HD 121504 b
Astrophysical Synthesis & Habitability Characterization
HD 121504 b is a confirmed gas giant exoplanet orbiting the star HD 121504, located approximately 135.9 light-years (41.66 parsecs) from the Solar System. First announced in 2003 by La Silla Observatory using the Radial Velocity detection technique (measuring periodic Doppler reflex motion in the host star's spectrum), this world measures 13.6 Earth radii (1.21 Jupiter radii) and has a measured mass of 479.923 Earth masses (1.51 Jupiter masses). Combining its mass and radius yields a mean bulk density of 1.05 g/cm³ (a low bulk density dominated by an extended hydrogen-helium gaseous envelope), a surface gravity of 2.59 g⊕ (25.4 m/s²), and an escape velocity of 66.4 km/s.
In its orbital architecture around HD 121504, HD 121504 b completes one full revolution every 63.33 Earth days at a semi-major axis of 0.33 AU (orbiting interior to Mercury's 0.387 AU benchmark in the Solar System). Its mean orbital velocity is approximately 56.7 km/s (compared to Earth's 29.8 km/s). With a near-circular eccentricity of e = 0.03 (ranging from 0.3201 AU at periapsis to 0.3399 AU at apoapsis), the planet experiences minimal orbital distance variation.
⚠️ Thermal Regime (Irradiated / Hot Zone): At an equilibrium temperature of 523 K (+250°C / +482°F) and receiving 14.22 S⊕ (~19,357 W/m²), HD 121504 b orbits interior to the runaway greenhouse boundary where surface liquid water cannot persist.
The host star HD 121504 is classified as a solar-type yellow star of spectral type G2 V with an effective photospheric temperature of 6,027 K, a radius of 1.07 R☉, a mass of 1.62 M☉, an estimated bolometric luminosity of 1.355 L☉. Stellar evolution modeling indicates an estimated stellar age of 1.2 Gyr and a metallicity of [Fe/H] = 0.15 dex (metal-rich relative to the Sun). From Earth, HD 121504 shines at apparent V-band magnitude 7.54 (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: La Silla Observatory · Catalogue Source: NASA Exoplanet Archive (IPAC / Caltech).
☀️ Host Star Telemetry
| Host Star Name | HD 121504 |
| Spectral Type | G2 V |
| Effective Temp (Teff) | 6,027 K |
| Stellar Radius | 1.0700 R☉ |
| Stellar Mass | 1.6200 M☉ |
| Bolometric Luminosity | 1.355 L☉ |
| Metallicity [Fe/H] | 0.150 dex |
| Stellar Age | 1.200 Gyr |
| Apparent Magnitude | V: 7.540 · G: 7.390 |
| System Architecture | 1 Star, 1 Planet |
🪐 Orbital & Physical Dynamics
| Semi-Major Axis (a) | 0.330000 AU |
| Orbital Period | 63.33 days |
| Mean Orbital Velocity | 56.7 km/s |
| Orbital Eccentricity (e) | 0.03000 |
| Orbital Inclination (i) | — |
| Mean Bulk Density | 1.05 g/cm³ (66.4 km/s vesc) |
| Insolation Flux | 14.2224 S⊕ (~19,357 W/m²) |
| Habitable Zone Status | Exterior / Non-HZ |
| Celestial Distance | 135.900 LY (41.659 pc) |