Abstract
We highlight the importance of gaseous TiO and VO opacity on the highly irradiated close-in giant planets. The day-side atmospheres of these planets naturally fall into two classes that are somewhat analogous to the M- and L-type dwarfs. Those that are warm enough to have appreciable opacity due to TiO and VO gases we term ``pM class'' planets, and those that are cooler we term ``pL class'' planets. 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), appear ``anomalously'' bright in the mid-infrared secondary eclipse, and feature molecular bands in emission rather than absorption. From simple physical arguments, we show that 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 will lead to cooler day sides, warmer night sides, and larger phase shifts in thermal emission light curves. The boundary between these classes (~0.04-0.05 AU from a Sun-like primary for solar composition) is particularly dependent on the incident flux from the parent star, and less so on other factors. We apply these results to several planets and note that the eccentric transiting planets HD 147506b and HD 17156b alternate between the classes. Thermal emission in the optical from pM class planets is significant redward of 400 nm, making these planets attractive targets for optical detection. The difference in the observed day/night contrast between υ And b (pM class) and HD 189733b (pL class) is naturally explained in this scenario.