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Planets migrate when close to their star

An international collaboration led by the Irfu has demonstrated that exoplanets in orbit close to their star are rapidly approaching or moving away from it under the combined effects of tidal forces and magnetism. The consequences of these two-way migrations (spreading apart or coming closer together) could soon be observed via the distribution of exoplanets closer to and farther from their star, by several space observatories: NASA's Tess (Transiting Exoplanet Survey Satellite) in 2018, or the ESA's Plato (PLAnetary Transits and Oscillations of stars) in 2024.

Published on 23 November 2017

Since their discovery in 1995, more than 3,600 exoplanets have been detected. These extra-solar systems are very different from our own solar system. In particular, many of them have one or more planets in very close orbit to their star.

Researchers have compared for the first time the mechanisms by which this type of planet can migrate. On the one hand, if the planet has a magnetic field, it can interact with the wind of magnetized matter coming from the star. In this case, the orbital motion of the planet slows or accelerates and migration begins. On the other hand, the tidal forces between the star and the planet can deform the star by extending it in the direction of the planet, producing waves similar to the tides on Earth. As a result, there is the possibility of a migration resembling that of the moon, which moves about four centimeters away from us every year.

Depending on the mass of the central star, the nature of the planet (a gaseous or telluric giant) and the rotation speed of the star, one of these mechanisms can dominate the other. The two effects sometimes add together and increase global migration to a previously unsuspected level. Finally, whatever the mechanism, the planet can also move closer to or further away from its star, depending on the distance between them.

These results provide a better understanding of the distance of exoplanets in close orbit around young stars because they possess intense magnetic fields and "rapidly" change their rotation speed at the beginning of their "life". They will also make it possible to predict the distribution of planets around red dwarf stars, in which the habitable zone compatible with the presence of liquid water is close to the star. A good reason for why these planets are actively sought after…

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