This PhD thesis details the development of a new 1D ionospheric model to describe the upper atmospheres of extrasolar giant plants. The upper atmospheres of Hot Jupiters are subject to extreme radiation conditions that can result in rapid atmospheric escape. The composition and structure of these planets' upper atmospheres are affected by high-energy emissions from the host star. The nature of these emissions depends on the stellar type and age, making them important factors in understanding the behaviour of exoplanetary atmospheres.