A three-dimensional coupled thermosphere-ionosphere model for extrasolar giant planets (EXOTIM)
has been developed. This is the first such model reported in the literature. This thesis contains an
extensive description of the model and the methods adopted in modelling the different physical processes
expected in the upper atmospheres and ionospheres of extrasolar giant planets. Modelling the upper
atmosphere is important because the stability of the atmosphere against thermal evaporation is controlled
by the conditions in the thermosphere. The thermosphere is heated by the absorption of EUV and X
ray (XUV) radiation emitted by the host star. The radiation also ionises the neutral species in the
upper atmosphere, which is expected to be composed mainly of molecular and atomic hydrogen, and
atomic helium. Ionisation and subsequent photochemistry leads to the formation of the H^+, H^+_2 , H^+_3 ,
and He^+ ions (and small quantities of HeH^+). H^+_3 emits strongly in the infrared and may act as a
significant coolant in gas giant thermospheres. Assuming photochemical equilibrium, the absorption of
XUV radiation and ion photochemistry were modelled in a self-consistent fashion. The 3D model can
also simulate strong winds affecting the upper atmosphere, and account for both advection and diffusion
of the neutral species around the planet. The results indicate that within 1.0 AU from a solar-type
host star, the upper atmospheres of Jupiter-type EGPs can be substantially cooler and more stable
than implied by studies that ignore the possibility of radiative (H^+_3 ) cooling. In this context, a limiting
distance, or a stability limit, was identified for such EGPs that depends on the composition of the upper
atmosphere and ionosphere, and within which the atmospheres of the planets undergo hydrodynamic
escape. Under restricted conditions, this limit is located around 0.15 AU from a Sun-like host star. The
model was also used to simulate a newly found transiting planet HD17156b, which orbits its host star
on a highly eccentric orbit