The Structure of Exoplanets

The hundreds of exoplanets that have been discovered in the past two decades offer a new perspective on planetary structure. Instead of being the archetypal examples of planets, those of our Solar System are merely possible outcomes of planetary system formation and evolution, and conceivably not even terribly common outcomes (although this remains an open question). Here, we review the diverse range of interior structures that are known to, and speculated to, exist in exoplanetary systems -- from mostly degenerate objects that are more than 10 times as massive as Jupiter, to intermediate-mass Neptune-like objects with large cores and moderate hydrogen/helium envelopes, to rocky objects with roughly the mass of the Earth.Comment: To be published in PNAS special issue on exoplanets. 6 pages, 3 figure

Analysis of Spitzer Spectra of Irradiated Planets: Evidence for Water Vapor?

Published mid infrared spectra of transiting planets HD 209458b and HD 189733b, obtained during secondary eclipse by the InfraRed Spectrograph (IRS) aboard the Spitzer Space Telescope, are predominantly featureless. In particular these flux ratio spectra do not exhibit an expected feature arising from water vapor absorption short-ward of 10 um. Here we suggest that, in the absence of flux variability, the spectral data for HD 189733b are inconsistent with 8 um-photometry obtained with Spitzer's InfraRed Array Camera (IRAC), perhaps an indication of problems with the challenging reduction of the IRS spectra. The IRAC point, along with previously published secondary eclipse photometry for HD 189733b, are in good agreement with a one-dimensional model of HD 189733b that clearly shows absorption due to water vapor in the emergent spectrum. We are not able to draw firm conclusions regarding the IRS data for HD 209458b, but spectra predicted by 1D and 3D atmosphere models fit the data adequately, without adjustment of the water abundance or reliance on cloud opacity. We argue that the generally good agreement between model spectra and IRS spectra of brown dwarfs with atmospheric temperatures similar to these highly irradiated planets lends confidence in the modeling procedure.Comment: Revised, Accepted to ApJ Letter

Exoplanet albedo spectra and colors as a function of planet phase, separation, and metallicity

First generation optical coronagraphic telescopes will obtain images of cool gas and ice giant exoplanets around nearby stars. The albedo spectra of exoplanets at planet-star separations larger than about 1 AU are dominated by reflected light to beyond 1 {\mu}m and are punctuated by molecular absorption features. We consider how exoplanet albedo spectra and colors vary as a function of planet-star separation, metallicity, mass, and observed phase for Jupiter and Neptune analogs from 0.35 to 1 {\mu}m. We model Jupiter analogs with 1x and 3x the solar abundance of heavy elements, and Neptune analogs with 10x and 30x. Our model planets orbit a solar analog parent star at separations of 0.8 AU, 2 AU, 5 AU, and 10 AU. We use a radiative-convective model to compute temperature-pressure profiles. The giant exoplanets are cloud-free at 0.8 AU, have H2O clouds at 2 AU, and have both NH3 and H2O clouds at 5 AU and 10 AU. For each model planet we compute moderate resolution spectra as a function of phase. The presence and structure of clouds strongly influence the spectra. Since the planet images will be unresolved, their phase may not be obvious, and multiple observations will be needed to discriminate between the effects of planet-star separation, metallicity, and phase. We consider the range of these combined effects on spectra and colors. For example, we find that the spectral influence of clouds depends more on planet-star separation and hence temperature than metallicity, and it is easier to discriminate between cloudy 1x and 3x Jupiters than between 10x and 30x Neptunes. In addition to alkalis and methane, our Jupiter models show H2O absorption features near 0.94 {\mu}m. We also predict that giant exoplanets receiving greater insolation than Jupiter will exhibit higher equator to pole temperature gradients than are found on Jupiter and thus may have differing atmospheric dynamics.Comment: 62 pages, 19 figures, 6 tables Accepted for publication in Ap

