Internal gravity waves are a common feature of stratified fluids. They facilitate transport of momentum and energy – thus influencing the evolution of the fluid. There is a large body of research addressing the behaviour of gravity waves in the terrestrial atmosphere. This thesis builds and extends the research to giant planets – in particular to close-in extrasolar giant planets and the solar system giant planet, Jupiter. Because the atmospheres of close-in giant planets are expected to be strongly stratified, knowledge of the behaviour of gravity waves in such atmospheres is especially important. Close-in giant planets are thought to have their rotations and orbital period 1:1 synchronised, i.e., they are “tidally locked”. Such planets do not exist in the Solar System. However, many are known from observations of extrasolar systems. Their synchronisation means that they have a permanent day-side and night-side leading to interesting atmospheric dynamics. Modelling these circulations with global circulation models (GCMs) and comparing these models with observations is an active research area. However, many GCMs filter some or all gravity waves removing their effects. This thesis addresses this by explicitly looking at the effects gravity waves can have on the circulation. It is shown that gravity waves provide a mechanism for accelerating, decelerating, and heating the flow. Further, horizontally propagating gravity waves are shown to provide a possible means for coupling the day- and night-sides of tidally locked planets. As well as affecting the dynamics of the atmosphere, gravity wave behaviour is affected by the dynamics of the atmosphere. Therefore, gravity waves can be used to explore atmospheric properties. In this thesis gravity waves observed in Jupiter’s atmosphere, by the Galileo probe, are used to identify features of Jupiter’s atmosphere such as the altitude of the turbopause and the vertical profile of zonal winds at the probe entry site.

The detection of more than 130 multiple planet systems makes it necessary to interpret a broader range of properties than are shown by our Solar system. This thesis covers aspects linked to the proliferation in recent years of multiple extrasolar planet systems. A narrow observational window, only partially covering the longest orbital period, can lead to solutions representing unrealistic scenarios. The best-fit solution for the three-planet extrasolar system of HD 181433 describes a highly unstable configuration. Taking into account the dynamical stability as an additional observable while interpreting the RV data, I have analysed the phase space in the neighbourhood of the statistical best-fit. The two giant companions are found to be locked in the 5:2 MMR in the stable best-fit model. I have analysed the dynamics of the system HD 181433 by assessing different scenarios that may explain the origin of these eccentric orbits, with particular focus on the innermost body. A scenario is considered in which the system previously contained an additional giant planet that was ejected during a period of dynamical instability among the planets. Also considered is a scenario in which the spin-down of the central star causes the system to pass through secular resonance. In its simplest form this latter scenario fails to produce the system observed. If additional short-period low mass planets are present in the system, I find that mutual scattering can release planet b from the secular resonance, leading to a system with orbital parameters similar to those observed today. Finally, I have studied the evolution of low mass planets interacting with a gas-giant planet embedded in a gaseous disc. The transit timing method allows the detection of non-transiting planets through their gravitational perturbations. I have investigated the detectability of low mass planets neighbouring short-period giants after protoplanetary disc dispersal.

Over two hundred extrasolar planets have been discovered to date with various methods. This thesis reports on searching for extrasolar planets and characterising them by simulating their atmospheres. We used open clusters as targets for deep transit searches, with specific emphasis on the University of St. Andrews Planet Search at the Isaac Newton Telescope. We reduced CCD photometry and described the algorithm we used to search for transits. We estimated the number of transits we expect from our data. We then reduced photometry for the open cluster NGC 6940. From that data we found 18 low-amplitude, short-duration events, though none are transiting planets. They are all eclipsing binary stars. However, our null result constrains the number of planets around M dwarfs, the most numerous stars in our sample. In order to characterise reflected light from extrasolar planets, we built a three-dimensional Monte Carlo based radiation transfer model of extrasolar planetary atmospheres. We detailed the input parameters of the model, and show results of various models, focusing especially on the fractal nature of the clouds of our models, because these are the first three dimensional radiation transfer models of extrasolar planet atmospheres. We found very low geometric albedos in our simulations. Using data specific to the transiting planet HD 209458b, we built a model atmosphere with Rayleigh-scattering hydrogen gas and clouds of enstatite and iron. We show in several models the rarity of a bright HD 209458b, and conclude with some explanations on why extrasolar planets are likely dark and not detected with reflected light.