HD 117618 b
Astrophysical Synthesis & Habitability Characterization
HD 117618 b is a confirmed gas giant exoplanet orbiting the star HD 117618, located approximately 123.2 light-years (37.78 parsecs) from the Solar System. First announced in 2004 by Anglo-Australian Telescope using the Radial Velocity detection technique (measuring periodic Doppler reflex motion in the host star's spectrum), this world measures 8.59 Earth radii (0.77 Jupiter radii) and has a measured mass of 55.302 Earth masses (0.17 Jupiter masses). Combining its mass and radius yields a mean bulk density of 0.48 g/cm³ (a low bulk density dominated by an extended hydrogen-helium gaseous envelope), a surface gravity of 0.75 g⊕ (7.35 m/s²), and an escape velocity of 28.4 km/s.
In its orbital architecture around HD 117618, HD 117618 b completes one full revolution every 25.8 Earth days at a semi-major axis of 0.18 AU (orbiting interior to Mercury's 0.387 AU benchmark in the Solar System). Its mean orbital velocity is approximately 75.9 km/s (compared to Earth's 29.8 km/s). Its moderate orbital eccentricity of e = 0.15 carries the world between 0.153 AU at periapsis and 0.207 AU at apoapsis, driving measurable seasonal insolation cycles.
⚠️ Thermal Regime (Irradiated / Hot Zone): At an equilibrium temperature of 746 K (+473°C / +883°F) and receiving 50.41 S⊕ (~68,605 W/m²), HD 117618 b orbits interior to the runaway greenhouse boundary where surface liquid water cannot persist.
The host star HD 117618 is classified as a solar-type yellow star of spectral type G2 V with an effective photospheric temperature of 5,990 K, a radius of 1.2 R☉, a mass of 1.077 M☉, an estimated bolometric luminosity of 1.663 L☉. Stellar evolution modeling indicates an estimated stellar age of 5.6 Gyr and a metallicity of [Fe/H] = 0.028 dex (near-solar heavy-element abundance). From Earth, HD 117618 shines at apparent V-band magnitude 7.17 (accessible to binoculars and small amateur telescopes) 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: Anglo-Australian Telescope · Catalogue Source: NASA Exoplanet Archive (IPAC / Caltech).
☀️ Host Star Telemetry
| Host Star Name | HD 117618 |
| Spectral Type | G2 V |
| Effective Temp (Teff) | 5,990 K |
| Stellar Radius | 1.2000 R☉ |
| Stellar Mass | 1.0770 M☉ |
| Bolometric Luminosity | 1.663 L☉ |
| Metallicity [Fe/H] | 0.028 dex |
| Stellar Age | 5.600 Gyr |
| Apparent Magnitude | V: 7.170 · G: 7.022 |
| System Architecture | 1 Star, 1 Planet |
🪐 Orbital & Physical Dynamics
| Semi-Major Axis (a) | 0.180000 AU |
| Orbital Period | 25.8 days |
| Mean Orbital Velocity | 75.9 km/s |
| Orbital Eccentricity (e) | 0.15000 |
| Orbital Inclination (i) | — |
| Mean Bulk Density | 0.48 g/cm³ (28.4 km/s vesc) |
| Insolation Flux | 50.4075 S⊕ (~68,605 W/m²) |
| Habitable Zone Status | Exterior / Non-HZ |
| Celestial Distance | 123.200 LY (37.778 pc) |