There are dark stars, cosmic orphans, planets without sun, heat or light. They are called in English free-floating planets (FFP) – which could be translated as “free-floating planets” – and in French wandering planets. These icy wanderers, lost in interstellar space and only lit of « this dark light that falls from the stars” dear to the Cid, could be considered the last place to look for life. But astrophysics tastes paradoxes, and an original model, published on March 20 by theInternational Journal of Astrobiology, shows that, under certain conditions, possible satellites of these wandering planets could see water flowing on their surface.
The role of tidal forces
CNRS researcher at the Côte d’Azur Observatory and co-signer of this article, Andrea Chiavassa recalls that only a few dozen of these FFPs have so far been discovered: “This is just the tip of the iceberg. According to a 2011 study, there could be at least as many of them in the galaxy as there are stars. » Several mechanisms explain their existence. The most common would be ejection: in a very young system comprising several giant planets around a star, the orbits are far from stable and gravitational disturbances can lead to the pure and simple expulsion of a planet.
However, if we stick to what we know in the Solar System, the giant planets are accompanied by numerous more or less massive moons such as, for example, for Jupiter, the satellite Ganymede, larger than the planet Mercury. Numerical simulation shows that an ejected gas giant could easily take one or more exile companions with it. But then a crucial question arises: how, without the rays of a sun, to obtain enough energy to have liquid water on the surface of these moons?
To be tidal forces come into play. If a satellite is in an elliptical orbit, it moves away then approaches the giant planet. The tidal forces that it exerts on it therefore vary ceaselessly, kneading its entrails permanently and producing internal frictions which result in the dissipation of a great quantity of energy. This is how Io, one of Jupiter’s moons, is shaken by furious volcanism. The study published by theInternational Journal of Astrobiology shows that, under particular conditions (an orbit close to the planet, a size similar to that of the Earth, an atmosphere of CO2 thick and high pressure), the satellite of an FFP can benefit from enough heat to keep liquid water on its surface for more than a billion years, roughly the time it took to life to emerge on Earth.
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