Ancient rivers and lakes discovered on Mars. Numerous temperate, Earth-sized exoplanets detected around nearby stars. Thanks to ground and space-based telescope observations and Solar System exploration missions, we now have a fantastic playground to explore how prevalent life is in the Universe. The main goal of my thesis work is to better understand the conditions necessary for a planet to maintain liquid water - a primary building block for life - on its surface. Using 3-D numerical climate models, as well as spectroscopic calculations and measurements, I conducted two major investigations during my thesis. First, I explored the environments of ancient Mars at multiple epochs in order to understand the conditions in which the enigmatic Martian rivers were carved. Apart from Earth, Mars is the only planet that has been habitable, but we don't know why. I showed that extreme events (outflow channel formation, meteoritic impacts) that scarred the surface of Mars cannot explain the formation of these valley networks. Nonetheless, I showed that the presence of reducing greenhouse gases such as hydrogen and methane offers a promising alternative solution. Secondly, I studied the possible atmospheres of solid, temperate exoplanets, with a particular focus on those orbiting small stars such as Proxima Centauri and TRAPPIST-1. I showed that some of these planets have characteristics that are highly favourable to the presence of liquid water on their surface. This result is really promising as it will be soon become possible - as demonstrated in my thesis for Proxima b - to characterize the atmosphere of these planets with the future JWST and ELTs astronomical observatories.