Plasma interactions of exoplanets with their parent star and associatedradio emissions

Abstract

The relatively high contrast between planetary and solar low frequencyradio emissions suggests that the low–frequency radio range may bewell adapted to the direct detection of exoplanets. We review the mostsignificant properties of planetary radio emissions (auroral as well assatellite–induced) and show that their primary engine is theinteraction of a plasma flow with an obstacle in the presence of astrong magnetic field (of the flow or of the obstacle). Scaling lawshave been derived from solar system planetary radio emissions thatrelate the emitted radio power to the power dissipated in the variouscorresponding flow–obstacle interactions. We generalize thesescaling laws into a “radio–magnetic” scaling law that seems to relate output radio powerto the magnetic energy flux convected on the obstacle, this obstaclebeing magnetized or unmagnetized. Extrapolating this scaling law to thecase of exoplanets, we find that hot Jupiters may produce very intenseradio emissions due to either magnetospheric interaction with a strongstellar wind or to unipolar interaction between the planet and amagnetic star (or strongly magnetized regions of the stellar surface).In the former case, similar to the magnetosphere–solar windinteractions in our solar system or to the Ganymede–Jupiterinteraction, a hecto–decameter emission is expected in the vicinityof the planet with an intensity possibly 10³ to10⁵ times that of Jupiter's low frequency radioemissions. In the latter case, which is a giant analogy of theIo–Jupiter system, emission in the decameter–to–meterwavelength range near the footprints of the star'smagnetic field lines interacting with the planet may reach10⁶ times that of Jupiter (unless some“saturation” mechanism occurs). Thesystem of HD 179949, where a hot spot has been tentatively detected invisible light near the sub–planetary point, is discussed in somedetails. Finally, we discuss the interests of direct radio detection,among which access to exoplanetary magnetic field measurements andcomparative magnetospheric physics.