This dissertation explores the consequences of atmospheric dynamics for observations of substellar mass objects (SMOs). Discussed first is the growth of cloud particles of various compositions in brown dwarfs of different surface gravities and effective temperatures. The structure of these objects is calculated with a one-dimensional radiative transfer model. To determine particle sizes, the timescales for microphysical growth processes, including nucleation, coagulation, and coalescence, are compared to the timescale for gravitational sedimentation. The model also allows for sustained uplifting of condensable vapor in convective regions. The results show that particle sizes vary greatly over the range of objects studied. In most cases, clouds on brown dwarfs do not dominate the opacity. Rather, they smooth the emergent spectrum and partially redistribute the radiative energy. The focus then shifts to extrasolar giant planets (EGPs). Results are presented from a three-dimensional model of atmospheric dynamics on the transiting Jupiter-like planet HD 209458b. As a close-in orbiter (known as a “roaster”), HD 209458b is super-heated on its dayside. Due to tidal locking of the interior, the dayside hemisphere faces the star in perpetuity, which leads to very different dynamics than is seen on Jupiter. The flow is characterized by an eastward supersonic jet (u ∼ 4 km s⁻¹) extending from the equator to the mid-latitudes. Temperature contrasts are ∼ 500 K at the photosphere. At 220 mbar, winds blow the hottest regions downstream from the substellar point by ∼ 60°, with direct implications for the infrared light curve. These simulations are extended to the study of carbon chemistry in HD 209458b’s atmosphere by coupling the CO/CH₄ reaction kinetics to the dynamics. Disequilibrium results from slow reaction rates at low temperatures and pressures. Effective vertical quenching near the ∼ 3 bar level leads to uniformly high concentrations of CO at the photosphere, even in cool regions where CH₄ is strongly favored thermodynamically. Observations are underway to detect these signatures of meteorology on HD 209458b and similar planets.

For centuries, an orderly view of planetary system architectures dominated the discourse on planetary systems. However, there is growing evidence that many planetary systems underwent a period of upheaval, during which giant planets "migrated" from where they formed. This thesis addresses a question key to understanding how planetary systems evolve: is planetary migration typically a smooth, disk-driven process or a violent process involving strong multi-body gravitational interactions? First, we analyze evidence from the dynamical structure of debris disks dynamically sculpted during planets' migration. Based on the orbital properties our own solar system's Kuiper belt, we deduce that Neptune likely underwent both planet-planet scattering and smooth migration caused by interactions with leftover planetesimals. In another planetary system, Beta Pictoris, we find that the giant planet discovered there must be responsible for the observed warp of the system's debris belt, reconciling observations that suggested otherwise. Second, we develop two new approaches for characterizing planetary orbits: one for distinguishing the signal of a planet's orbit from aliases, spurious signals caused by gaps in the time sampling of the data, and another to measure the eccentricity of a planet's orbit from transit photometry, "the photoeccentric effect." We use the photoeccentric effect to determine whether any of the giant planets discovered by the Kepler Mission are currently undergoing planetary migration on highly elliptical orbits. We find a lack of such "super-eccentric" Jupiters, allowing us to place an upper limit on the fraction of hot Jupiters created by the stellar binary Kozai mechanism. Finally, we find new correlations between the orbital properties of planets and the metallicity of their host stars. Planets orbiting metal-rich stars show signatures of strong planet-planet gravitational interactions, while those orbiting metal-poor stars do not. Taken together, the results of thesis suggest that suggest that both disk migration and planet-planet interactions likely play a role in setting the architectures of planetary systems. / Astronomy

Thanks to a range of discovery methods that are sensitive to different regions of parameter space, we now know of over 900 planets in over 700 planetary systems. This large population has allowed exoplanetary scientists to move away from a focus on simple discovery, and towards efforts to study the bigger pictures of planetary system formation and evolution. The interactions between planets and their host stars have proven to be varied in both mechanisms and scope. In particular, tidal interactions seem to affect both the physical and dynamical properties of planetary systems, but characterising the broader implications of this has proven challenging. In this thesis I present work that investigates different aspects of tidal interactions, in order to uncover the scope of their influence of planetary system evolution. I compare two different age calculation methods using a large sample of exoplanet and brown dwarf host stars, and find a tendency for stellar model fitting to supply older age estimates than gyrochronology, the evaluation of a star's age through its rotation (Barnes 2007). Investigating possible sources of this discrepancy suggests that angular momentum exchange through the action of tidal forces might be the cause. I then select two systems from my sample, and investigate the effect of tidal interactions on their planetary orbits and stellar spin using a forward integration scheme. By fitting the resulting evolutionary tracks to the observed eccentricity, semi-major axis and stellar rotation rate, and to the stellar age derived from isochronal fitting, I am able to place constraints on tidal dissipation in these systems. I find that the majority of evolutionary histories consistent with my results imply that the stars have been spun up through tidal interactions as the planets spiral towards their Roche limits. I also consider the influence of tidal interactions on the alignment between planetary orbits and stellar spin, presenting new measurements of the projected spin-orbit alignment angle, λ, for six hot Jupiters. I consider my results in the context of the full ensemble of measurements, and find that they support a previously identified trend in alignment angle with tidal timescale, implying that tidal realignment might be responsible for patterns observed in the λ distribution.