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Understanding the structure and composition of gas giants is of basic importance to planetary astrophysics. Our local exemplars Jupiter and Saturn permit spatially resolved observations from Earth as well as sensitive in situ observations by spacecraft. This intersection of physical accessibility and conceptual importance for the planet formation process renders Jupiter and Saturn essential for establishing baseline truth for exploration of the assembly and evolution of planetary systems in general. In this thesis I develop new models for the structure and evolution of these solar system gas giants, bringing spacecraft observations to bear on our understanding of the physical processes at work in giant planet interiors.I first describe evolutionary models for Jupiter and Saturn that incorporate results from first-principles simulations of hydrogen-helium mixtures at high pressures to address how the helium distribution is likely to evolve in these planets as they cool. Bayesian parameter estimation is used to retrieve the distribution of likely thermal histories for Jupiter and Saturn. I present models that reconcile Jupiter and Saturn’s observed heat flow, and Jupiter’s atmospheric helium depletion, at the solar age. These solutions put stringent limits on the uncertain physics of helium immiscibility, and translate to a precise prediction for the unknown atmospheric helium content of Saturn.Second, I describe seismology of Saturn using ring waves driven by gravitational perturbations from the planet's nonradial oscillations. I present a family of Saturn interior models together with their normal mode eigenfrequencies and corresponding resonances with orbits in Saturn’s C ring, where more than twenty otherwise unexplained waves have been characterized using Cassini stellar occultation data. I identify the fundamental modes of Saturn at the origin of these ring waves, and use their observed frequencies and azimuthal wavenumbers to estimate Saturn’s rotation period t

Understanding the structure and composition of gas giants is of basic importance to planetary astrophysics. Our local exemplars Jupiter and Saturn permit spatially resolved observations from Earth as well as sensitive in situ observations by spacecraft. This intersection of physical accessibility and conceptual importance for the planet formation process renders Jupiter and Saturn essential for establishing baseline truth for exploration of the assembly and evolution of planetary systems in general. In this thesis I develop new models for the structure and evolution of these solar system gas giants, bringing spacecraft observations to bear on our understanding of the physical processes at work in giant planet interiors.I first describe evolutionary models for Jupiter and Saturn that incorporate results from first-principles simulations of hydrogen-helium mixtures at high pressures to address how the helium distribution is likely to evolve in these planets as they cool. Bayesian parameter estimation is used to retrieve the distribution of likely thermal histories for Jupiter and Saturn. I present models that reconcile Jupiter and Saturn’s observed heat flow, and Jupiter’s atmospheric helium depletion, at the solar age. These solutions put stringent limits on the uncertain physics of helium immiscibility, and translate to a precise prediction for the unknown atmospheric helium content of Saturn.Second, I describe seismology of Saturn using ring waves driven by gravitational perturbations from the planet's nonradial oscillations. I present a family of Saturn interior models together with their normal mode eigenfrequencies and corresponding resonances with orbits in Saturn’s C ring, where more than twenty otherwise unexplained waves have been characterized using Cassini stellar occultation data. I identify the fundamental modes of Saturn at the origin of these ring waves, and use their observed frequencies and azimuthal wavenumbers to estimate Saturn’s rotation period t

The study of planets around other stars has entered a science-rich era of characterization, in which detailed information about individual planets can be inferred from observations beyond discovery and confirmation, which only yield bulk properties like mass or radius. Characterization probes more revealing quantities such as chemical abundances, albedo, and temperature/pressure profiles, allowing us to address larger questions of planet formation mechanisms, planetary evolution, and, eventually, presence of biosignature gases. The primary method for characterization of close-in planets is transit spectroscopy. My dissertation comprises transiting exoplanet case studies using the Hubble Space Telescopes Wide-Field Camera-3 (HST/WFC3) as a tool of exoplanet characterization in a near-infrared band dominated by broad water absorption. Much of my efforts went toward a characterization of the WFC3 systematic effects that must be mitigated to extract the incredibly small (tens to 200 parts per million) signals. The case study subjects in this dissertation are CoRoT-2b (in emission), WASP-18b (in transmission and emission), and HATS-7b (in transmission), along with some partial/preliminary analyses of HAT-p-3b and HD 149026b (both in transmission). I also present an analysis of transit timing of WASP-18b with HST and other observatories as another clue to its evolution as a close-in, extremely massive planet purported to be spiraling in to its host star. The five planets range from super Neptunes to Super-Jupiter in size/mass. The observability of such planets – i.e. giants across a continuum of mass/size in extreme local environments close to their respective host stars, – is a unique opportunity to probe planet formation and evolution, as well as atmospheric structures in a high-irradiation environment. This genre of observations reveal insights into aerosols in the atmosphere; clouds and/or hazes can significantly impact atmospheric chemistry and observational signatur

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 especially common outcomes (although this remains an open question). Here, we review the diverse range of interior structures that are both known and speculated to exist in exoplanetary systems--from mostly degenerate objects that are more than 10× 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 Earth.

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 especially common outcomes (although this remains an open question). Here, we review the diverse range of interior structures that are both known and speculated to exist in exoplanetary systems--from mostly degenerate objects that are more than 10× 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 Earth.