Cassini Ring Seismology as a Probe of Saturn's Interior I: Rigid Rotation

Seismology of the gas giants holds the potential to resolve long-standing questions about their internal structure and rotation state. We construct a family of Saturn interior models constrained by the gravity field and compute their adiabatic mode eigenfrequencies and corresponding Lindblad and vertical resonances in Saturn's C ring, where more than twenty waves with pattern speeds faster than the ring mean motion have been detected and characterized using high-resolution Cassini Visual and Infrared Mapping Spectrometer (VIMS) stellar occultation data. We present identifications of the fundamental modes of Saturn that appear to be the origin of these observed ring waves, and use their observed pattern speeds and azimuthal wavenumbers to estimate the bulk rotation period of Saturn's interior to be $10{\rm h}\, 33{\rm m}\, 38{\rm s}^{+1{\rm m}\, 52{\rm s}}_{-1{\rm m}\, 19{\rm s}}$ (median and 5%/95% quantiles), significantly faster than Voyager and Cassini measurements of periods in Saturn's kilometric radiation, the traditional proxy for Saturn's bulk rotation period. The global fit does not exhibit any clear systematics indicating strong differential rotation in Saturn's outer envelope.Comment: 19 pages, 6 figures, 3 tables, accepted to ApJ; a bug fix improves the fit, predicts faster bulk spin periods (Figure 4) and virtually eliminates evidence for strong radial differential rotation (Figure 5

Spectral Variability from the Patchy Atmospheres of T and Y Dwarfs

Brown dwarfs of a variety of spectral types have been observed to be photometrically variable. Previous studies have focused on objects at the L/T transition, where the iron and silicate clouds in L dwarfs break up or dissipate. However, objects outside of this transitional effective temperature regime also exhibit variability. Here, we present models for mid-late T dwarfs and Y dwarfs. We present models that include patchy salt and sulfide clouds as well as water clouds for the Y dwarfs. We find that for objects over 375 K, patchy cloud opacity would generate the largest amplitude variability within near-infrared spectral windows. For objects under 375 K, water clouds also become important and generate larger amplitude variability in the mid-infrared. We also present models in which we perturb the temperature structure at different pressure levels of the atmosphere to simulate hot spots. These models show the most variability in the absorption features between spectral windows. The variability is strongest at wavelengths that probe pressure levels at which the heating is the strongest. The most illustrative types of observations for understanding the physical processes underlying brown dwarf variability are simultaneous, multi-wavelength observations that probe both inside and outside of molecular absorption features.Comment: 6 pages, 5 figures, Accepted for publication in ApJ Letter

Transmission Spectra of Transiting Planet Atmospheres: Model Validation and Simulations of the Hot Neptune GJ 436b for JWST

We explore the transmission spectrum of the Neptune-class exoplanet GJ 436b, including the possibility that its atmospheric opacity is dominated by a variety of non- equilibrium chemical products. We also validate our transmission code by demonstrating close agreement with analytic models that use only Rayleigh scattering or water vapor opacity. We find broad disagreement with radius variations predicted by another published model. For GJ 436b, the relative coolness of the planet's atmosphere, along with its implied high metallicity, may make it dissimilar in character compared to "hot Jupiters." Some recent observational and modeling efforts suggest low relative abundances of H2O and CH4 present in GJ 436b's atmosphere, compared to calculations from equilibrium chemistry. We include these characteristics in our models and examine the effects of absorption from methane-derived higher order hydrocarbons. Significant absorption from HCN and C2H2 are found throughout the infrared, while C2H4 and C2H6 are less easily seen. We perform detailed simulations of JWST observations, including all likely noise sources, and find that we will be able to constrain chemical abundance regimes from this planet's transmission spectrum. For instance, the width of the features at 1.5, 3.3, and 7 microns indicates the amount of HCN versus C2H2 present. The NIRSpec prism mode will be useful due to its large spectral range and the relatively large number of photo-electrons recorded per spectral resolution element. However, extremely bright host stars like GJ 436 may be better observed with a higher spectroscopic resolution mode in order to avoid detector saturation. We find that observations with the MIRI low resolution spectrograph should also have high signal-to-noise in the 5 - 10 micron range due to the brightness of the star and the relatively low spectral resolution (R ~ 100) of this mode.Comment: 33 pages, 12 figures, Accepted to Ap

