The relatively high contrast between planetary and solar low frequency
radio emissions suggests that the low–frequency radio range may be
well adapted to the direct detection of exoplanets. We review the most
significant properties of planetary radio emissions (auroral as well as
satellite–induced) and show that their primary engine is the
interaction of a plasma flow with an obstacle in the presence of a
strong magnetic field (of the flow or of the obstacle). Scaling laws
have been derived from solar system planetary radio emissions that
relate the emitted radio power to the power dissipated in the various
corresponding flow–obstacle interactions. We generalize these
scaling laws into a “radio–magnetic” scaling law that seems to relate output radio power
to the magnetic energy flux convected on the obstacle, this obstacle
being magnetized or unmagnetized. Extrapolating this scaling law to the
case of exoplanets, we find that hot Jupiters may produce very intense
radio emissions due to either magnetospheric interaction with a strong
stellar wind or to unipolar interaction between the planet and a
magnetic star (or strongly magnetized regions of the stellar surface).
In the former case, similar to the magnetosphere–solar wind
interactions in our solar system or to the Ganymede–Jupiter
interaction, a hecto–decameter emission is expected in the vicinity
of the planet with an intensity possibly 103 to
105 times that of Jupiter's low frequency radio
emissions. In the latter case, which is a giant analogy of the
Io–Jupiter system, emission in the decameter–to–meter
wavelength range near the footprints of the star's
magnetic field lines interacting with the planet may reach
106 times that of Jupiter (unless some
“saturation” mechanism occurs). The
system of HD 179949, where a hot spot has been tentatively detected in
visible light near the sub–planetary point, is discussed in some
details. Finally, we discuss the interests of direct radio detection,
among which access to exoplanetary magnetic field measurements and
comparative magnetospheric physics.