HD 81817 c
Astrophysical Synthesis & Habitability Characterization
HD 81817 c is a confirmed gas giant exoplanet orbiting the star HD 81817, located approximately 877.8 light-years (269.13 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 12 Earth radii (1.07 Jupiter radii) and has a measured mass of 7185.782 Earth masses (22.61 Jupiter masses). Combining its mass and radius yields a mean bulk density of 22.93 g/cm³ (an iron-enriched, high-compression rocky core), a surface gravity of 49.9 g⊕ (489.35 m/s²), and an escape velocity of 273.7 km/s.
In its orbital architecture around HD 81817, HD 81817 c completes one full revolution every 622.978 Earth days (1.71Julian years) at a semi-major axis of 2.325 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 40.6 km/s (compared to Earth's 29.8 km/s). Its moderate orbital eccentricity of e = 0.095 carries the world between 2.1041 AU at periapsis and 2.5459 AU at apoapsis, driving measurable seasonal insolation cycles.
⚠️ Thermal Regime (Irradiated / Hot Zone): At an equilibrium temperature of 1199 K (+926°C / +1699°F) and receiving 337.22 S⊕ (~458,960 W/m²), HD 81817 c orbits interior to the runaway greenhouse boundary where surface liquid water cannot persist.
The host star HD 81817 is classified as an orange K-type star of spectral type K3 III with an effective photospheric temperature of 4,140 K, a radius of 83.8 R☉, a mass of 4.3 M☉, an estimated bolometric luminosity of 1850.884 L☉. Stellar evolution modeling indicates an estimated stellar age of 0.15 Gyr and a metallicity of [Fe/H] = -0.17 dex (metal-poor relative to the Sun). From Earth, HD 81817 shines at apparent V-band magnitude 4.265 (visible to the unaided human eye under dark skies) in the Northern celestial hemisphere. The HD 81817 system is a multi-planet architecture hosting 2 confirmed worlds, with HD 81817 c sharing its stellar neighborhood alongside HD 81817 b.
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 81817 |
| Spectral Type | K3 III |
| Effective Temp (Teff) | 4,140 K |
| Stellar Radius | 83.8000 R☉ |
| Stellar Mass | 4.3000 M☉ |
| Bolometric Luminosity | 1850.884 L☉ |
| Metallicity [Fe/H] | -0.170 dex |
| Stellar Age | 0.150 Gyr |
| Apparent Magnitude | V: 4.265 · G: 3.801 |
| System Architecture | 1 Star, 2 Planets |
🪐 Orbital & Physical Dynamics
| Semi-Major Axis (a) | 2.325000 AU |
| Orbital Period | 622.978 days |
| Mean Orbital Velocity | 40.6 km/s |
| Orbital Eccentricity (e) | 0.09500 |
| Orbital Inclination (i) | — |
| Mean Bulk Density | 22.93 g/cm³ (273.7 km/s vesc) |
| Insolation Flux | 337.2228 S⊕ (~458,960 W/m²) |
| Habitable Zone Status | Exterior / Non-HZ |
| Celestial Distance | 877.800 LY (269.126 pc) |