A Unified Theory for the Atmospheres of the Hot and Very Hot Jupiters: Two Classes of Irradiated Atmospheres

We highlight the importance of gaseous TiO and VO opacity on the highly irradiated close-in giant planets. The atmospheres of these planets naturally fall into two classes that are somewhat analogous to the Mand Ltype dwarfs. Those that are warm enough to have appreciable opacity due to TiO and VO gases we term the “pM Class” planets, and those that are cooler, such that Ti and V are predominantly in solid condensates, we term “pL Class” planets. The optical spectra of pL Class planets are dominated by neutral atomic Na and K absorption. We calculate model atmospheres for these planets, including pressure-temperature profiles, spectra, and characteristic radiative time constants. We show that pM Class planets have temperature inversions (hot stratospheres) ∼2000 K and appear “anomalously” bright in the mid infrared secondary eclipse, as was recently found for planets HD 149026b and HD 209458b. Molecular bands of TiO, VO, H2O, and CO will be seen in emission, rather than absorption. This class of planets absorbs incident flux and emits thermal flux from high in their atmospheres. Consequently, they will have large day/night temperature contrasts and negligible phase shifts between orbital phase and thermal emission light curves, because radiative timescales are much shorter than possible dynamical timescales. The pL Class planets absorb incident flux deeper in the atmosphere where atmospheric dynamics will more readily redistribute absorbed energy. This leads to cooler day sides, warmer night sides, and larger phase shifts in thermal emission light curves. We briefly examine the transit radii for both classes of planets. The boundary between these classes is particularly dependent on the incident flux from the parent star, and less so on the temperature of the planet’s internal adiabat (which depends on mass and age), and surface gravity. Around a Sun-like primary, for solar composition, this boundary occurs at ∼0.04-0.05 AU. We apply these results to pM Class transiting planets that are observable with the Spitzer Space Telescope, including HD 209458b, WASP-1b, TrES-3b, TrES-4b, HD 149026b, and others. The eccentric transiting planets HD 147506b and HD 17156b alternate between the classes during their orbits. Thermal emission in the optical from pM Class planets is significant red-ward of 400 nm, making these planets attractive targets for optical detection via Kepler, COROT, and from the ground. The difference in the observed day/night contrast between υ Andromeda b (pM Class) and HD 189733b (pL Class) is naturally explained in this scenario. Subject headings: planetary systems, radiative transfer