A giant planet embedded in the magnetosphere of its star

The catalogue of extrasolar planets is growing almost every day, containing more than 200 objects, and the detection of these exoplanets has almost become a routine. But what are the charcateristics of the stellar hosts, how can we explain the formation of these planetary systems, or why are some of these giant exoplanets, which are called ’hot jupiters’, migrating down to very close-in orbits ? Astrophysicists suspect the magnetic field to play a crucial role in some of these questions. However, although indirect effects of magnetic fields have already been detected on stars hosting giant extrasolar planets, no direct measurement has ever been realized until recently. Now, it has been done !

This first measurement has been obtained by an international team of astronomers with the ESPaDOnS spectropolarimeter located on the Canada-France-Hawaii telescope. They detected the magnetic field of tau Bootis, a one billion year old star, with one and a half solar masses and located at nearly 50 light years from the Earth. This cool and weakly active star, orbited by a giant planet with 4.4 Jupiter masses on a very close-in orbit at 0.049 AU (i.e. 5% of the Sun-Earth distance), possess a magnetic field of a few gauss, just a little more than the Sun’s, but showing a more complex structure. Moreover, astronomers have also measured the level of differential rotation of the star, because they know that this parameter is crucial in the generation of the magnetic field. In the present case, the matter located at the equator rotates 18% faster than that located at the poles, leading to one full turn in approximately 15 days. By comparing the differential rotation of the star with the revolution of the giant extrasolar planet, astronomers have noticed that the planet is synchronized with stellar material located at about 45 degrees. This observation suggests very complex interactions between the magnetosphere of the star and its companion, perhaps similar to the interaction of the magnetosphere of Jupiter with its satellite Io, giving rise to the so-called "Io torus".

The data collected for this study are not sufficient to describe precisely these interactions, but this first measurement is opening new prospects for detailed studies of star-planet systems.

Dernière modification le 21 décembre 2021