Young, rocky planets acquire atmospheres if gases escape their core. We will calculate which gases escape planets, from young to old, and the role that the core of a planet, plate tectonics and volcanism play in this. We will investigate how the climate on such planets develop and whether liquid surface water arises and how much, as this is vital for life like that on Earth. The underlying question is why Earth has oceans and life, whereas Venus is as hot as an oven, dry and dead. Which astronomical observations are needed to determine whether an exoplanet is an "Earth" or a "Venus"? ( Credits)