WASP-121 b
Astrophysical Synthesis & Habitability Characterization
WASP-121 b is a confirmed gas giant exoplanet orbiting the star WASP-121, located approximately 880.3 light-years (269.9 parsecs) from the Solar System. First announced in 2016 by SuperWASP using the Transit detection technique (producing an estimated primary transit depth of 1.5013% across the stellar disk), this world measures 19.526 Earth radii (1.74 Jupiter radii) and has a measured mass of 371.859 Earth masses (1.17 Jupiter masses). Combining its mass and radius yields a mean bulk density of 0.28 g/cm³ (a low bulk density dominated by an extended hydrogen-helium gaseous envelope), a surface gravity of 0.98 g⊕ (9.61 m/s²), and an escape velocity of 48.8 km/s.
In its orbital architecture around WASP-121, WASP-121 b completes one full revolution every 1.275 Earth days at a semi-major axis of 0.02571 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 219.4 km/s (compared to Earth's 29.8 km/s). With a near-circular eccentricity of e = 0.0085 (ranging from 0.0255 AU at periapsis to 0.0259 AU at apoapsis), the planet experiences minimal orbital distance variation.
⚠️ Thermal Regime (Irradiated / Hot Zone): At an equilibrium temperature of 2409 K (+2136°C / +3877°F) and receiving 6,216.41 S⊕ (~8,460,534 W/m²), WASP-121 b orbits interior to the runaway greenhouse boundary where surface liquid water cannot persist.
The host star WASP-121 is classified as a warm F-type star of spectral type F6 V with an effective photospheric temperature of 6,628 K, a radius of 1.461 R☉, a mass of 1.33 M☉, an estimated bolometric luminosity of 3.696 L☉. Stellar evolution modeling indicates an estimated stellar age of 1.11 Gyr and a metallicity of [Fe/H] = 0.17 dex (metal-rich relative to the Sun). From Earth, WASP-121 shines at apparent V-band magnitude 10.514 (faint and requiring research-grade or CCD telescopic photometry) 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: SuperWASP · Catalogue Source: NASA Exoplanet Archive (IPAC / Caltech).
☀️ Host Star Telemetry
| Host Star Name | WASP-121 |
| Spectral Type | F6 V |
| Effective Temp (Teff) | 6,628 K |
| Stellar Radius | 1.4610 R☉ |
| Stellar Mass | 1.3300 M☉ |
| Bolometric Luminosity | 3.696 L☉ |
| Metallicity [Fe/H] | 0.170 dex |
| Stellar Age | 1.110 Gyr |
| Apparent Magnitude | V: 10.514 · G: 10.375 |
| System Architecture | 1 Star, 1 Planet |
🪐 Orbital & Physical Dynamics
| Semi-Major Axis (a) | 0.025710 AU |
| Orbital Period | 1.275 days |
| Mean Orbital Velocity | 219.4 km/s |
| Orbital Eccentricity (e) | 0.00850 |
| Orbital Inclination (i) | 88.4900° |
| Mean Bulk Density | 0.28 g/cm³ (48.8 km/s vesc) |
| Insolation Flux | 6216.4100 S⊕ (~8,460,534 W/m²) |
| Habitable Zone Status | Exterior / Non-HZ |
| Celestial Distance | 880.300 LY (269.898 pc) |