Exoplanets

Giant Planet Atmospheres

1. INTRODUCTION Our understanding of gas giant planets was informed for many decades by remote telescopic observations and in situ measurements of Jupiter and Saturn. These detailed investigations provided a fine-grained view of their atmospheric compositions, temperatures, dynamics, and cloud structures. However, they left us with a parochial view of the range of possible orbits, masses, and compositions that has now been shattered by the discovery of extrasolar giant planets (EGPs) in the hundreds. We have found gas giants in orbits from ~0.02 AU to many AU, with masses from below Neptune’s to ~10 MJup, and around stars from M to F dwarfs. The corresponding stellar irradiation fluxes at the planet vary by a factor of ~105, and this variation translates into variations in atmosphere temperatures from ~100 K to ~2500 K. With such a range of temperatures and of orbital distances, masses, and ages, atmospheres can have starkly different compositions, can be clear or cloudy, and can evince dramatic day-night contrasts. One must distinguish imaging of the planet itself by separating the light of planet and star, something that can currently be contemplated only for wide-separation planets, from measurements of the summed light when the orbit is tight and the planet cannot be separately imaged. In the latter case, the planet’s light can be a nontrivial fraction of the total, particularly in the infrared (IR). When transiting, such hot Jupiter systems provide an unprecedented opportunity to measure the planet’s emissions by the difference in the summed light of planet and star in and out of secondary eclipse and by the phase variation of that sum. Generally, the star itself will not vary with the period of the planet’s orbit. Moreover, in a complementary, but different, fashion, the wavelength dependence of the transit depth is now being used to probe the composition of the planet’s atmosphere near the terminators. The EGPs, by dint of their mass and luminosity, have been the first discovered, and will serve as stepping stones to the terrestrial exoplanets. To understand in physical detail the growing bestiary of EGPs requires chemistry to determine compositions, molecular and atomic spectroscopy to derive opacities, radiative transfer to predict spectra, hydrodynamics to constrain atmospheric dynamics and heat redistribution, and cloud physics. In short, global three-dimensional radiation-hydrodynamic general circulation models (GCMs) with multispectral, multiangle, and nonequilibrium chemistry and kinetics will be needed. We are not there yet, but basic treatments have emerged that allow us to interpret and constrain day-night differences, profiles, molecular compositions, and phase lightcurves. In this chapter we lay out some of the basic elements of any theoretical treatment of the atmospheres, spectra, and lightcurves of EGPs. This theory provides the necessary underpinnings for any progress in EGP studies, a subject that is engaging an increasing fraction of the world’s astronomical and planetary science communities. In section 2.1, we summarize the techniques for calculating molecular abundances. We follow in section 2.2 with an explication of general methods for assembling opacity tables. Section 2.3 touches on Rayleigh scattering, and then we continue in section 2.4 with a tutorial on albedos and phase functions. In section 2.5, we explain the nature of the transit radius. Section 2.7 contains a very useful analytic model for the atmospheric thermal profile of EGPs, which is a generalization for irradiated atmospheres of 419 Giant Planet Atmospheres Adam Burrows Princeton University Glenn Orton Jet Propulsion Laboratory, California Institute of Technology Direct measurements of the spectra of giant exoplanets are the keys to determining their physical and chemical nature. The goal of theory is to provide the tools and context with which such data are understood. It is only by putting spectral observations through the sieve of theory that the promise of exoplanet research can be realized. With the new Spitzer and Hubble Space Telescope data of transiting “hot Jupiters,” we have now dramatically entered the era of remote sensing. We are probing their atmospheric compositions and temperature profiles, are constraining their atmospheric dynamics, and are investigating their phase lightcurves. Soon, many nontransiting exoplanets with wide separations (analogs of Jupiter) will be imaged and their lightcurves and spectra measured. In this paper, we present the basic physics, chemistry, and spectroscopy necessary to model the current direct detections and to develop the more sophisticated theories for both close-in and wide-separation giant exoplanets that will be needed in the years to come as exoplanet research accelerates into...