HD 209458 b
Astrophysical Synthesis & Habitability Characterization
HD 209458 b is a confirmed gas giant exoplanet orbiting the star HD 209458, located approximately 157.5 light-years (48.3 parsecs) from the Solar System. First announced in 1999 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 15.581 Earth radii (1.39 Jupiter radii) and has a measured mass of 232.016 Earth masses (0.73 Jupiter masses). Combining its mass and radius yields a mean bulk density of 0.34 g/cm³ (a low bulk density dominated by an extended hydrogen-helium gaseous envelope), a surface gravity of 0.96 g⊕ (9.41 m/s²), and an escape velocity of 43.2 km/s.
In its orbital architecture around HD 209458, HD 209458 b completes one full revolution every 3.525 Earth days at a semi-major axis of 0.04707 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 145.3 km/s (compared to Earth's 29.8 km/s). The orbit is modeled as circular (e = 0.00), maintaining a constant stellar distance throughout its year.
⚠️ Thermal Regime (Irradiated / Hot Zone): At an equilibrium temperature of 1459 K (+1186°C / +2167°F) and receiving 587.10 S⊕ (~799,040 W/m²), HD 209458 b orbits interior to the runaway greenhouse boundary where surface liquid water cannot persist.
The host star HD 209458 is classified as a solar-type yellow star of spectral type G0 V with an effective photospheric temperature of 6,091 K, a radius of 1.19 R☉, a mass of 1.23 M☉, an estimated bolometric luminosity of 1.749 L☉. Stellar evolution modeling indicates an estimated stellar age of 3.1 Gyr and a metallicity of [Fe/H] = 0.01 dex (near-solar heavy-element abundance). From Earth, HD 209458 shines at apparent V-band magnitude 7.65 (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: W. M. Keck Observatory · Catalogue Source: NASA Exoplanet Archive (IPAC / Caltech).
☀️ Host Star Telemetry
| Host Star Name | HD 209458 |
| Spectral Type | G0 V |
| Effective Temp (Teff) | 6,091 K |
| Stellar Radius | 1.1900 R☉ |
| Stellar Mass | 1.2300 M☉ |
| Bolometric Luminosity | 1.749 L☉ |
| Metallicity [Fe/H] | 0.010 dex |
| Stellar Age | 3.100 Gyr |
| Apparent Magnitude | V: 7.650 · G: 7.509 |
| System Architecture | 1 Star, 1 Planet |
🪐 Orbital & Physical Dynamics
| Semi-Major Axis (a) | 0.047070 AU |
| Orbital Period | 3.525 days |
| Mean Orbital Velocity | 145.3 km/s |
| Orbital Eccentricity (e) | 0.00000 |
| Orbital Inclination (i) | 86.7100° |
| Mean Bulk Density | 0.34 g/cm³ (43.2 km/s vesc) |
| Insolation Flux | 587.0980 S⊕ (~799,040 W/m²) |
| Habitable Zone Status | Exterior / Non-HZ |
| Celestial Distance | 157.500 LY (48.302 pc) |