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

Abstract

We highlight the importance of gaseous TiO and VO opacity on the highly irradiated close-in giant planets. The dayside 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 ($0.04Y0.05 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 And b ( pM class) and HD 189733b (pL class) is naturally explained in this scenario. Subject headingg s: planetary systems -radiative transfer

Characterizing the Atmospheres of Hot Jupiters: From Spectra to Multi-Color Maps

“…Similarly, it has been suggested , Burrows et al 2007, Fortney et al 2008) that the presence of gas-phase TiO or VO in the atmosphere might also provide the necessary opacity. These compounds are predicted to have condensed out of the relatively cool atmospheres of HD 189733b and TrES-1, whereas they might still remain in gas phase in the hottest regions on the day side of HD 209458b.…”

Two Classes of Hot Jupiter Atmospheres

“…A key difference between the atmospheres of the pL Class planets and pM Class planets is the pressure at which the absorption and emission of flux occurs (Fortney et al 2008). This can be shown by plotting the pressure that corresponds to a calculated brightness temperature.…”

“…What effect this may have on the dynamical redistribution of energy can be considered after calculation of the radiative time constant, τ rad . In the Newtonian cooling approximation a temperature disturbance relaxes exponentially toward radiative equilibrium with a characteristic time constant (see Showman & Guillot 2002;Fortney et al 2008).…”

“…The first known transiting planet, HD 209458b, is seen to be fairly representative in terms of incident flux. Fortney et al (2008) argue that based on the examination of a few physical processes, two classes of hot Jupiter atmospheres emerge. Equilibrium chemistry abundances, 248 Jonathan J. Fortney the depth to which incident flux will penetrate into a planet's atmosphere, and the radiative time constant as a function of pressure and temperature in the atmosphere all naturally define two classes these irradiated planets.…”

Testing substellar models with dynamical mass measurements

“…At temperatures below ∼2000 K, dust and H 2 O are the major sources of opacity in the photosphere, and below ∼1400 K methane absorption becomes important. Over the last decade, new theory has been developed to describe such low-temperature objects, which encompass brown dwarfs and gas-giant planets (e.g., [1,2,[4][5][6]13]). These models now form the basis of our understanding of all low-mass gaseous objects, from stars at the bottom of the main sequence to extrasolar giant planets.…”