Planets interact with their host stars through gravity , radiation and magnetic fields , and for those giant planets that orbit their stars within \sim 10 stellar radii ( \sim 0.1 AU for a sun-like star ) , star-planet interactions ( SPI ) are observable with a wide variety of photometric , spectroscopic and spectropolarimetric studies .
At such close distances , the planet orbits within the sub-alfvénic radius of the star in which the transfer of energy and angular momentum between the two bodies is particularly efficient .
The magnetic interactions appear as enhanced stellar activity modulated by the planet as it orbits the star rather than only by stellar rotation .
These SPI effects are informative for the study of the internal dynamics and atmospheric evolution of exoplanets .
The nature of magnetic SPI is modeled to be strongly affected by both the stellar and planetary magnetic fields , possibly influencing the magnetic activity of both , as well as affecting the irradiation and even the migration of the planet and rotational evolution of the star .
As phase-resolved observational techniques are applied to a large statistical sample of hot Jupiter systems , extensions to other tightly orbiting stellar systems , such as smaller planets close to M dwarfs become possible .
In these systems , star-planet separations of tens of stellar radii begin to coincide with the radiative habitable zone where planetary magnetic fields are likely a necessary condition for surface habitability .