Rogue planets, or free-floating planets (FFPs), are dark stars, cosmic orphans, without heat or light – or the sun. These icy wanderers, lost in interstellar space and only lit by "that darkling brightness which falls from the stars," as described in Pierre Corneille's Le Cid (1637), might be thought to be the last place to look for life. But astrophysics has a taste for paradoxes, so some original modeling, published on March 20 by the International Journal of Astrobiology, shows that, under certain conditions, the potential satellites of these rogue planets could have water flowing on their surfaces.
The role of tidal forces
CNRS researcher at the Observatoire de la Côte d'Azur and co-author of the article, Andrea Chiavassa said that only a few dozen of these FFPs have been discovered so far: "This is just the tip of the iceberg. According to a 2011 study, there could be at least as many in the galaxy as there are stars." Several reasons can 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 result in the outright expulsion of a planet.
However, knowing what is understood about the Solar System, the giant planets are escorted by many moons, sometimes of enormous size, like, for example, Jupiter's satellite Ganymede, which is larger than the planet Mercury. Numerical simulation shows that an ejected gas giant could certainly take with it one or more companions in exile. But then a crucial question arises: without the rays of a sun, how could enough energy be obtained in order to have liquid water on the surface of these moons?
This is where tidal forces come in. If a satellite evolves in an elliptical orbit, it moves away from and then closer to the giant planet. The tidal forces that the latter exerts on it, therefore, vary incessantly, kneading its insides constantly and producing internal friction that results in the dissipation of a large amount of energy.
This is why Io, one of Jupiter's moons, experiences such violent volcanic activity. The study published by the International Journal of Astrobiology shows that, under particular conditions (close orbit to the planet, a similar size to Earth, a thick CO2 atmosphere, and significant 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 length of time it took life to emerge on Earth.
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