HD 187123 b
Astrophysical Synthesis & Habitability Characterization
HD 187123 b is a confirmed gas giant exoplanet orbiting the star HD 187123, located approximately 149.9 light-years (45.96 parsecs) from the Solar System. First announced in 1998 by W. M. Keck Observatory using the Radial Velocity detection technique (measuring periodic Doppler reflex motion in the host star's spectrum), this world measures 14.2 Earth radii (1.27 Jupiter radii) and has a measured mass of 166.218 Earth masses (0.52 Jupiter masses). Combining its mass and radius yields a mean bulk density of 0.32 g/cm³ (a low bulk density dominated by an extended hydrogen-helium gaseous envelope), a surface gravity of 0.82 g⊕ (8.04 m/s²), and an escape velocity of 38.3 km/s.
In its orbital architecture around HD 187123, HD 187123 b completes one full revolution every 3.097 Earth days at a semi-major axis of 0.0426 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 149.7 km/s (compared to Earth's 29.8 km/s). With a near-circular eccentricity of e = 0.0103 (ranging from 0.0422 AU at periapsis to 0.043 AU at apoapsis), the planet experiences minimal orbital distance variation.
⚠️ Thermal Regime (Irradiated / Hot Zone): At an equilibrium temperature of 1451 K (+1178°C / +2152°F) and receiving 738.43 S⊕ (~1,004,997 W/m²), HD 187123 b orbits interior to the runaway greenhouse boundary where surface liquid water cannot persist.
The host star HD 187123 is classified as a solar-type yellow star of spectral type G3 V with an effective photospheric temperature of 5,790 K, a radius of 1.1504 R☉, a mass of 1.04 M☉, an estimated bolometric luminosity of 1.334 L☉. Stellar evolution modeling indicates an estimated stellar age of 7.4 Gyr and a metallicity of [Fe/H] = 0.09 dex (metal-rich relative to the Sun). From Earth, HD 187123 shines at apparent V-band magnitude 7.83 (accessible to binoculars and small amateur telescopes) in the Northern celestial hemisphere. The HD 187123 system is a multi-planet architecture hosting 2 confirmed worlds, with HD 187123 b sharing its stellar neighborhood alongside HD 187123 c.
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: W. M. Keck Observatory · Catalogue Source: NASA Exoplanet Archive (IPAC / Caltech).
☀️ Host Star Telemetry
| Host Star Name | HD 187123 |
| Spectral Type | G3 V |
| Effective Temp (Teff) | 5,790 K |
| Stellar Radius | 1.1504 R☉ |
| Stellar Mass | 1.0400 M☉ |
| Bolometric Luminosity | 1.334 L☉ |
| Metallicity [Fe/H] | 0.090 dex |
| Stellar Age | 7.400 Gyr |
| Apparent Magnitude | V: 7.830 · G: 7.673 |
| System Architecture | 1 Star, 2 Planets |
🪐 Orbital & Physical Dynamics
| Semi-Major Axis (a) | 0.042600 AU |
| Orbital Period | 3.097 days |
| Mean Orbital Velocity | 149.7 km/s |
| Orbital Eccentricity (e) | 0.01030 |
| Orbital Inclination (i) | 79.0000° |
| Mean Bulk Density | 0.32 g/cm³ (38.3 km/s vesc) |
| Insolation Flux | 738.4255 S⊕ (~1,004,997 W/m²) |
| Habitable Zone Status | Exterior / Non-HZ |
| Celestial Distance | 149.900 LY (45.955 pc) |