in 2007 by Okayama Astrophysical Observatory using the Radial Velocity detection technique (measuring periodic Doppler reflex motion in the host star's spectrum), this world measures 13.5 Earth radii (1.2 Jupiter radii) and has a measured mass of 525.688 Earth masses (1.65 Jupiter masses). Combining its mass and radius yields a mean bulk density of 1.18 g/cm³ (a low bulk density dominated by an extended hydrogen-helium gaseous envelope), a surface gravity of 2.88 g⊕ (28.24 m/s²), and an escape velocity of 69.8 km/s.
In its orbital architecture around HD 81688, HD 81688 b completes one full revolution every 183.93 Earth days at a semi-major axis of 0.648 AU (situated at an orbital separation between Mercury's 0.39 AU and Venus's 0.72 AU). Its mean orbital velocity is approximately 38.3 km/s (compared to Earth's 29.8 km/s). With a near-circular eccentricity of e = 0.04 (ranging from 0.6221 AU at periapsis to 0.6739 AU at apoapsis), the planet experiences minimal orbital distance variation.
⚠️ Thermal Regime (Irradiated / Hot Zone): At an equilibrium temperature of 959 K (+686°C / +1267°F) and receiving 143.60 S⊕ (~195,445 W/m²), HD 81688 b orbits interior to the runaway greenhouse boundary where surface liquid water cannot persist.
The host star HD 81688 is classified as an orange K-type star of spectral type K0 III-IV with an effective photospheric temperature of 4,797 K, a radius of 11.13 R☉, a mass of 1.07 M☉, an estimated bolometric luminosity of 58.852 L☉. Stellar evolution modeling indicates an estimated stellar age of 1.48 Gyr and a metallicity of [Fe/H] = -0.32 dex (metal-poor relative to the Sun). From Earth, HD 81688 shines at apparent V-band magnitude 5.398 (visible to the unaided human eye under dark skies) in the Northern 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 | HD 81688 |
| Spectral Type | K0 III-IV |
| Effective Temp (Teff) | 4,797 K |
| Stellar Radius | 11.1300 R☉ |
| Stellar Mass | 1.0700 M☉ |
| Bolometric Luminosity | 58.852 L☉ |
| Metallicity [Fe/H] | -0.320 dex |
| Stellar Age | 1.480 Gyr |
| Apparent Magnitude | V: 5.398 · G: 5.126 |
| System Architecture | 1 Star, 1 Planet |
🪐 Orbital & Physical Dynamics
| Semi-Major Axis (a) | 0.648000 AU |
| Orbital Period | 183.93 days |
| Mean Orbital Velocity | 38.3 km/s |
| Orbital Eccentricity (e) | 0.04000 |
| Orbital Inclination (i) | — |
| Mean Bulk Density | 1.18 g/cm³ (69.8 km/s vesc) |
| Insolation Flux | 143.6043 S⊕ (~195,445 W/m²) |
| Habitable Zone Status | Exterior / Non-HZ |
| Celestial Distance | 279.600 LY (85.718 pc) |