The architecture of extrasolar planetary systems often differs strongly from our own solar system. Many exoplanets orbit their host stars at close distances, with semi-major axes of only a few stellar radii; this can influence the evolution of the planetary atmospheres as well as the evolution of the host stars themselves. It is a long-standing question if massive close-in planets can influence the physical properties of their host stars, especially their magnetic activity level. While cool stars usually spin down with age and become inactive, an input of angular momentum through tidal interaction, as seen for example in close binaries, can preserve high activity levels over time. This may also be the case for cool stars hosting massive, close-in planets. A variety of magnetic interaction scenarios has also been explored in models. However, selection effects from planet detection methods may skew the activity levels seen in samples of exoplanet host stars, so caution is warranted. Recent observational advances have made it possible to study these effects in an unambiguous manner, with intriguing results on the evolution of host star activity and mass loss of exoplanetary atmospheres. I will conclude with an outlook of how our emerging insights into stellar ages, magnetic activity, and exoplanet atmospheres will lead to a comprehensive understanding of exoplanet systems.
Followed by wine and cheese in Pupin 1402.