KELT-21 b

Astrophysical Synthesis & Habitability Characterization

KELT-21 b is a confirmed gas giant exoplanet orbiting the star KELT-21, located approximately 1535.8 light-years (470.88 parsecs) from the Solar System. First announced in 2018 by KELT-North using the Transit detection technique (producing an estimated primary transit depth of 0.99% across the stellar disk), this world measures 17.777 Earth radii (1.59 Jupiter radii) and has a measured mass of 1242.715 Earth masses (3.91 Jupiter masses). Combining its mass and radius yields a mean bulk density of 1.22 g/cm³ (a low bulk density dominated by an extended hydrogen-helium gaseous envelope), a surface gravity of 3.93 g⊕ (38.54 m/s²), and an escape velocity of 93.5 km/s.

In its orbital architecture around KELT-21, KELT-21 b completes one full revolution every 3.613 Earth days at a semi-major axis of 0.05224 AU (orbiting interior to Mercury's 0.387 AU benchmark in the Solar System). Its mean orbital velocity is approximately 157.3 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 2051 K (+1778°C / +3232°F) and receiving 2,950.00 S⊕ (~4,014,950 W/m²), KELT-21 b orbits interior to the runaway greenhouse boundary where surface liquid water cannot persist.

The host star KELT-21 is classified as a luminous A-type star of spectral type A8 V with an effective photospheric temperature of 7,598 K, a radius of 1.638 R☉, a mass of 1.458 M☉, an estimated bolometric luminosity of 8.023 L☉. Stellar evolution modeling indicates an estimated stellar age of 1.6 Gyr and a metallicity of [Fe/H] = -0.405 dex (metal-poor relative to the Sun). From Earth, KELT-21 shines at apparent V-band magnitude 10.484 (faint and requiring research-grade or CCD telescopic photometry) in the Northern celestial hemisphere.

Discovery Facility: KELT-North · Catalogue Source: NASA Exoplanet Archive (IPAC / Caltech).