HD 217786 c
Astrophysical Synthesis & Habitability Characterization
HD 217786 c is a confirmed sub-neptune exoplanet orbiting the star HD 217786, located approximately 181 light-years (55.49 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 2.61 Earth radii and has a measured mass of 7.31 Earth masses. Combining its mass and radius yields a mean bulk density of 2.27 g/cm³ (an intermediate density indicative of a deep water-ice mantle or dense volatile envelope), a surface gravity of 1.07 g⊕ (10.49 m/s²), and an escape velocity of 18.7 km/s.
In its orbital architecture around HD 217786, HD 217786 c completes one full revolution every 2.522 Earth days at a semi-major axis of 0.038 AU (an ultra-close orbit less than one-seventh of Mercury's 0.387 AU distance from the Sun, where strong stellar tidal forces typically induce synchronous rotation). Its mean orbital velocity is approximately 163.9 km/s (compared to Earth's 29.8 km/s). The planet follows a highly eccentric trajectory (e = 0.399, swinging from 0.0228 AU at periapsis out to 0.0532 AU at apoapsis), pointing to past dynamical scattering or secular gravitational perturbations.
⚠️ Thermal Regime (Irradiated / Hot Zone): At an equilibrium temperature of 1663 K (+1390°C / +2534°F) and receiving 1,308.86 S⊕ (~1,781,364 W/m²), HD 217786 c orbits interior to the runaway greenhouse boundary where surface liquid water cannot persist.
The host star HD 217786 is classified as a warm F-type star of spectral type F8 V with an effective photospheric temperature of 5,966 K, a radius of 1.27 R☉, a mass of 1.14 M☉, an estimated bolometric luminosity of 1.833 L☉. Stellar evolution modeling indicates an estimated stellar age of 7.6 Gyr and a metallicity of [Fe/H] = -0.135 dex (metal-poor relative to the Sun). From Earth, HD 217786 shines at apparent V-band magnitude 7.78 (accessible to binoculars and small amateur telescopes) in the Southern celestial hemisphere. The HD 217786 system is a multi-planet architecture hosting 2 confirmed worlds, with HD 217786 c sharing its stellar neighborhood alongside HD 217786 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 217786 |
| Spectral Type | F8 V |
| Effective Temp (Teff) | 5,966 K |
| Stellar Radius | 1.2700 R☉ |
| Stellar Mass | 1.1400 M☉ |
| Bolometric Luminosity | 1.833 L☉ |
| Metallicity [Fe/H] | -0.135 dex |
| Stellar Age | 7.600 Gyr |
| Apparent Magnitude | V: 7.780 · G: 7.636 |
| System Architecture | 2 Stars, 2 Planets |
🪐 Orbital & Physical Dynamics
| Semi-Major Axis (a) | 0.038000 AU |
| Orbital Period | 2.522 days |
| Mean Orbital Velocity | 163.9 km/s |
| Orbital Eccentricity (e) | 0.39900 |
| Orbital Inclination (i) | — |
| Mean Bulk Density | 2.27 g/cm³ (18.7 km/s vesc) |
| Insolation Flux | 1308.8643 S⊕ (~1,781,364 W/m²) |
| Habitable Zone Status | Exterior / Non-HZ |
| Celestial Distance | 181.000 LY (55.485 pc) |