A hot Jupiter is a gas-giant exoplanet orbiting very close to its host star — often closer than Mercury orbits the Sun — and therefore intensely heated and irradiated. Such planets are typically tidally locked, with a permanent dayside and nightside separated by extreme limb-to-limb temperature contrasts. Their hot, cloud- and haze-prone atmospheres have historically confounded transmission-spectroscopy composition retrievals.
Significance for atmosphere studies
Hot Jupiters are the most accessible exoplanets for atmospheric characterization: large, hot, and frequently transiting bright stars. They serve as laboratories for cloud physics and 3D atmospheric dynamics — see exoplanet-atmosphere-models.
The jwst limb-resolved study of wasp-94ab (src-jwst-wasp-94ab-weather-cycle-2026-05) showed that a morning-cloud / evening-clear asymmetry recurs across multiple hot Jupiters (WASP-94Ab, WASP-39b, WASP-17b). Because earlier instruments averaged both limbs together, this asymmetry biased inferred metallicity — WASP-94Ab’s oxygen/carbon abundance was overstated by orders of magnitude until the limbs were separated. The lesson generalizes: limb asymmetry must be accounted for in any transit spectrum, including those of smaller biosignature targets.
wasp-121b is an example ultra-hot Jupiter (dayside ~2,770 K) where jwst resolved a dawn–dusk terminator asymmetry (Gapp et al., Nature Astronomy 2026) (src-jwst-wasp-121b-dawn-dusk-2026-06).