Classically speaking, we have available three criteria (necessary but maybe not sufficient) to declare a zone habitable: the presence of an energy source, the chemical elements indispensable for life and liquid water. The energy source can be the proximity of a star or a planet’s geological activity. As for the chemical elements indispensable for life  (C, H, O, N, and P), they were the principal constituents of the primeval soup on Earth and are commonplace across the universe.

Finally, the potential presence of water is often connected to the distance the planet finds itself from its star. ‘This practical definition for the presence of liquid water is too limited,’ explains our micro-palaeontologist. ‘It is already brought into disrepute in our solar system. For example, the Moon has no liquid water whilst it finds itself within the Sun’s habitable zone. On the other hand, the ice moons of other planets have an ocean under their surface, despite the fact that they indeed gravitate far from this same habitable zone.’

What characteristics can thus be added to this partial definition of habitability to render it more relevant? If the concept of liquid water is necessary to the concept of habitability, geophysical processes could also be required just as much. Let us analyse more precisely the case of our planet. ‘The presence of liquid water demands not only a suitable temperature, set notably by the distance from the star,’ picks up Emmanuelle Javaux, ‘but sufficient pressure is also necessary in order to prevent its volatilisation. The atmosphere plays a role. Yet our atmosphere is protected by magnetosphere which owes its existence to the mixed core (solid/liquid) of our planet. The structure of a planet’s core thus maybee influences its habitability.’