A star’s magnetic activity allows it to extend its influence far beyond its surface. The Sun’s magnetic field extends billions of kilometres into space, far beyond the orbit of Pluto. Phenomena such as stellar flares and coronal mass ejections are magnetically induced processes that can profoundly influence the atmospheres of close-in exoplanets. It is well established that the magnetic activity of a star depends on its rate of rotation. A star’s rotation gradually slows as angular momentum is carried away by the stellar wind. As a result, the magnetic activity of stars like the Sun decreases over billions of years.
Nevertheless, stars hosting hot-Jupiters (i.e. massive planets in close-in orbits) can maintain fast rotation rates and hence high magnetic activity. In a study by K. Poppenhaeger and S.J. Wolk (2014), 5 binary star systems with widely-spaced stars (over 100 AU apart) were observed in X-rays. For each binary system, one star is known to host a close-in massive planet, while the other star does not have a detected planet and acts as a negative control. Using X-ray emission as an observational proxy for stellar magnetic activity, the stars in each binary system were observed with Chandra and XMM-Newton.
The 5 binary star systems involved in the study are HD189733 AB, CoRoT-2 AB, Tau Boötis AB, Upsilon Andromedae AB and 55 Cancri AB. For 2 of the systems - HD189733 AB and CoRoT-2 AB, where the strongest tidal interaction is expected between planet and host star, the X-ray emission of the planet-hosting star is stronger than expected when compared to the companion star. It implies that when compared to its companion star, the planet-hosting star is over-rotating.
This study shows that hot-Jupiters may inhibit the spin-down of their host stars. Two possible mechanisms can drive this process. One involves the transfer of angular momentum from the planet’s orbit to the star’s rotation through tidal interaction, spinning-up the star as a result. The second mechanism involves the hot-Jupiter opening a gap in the protoplanetary disk during the early evolution of the star, resulting in a weaker star-disk coupling and causing a smaller rate of spin-down.
K. Poppenhaeger and S.J. Wolk (2014), “Indications for an influence of Hot Jupiters on the rotation and activity of their host stars”, arXiv:1404.1073 [astro-ph.SR]