​Saturn's satellites exert a stronger attraction on the nearer face of their host planet than on the more distant face. Such tidal effect slightly deforms Saturn in an ellipsoidal shape, in the direction of the companion star. In return, the deformed planet slightly disrupts the orbits of its satellites.

An international team of researchers involving IRFU had the idea to observe the trajectories of small moons framing Tethys and Dione, two satellites of Saturn. Each of these satellites is framed by two small moons that are located at a 60° angle along its orbit. Because of this offset positioning, the trajectories of these moons are affected by the deformation of Saturn, itself resulting from the tide exerted by Theys (or Dione). No matter how minuscule, these gaps can be measured since the geometry of the whole does not vary and the effects can accumulate to reach a few tens of kilometers over ten years. By measuring these deviations, the scientists are able to trace back the tiny fluctuations of the gravitational field inside Saturn over this period.

Coupled with other measurements by the Cassini spacecraft expected during its plunge into Saturn's atmosphere, scheduled for September 2017, the data collected should make it possible to determine if the central core of  Saturn is rocky.

These results were obtained by the Encelade working group, led by a researcher of the Institute of Celestial Mechanics and Ephemerides Calculation (Paris Observatory/CNRS/UPMC/Lille 1 University).