Since the dawn of humanity, one question that has animated philosophers is that of life. The debate about its origins, and its uniqueness on Earth, is at the heart of almost all religions. Ancient philosophers tried to answer this question with the limited technical means at their disposal. Nowadays, the development of technology allows us to observe the Universe with unprecedented precision. Since the detection of the first extra-solar planet orbiting a solar-type star named Pegasi 51b, in 1995, an impressive number of planets with various characteristics have been discovered. Some are similar to Jupiter but with extreme temperatures (so-called Hot-Jupiter), while others are similar in size and mass to our Earth. The study of these small, rocky planets raises questions about their habitability. Are they really Earth-like? Or more like Venus? Or even Mars? Can they have liquid water on their surface? This is an essential condition for life as we know it.

This is the context of my PhD work. Using different 1-D and 3-D numerical climate models, I have studied the major processes, particularly the runaway greenhouse effect, which can turn a habitable planet into a totally uninhabitable planet. Or should I say, removing any possibility of water in liquid phase on the surface of these planets. The conditions governing the existence of a "habitable" climate are multiple and induce climate multistability. Therefore, I also studied the possible transition between these states, via a perturbation induced by a stochastic event. However, climate modelling is not an isolated science. The atmosphere interacts with the rocky interior of the planet, as well as with its host star. Climate is therefore influenced by numerous external processes, which must be understood in order to model reality as accurately as possible. Moreover, the atmospheric composition is absolutely pivotal to the study of climate. As we can see, the Earth's climate change is induced by an increase in anthropogenic CO₂, modifying the climatic response. Therefore, I have also studied in detail the molecular spectroscopy needed to produce the data essential for climate models. This is an important preliminary step to guarantee the robustness and accuracy of the models we use to describe exoplanets.

Finally, the aim to better understand the processes governing the climate of small, temperate, rocky planets is part of a broader effort to strengthen our knowledge for promising future observational missions. The quantity and precision of the expected data will require high-performance tools for analysis. Climate models will certainly play a key role in this process.