HD 17156 b
Astrophysical Synthesis & Habitability Characterization
HD 17156 b is a confirmed gas giant exoplanet orbiting the star HD 17156, located approximately 254.7 light-years (78.09 parsecs) from the Solar System. First announced in 2007 by Subaru Telescope using the Radial Velocity detection technique (measuring periodic Doppler reflex motion in the host star's spectrum), this world measures 12.263 Earth radii (1.09 Jupiter radii) and has a measured mass of 1036.121 Earth masses (3.26 Jupiter masses). Combining its mass and radius yields a mean bulk density of 3.1 g/cm³ (an intermediate density indicative of a deep water-ice mantle or dense volatile envelope), a surface gravity of 6.89 g⊕ (67.57 m/s²), and an escape velocity of 102.8 km/s.
In its orbital architecture around HD 17156, HD 17156 b completes one full revolution every 21.216 Earth days at a semi-major axis of 0.1632 AU (orbiting interior to Mercury's 0.387 AU benchmark in the Solar System). Its mean orbital velocity is approximately 83.7 km/s (compared to Earth's 29.8 km/s). The planet follows a highly eccentric trajectory (e = 0.6772, swinging from 0.0527 AU at periapsis out to 0.2737 AU at apoapsis), pointing to past dynamical scattering or secular gravitational perturbations.
⚠️ Thermal Regime (Irradiated / Hot Zone): At an equilibrium temperature of 888 K (+615°C / +1139°F) and receiving 68.80 S⊕ (~93,637 W/m²), HD 17156 b orbits interior to the runaway greenhouse boundary where surface liquid water cannot persist.
The host star HD 17156 is classified as a solar-type yellow star of spectral type G5 with an effective photospheric temperature of 6,046 K, a radius of 1.517 R☉, a mass of 1.285 M☉, an estimated bolometric luminosity of 2.759 L☉. Stellar evolution modeling indicates an estimated stellar age of 3.3 Gyr and a metallicity of [Fe/H] = 0.208 dex (metal-rich relative to the Sun). From Earth, HD 17156 shines at apparent V-band magnitude 8.17 (accessible to binoculars and small amateur telescopes) 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: Subaru Telescope · Catalogue Source: NASA Exoplanet Archive (IPAC / Caltech).
☀️ Host Star Telemetry
| Host Star Name | HD 17156 |
| Spectral Type | G5 |
| Effective Temp (Teff) | 6,046 K |
| Stellar Radius | 1.5170 R☉ |
| Stellar Mass | 1.2850 M☉ |
| Bolometric Luminosity | 2.759 L☉ |
| Metallicity [Fe/H] | 0.208 dex |
| Stellar Age | 3.300 Gyr |
| Apparent Magnitude | V: 8.170 · G: 8.033 |
| System Architecture | 1 Star, 1 Planet |
🪐 Orbital & Physical Dynamics
| Semi-Major Axis (a) | 0.163200 AU |
| Orbital Period | 21.216 days |
| Mean Orbital Velocity | 83.7 km/s |
| Orbital Eccentricity (e) | 0.67720 |
| Orbital Inclination (i) | 86.5100° |
| Mean Bulk Density | 3.1 g/cm³ (102.8 km/s vesc) |
| Insolation Flux | 68.8000 S⊕ (~93,637 W/m²) |
| Habitable Zone Status | Exterior / Non-HZ |
| Celestial Distance | 254.700 LY (78.086 pc) |