The atmospheric circulation of the super Earth GJ 1214b: Dependence on composition and metallicity

We present three-dimensional atmospheric circulation models of GJ 1214b, a 2.7 Earth-radius, 6.5 Earth-mass super Earth detected by the MEarth survey. Here we explore the planet's circulation as a function of atmospheric metallicity and atmospheric composition, modeling atmospheres with a low mean-molecular weight (i.e., H2-dominated) and a high mean-molecular weight (i.e. water- and CO2-dominated). We find that atmospheres with a low mean-molecular weight have strong day-night temperature variations at pressures above the infrared photosphere that lead to equatorial superrotation. For these atmospheres, the enhancement of atmospheric opacities with increasing metallicity lead to shallower atmospheric heating, larger day-night temperature variations and hence stronger superrotation. In comparison, atmospheres with a high mean-molecular weight have larger day-night and equator-to-pole temperature variations than low mean-molecular weight atmospheres, but differences in opacity structure and energy budget lead to differences in jet structure. The circulation of a water-dominated atmosphere is dominated by equatorial superrotation, while the circulation of a CO2-dominated atmosphere is instead dominated by high-latitude jets. By comparing emergent flux spectra and lightcurves for 50x solar and water-dominated compositions, we show that observations in emission can break the degeneracy in determining the atmospheric composition of GJ 1214b. The variation in opacity with wavelength for the water-dominated atmosphere leads to large phase variations within water bands and small phase variations outside of water bands. The 50x solar atmosphere, however, yields small variations within water bands and large phase variations at other characteristic wavelengths. These observations would be much less sensitive to clouds, condensates, and hazes than transit observations.Comment: 12 pages, 11 figures, 2 tables, accepted to Ap

Three-dimensional atmospheric circulation of hot Jupiters on highly eccentric orbits

Of the over 800 exoplanets detected to date, over half are on non-circular orbits, with eccentricities as high as 0.93. Such orbits lead to time-variable stellar heating, which has implications for the planet's atmospheric dynamical regime. However, little is known about this dynamical regime, and how it may influence observations. Therefore, we present a systematic study of hot Jupiters on highly eccentric orbits using the SPARC/MITgcm, a model which couples a three-dimensional general circulation model with a plane-parallel, two-stream, non-grey radiative transfer model. In our study, we vary the eccentricity and orbit-average stellar flux over a wide range. We demonstrate that the eccentric hot Jupiter regime is qualitatively similar to that of planets on circular orbits; the planets possess a superrotating equatorial jet and exhibit large day-night temperature variations. We show that these day-night heating variations induce momentum fluxes equatorward to maintain the superrotating jet throughout its orbit. As the eccentricity and/or stellar flux is increased, the superrotating jet strengthens and narrows, due to a smaller Rossby deformation radius. For a select number of model integrations, we generate full-orbit lightcurves and find that the timing of transit and secondary eclipse viewed from Earth with respect to periapse and apoapse can greatly affect what we see in infrared (IR) lightcurves; the peak in IR flux can lead or lag secondary eclipse depending on the geometry. For those planets that have large day-night temperature variations and rapid rotation rates, we find that the lightcurves exhibit "ringing" as the planet's hottest region rotates in and out of view from Earth. These results can be used to explain future observations of eccentric transiting exoplanets.Comment: 20 pages, 18 figures, 2 tables; Accepted to Ap

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 M- and L-type 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 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 hot stratospheres $\sim$2000 K and appear ``anomalously'' bright in the mid infrared secondary eclipse, as was recently found for planets HD 149026b and HD 209458b. 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 will lead to cooler day sides, warmer night sides, and larger phase shifts in thermal emission light curves. Around a Sun-like primary this boundary occurs at $\sim$0.04-0.05 AU. The eccentric transiting planets HD 147506b and HD 17156b alternate between the classes. Thermal emission in the optical from pM Class planets is significant red-ward of 400 nm, making these planets attractive targets for optical detection. The difference in the observed day/night contrast between ups Andromeda b (pM Class) and HD 189733b (pL Class) is naturally explained in this scenario. (Abridged.)Comment: Accepted to the Astrophysical Journa