The briefness of this ‘habitable’ period does not exclude an effective habitability in the past. In effect, on Earth, life appeared over the course of the first billion years: ‘ If all the conditions come together, life can appear very quickly,’ explains Emmanuelle Javaux. ‘On Mars, it could also have appeared rapidly, when the planet was habitable, and then disappeared...but we just don’t know. To know if that is the case, it is necessary to carry out in situ studies. Every Mars mission begins with a study of the planet’s geology and the evolution of paleo-environments in order to determine when and where Mars could have been habitable.’

The second major target for astrobiology is Europa, an ice satellite of Jupiter. The absence of atmosphere prevents the presence of liquid water on its surface. Nonetheless an ocean is hidden under its icy crust. ‘This could be an interesting place to look for life,’ imagines Emmanuelle Javaux, ‘on condition that this water is in contact with a rocky substrate. In effect, a potential ice layer between the ocean and the rock would prevent life from drawing on the chemical elements of the rocks in order to ‘feed itself’.’

Finally, with the hundreds of exoplanets which have been discovered to date, the search for extraterrestrial life will not be restricted to our solar system, even if it is necessary to remain cautious: ‘the problem is that we will not be able to go to these exoplanets. Moreover, discovering the traces of water vapour would not necessarily mean that life abounds there. Conversely, the absence of ozone would not enable us to exclude every presence of life. In effect our atmosphere did not possess ozone for the first 2.5 billions of years, whilst the Earth was already teeming with life. Given all these uncertainties, it seems difficult to be able to affirm one day that there is life on an exoplanet. That takes nothing away from the interest of studying the diversity of the planets in the universe, as well as the ordinariness or the specificity of our solar system.’