Abstract. We discuss the applicability of Kolmogorov's theorem on existence of invariant tori to the real Sun-Jupiter-Saturn system. Using computer algebra, we construct a Kolmogorov's normal form defined in a neighborhood of the actual orbit in the phase space, giving a sharp evidence of the convergence of the algorithm. If not a rigorous proof, we consider our calculation as a strong indication that Kolmogorov's theorem applies to the motion of the two biggest planets of our solar system.
Introduction.We reconsider the problem of the applicability of KAM theory (see [13], [22] and [1]) to physical systems and, in particular, to planetary systems. As it is well known, the announcement of Kolmogorov's theorem has been immediately considered by the scientific community as a major step towards the solution of the classical problem of the stability of planetary motions. On the other hand, the results of several numerical calculations of the orbits seem to indicate that the applicability of the theorem to the whole solar system is an hopeless task. Indeed, Kolmogorov's theorem states that for most initial conditions the orbits of the system should be quasi-periodic with fixed frequencies, while the numerical calculations strongly indicate that the actual orbits exhibit a limited chaotic behavior which is superimposed to the quasi-periodic motion (see, e.g., [28], [15] and [23]). As a matter of fact, a rigorous check of the applicability of Kolmogorov's theorem at least to the main part of our solar system is still lacking.In this paper, we make a further step in this research field by considering the SunJupiter-Saturn (hereafter SJS) system in the framework of the general problem of three bodies. For this particular case, we produce a strong evidence of the existence of KAM tori, which are close to the actual orbits of the two biggest planets of our solar system.