Tidal instability in exoplanetary systems

Due to their observational method, many of the discovered exo-planets are massive gas giants called ``hot Jupiters'' orbiting rapidly very close to their stars. Because of this proximity, these celestial bodies (stars and planets) are strongly deformed by gravitational tides. Therefore, a certain number of them must be the site of an hydrodynamic instability, called the tidal instability. Starting from measured astrophysical characteristics of these systems (masses, orbit radius, eccentricity and period, spin velocity...), we show that this instability is, as expected, present in some of the stars when the ratio of the planet orbiting period to the star spinning period is not in a ``forbidden range.'' In this case, the instability should drive strong flows in the different fluid layers of both bodies. These flows must be taken into account to model the bodies interiors and subsequent properties (synchronization, dynamos, zonal winds...). Of particular interest is the possibility of modifying the alignment of the rotation axes of stars and planets by this tidal instability.