WASP-21 b

Astrophysical Synthesis & Habitability Characterization

WASP-21 b is a confirmed gas giant exoplanet orbiting the star WASP-21, located approximately 842.9 light-years (258.43 parsecs) from the Solar System. First announced in 2010 by SuperWASP using the Transit detection technique (producing an estimated primary transit depth of 1.0754% across the stellar disk), this world measures 11.99 Earth radii (1.07 Jupiter radii) and has a measured mass of 95.345 Earth masses (0.3 Jupiter masses). Combining its mass and radius yields a mean bulk density of 0.31 g/cm³ (a low bulk density dominated by an extended hydrogen-helium gaseous envelope), a surface gravity of 0.66 g⊕ (6.47 m/s²), and an escape velocity of 31.5 km/s.

In its orbital architecture around WASP-21, WASP-21 b completes one full revolution every 4.322 Earth days at a semi-major axis of 0.052 AU (orbiting interior to Mercury's 0.387 AU benchmark in the Solar System). Its mean orbital velocity is approximately 130.9 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 1333 K (+1060°C / +1940°F) and receiving 705.04 S⊕ (~959,565 W/m²), WASP-21 b orbits interior to the runaway greenhouse boundary where surface liquid water cannot persist.

The host star WASP-21 is classified as a solar-type yellow star of spectral type G3 V with an effective photospheric temperature of 5,800 K, a radius of 1.06 R☉, a mass of 1.01 M☉, an estimated bolometric luminosity of 1.141 L☉. Stellar evolution modeling indicates an estimated stellar age of 12 Gyr and a metallicity of [Fe/H] = -0.22 dex (metal-poor relative to the Sun). From Earth, WASP-21 shines at apparent V-band magnitude 11.59 (faint and requiring research-grade or CCD telescopic photometry) in the Northern celestial hemisphere.

Discovery Facility: SuperWASP · Catalogue Source: NASA Exoplanet Archive (IPAC / Caltech).