Hot Jupiter

A hot Jupiter is a giant exoplanet with a short orbital period and strong stellar irradiance, typically orbiting much closer to its star than Mercury orbits the Sun. Large radius and short orbital period make these bodies favourable for exoplanet transit photometry; large mass and small orbital separation also increase the stellar radial-velocity semi-amplitude.

Strong external heating makes an exoplanet atmosphere very different from that of Jupiter. Tidal locking can maintain nearly permanent illuminated and dark hemispheres, creating a large driving force for day-night heat redistribution. HD 209458 b illustrates transit spectroscopy, while HD 189733 b illustrates an exoplanet thermal phase curve. Their proximity to their stars also motivates planetary migration models: present orbital location need not be the location of formation.

HD 209458 b is a transiting hot Jupiter that illustrates how an unresolved exoplanet atmosphere can be measured. In the sodium experiment reported in 2002, four exoplanet transits were observed around the sodium doublet near $589.3\mathrm{nm}$. The exoplanet transit depth in that narrow band exceeded the depth in neighbouring bands by approximately $2.3\times 10^{-4}$, interpreted as additional sodium absorption in the planetary limb.^[ref]

The observable is a difference in blocked area with wavelength, rather than an isolated planetary emission spectrum. Its interpretation requires radiative transfer through the limb and a model of the illuminating star. The original sodium signal was weaker than a simple clear-atmosphere prediction, already illustrating why exoplanet clouds, composition and photoionization matter.

HD 189733 b is a transiting hot Jupiter used to study day-night heat redistribution. Observations with the Spitzer Space Telescope at $8\mu \mathrm{m}$ followed the changing combined light over half an orbital period. Subtracting the star's contribution yielded an exoplanet thermal phase curve, from which a broad longitudinal brightness distribution could be inferred.^[ref]

The bright hemisphere reached its maximum visibility before exoplanet secondary eclipse, implying a hot region displaced from the point directly facing the star. A warm nightside supports transport of absorbed energy by the exoplanet atmosphere. These are brightness temperature measurements in one band; they neither resolve small weather features directly nor uniquely specify the full atmospheric pressure-temperature profile.