Introduction

Since the beginning of this century, astronomers have discovered hundreds of exoplanets, almost all of them being giant planets. However, we expect the possibility of detecting relatively smaller exoplanets similar in size to our Earth soon. Among the thousands of exoplanets that will be discovered by the end of this century, some may host life. Obviously the possibility of finding life on another planet is not only a function of the number of discovered planets, but also of the stability of life on those planets: if life is only a glimpse, the search for life will be much harder! We have under our feet a marvellous example demonstrating that one kind of life may be hosted on a planet (Earth) for billions of years. This relative stability of life on Earth seems to be strongly correlated with the stable environmental conditions that have prevailed on the surface of our planet for several billion years (1 Ga = 109 years). This is the reason why this chapter is devoted to deciphering and understanding the ‘stable’ climate conditions on Earth since 3.8 Ga. Observation of our neighbouring planets in the Solar System teaches us that the conditions for the development of life (habitability) and sustainability at the surface of a planet are not widespread – at least in the Solar System. Nowadays, Mars is a very cold desert experiencing dust storms, whereas Venus is a burning hell whose surface is totally hidden by a thick greenhouse-gas atmosphere.