  We give a constructive proof of the existence of lower dimensional elliptic
tori in nearly integrable Hamiltonian systems. In particular we adapt the
classical Kolmogorov's normalization algorithm to the case of planetary
systems, for which elliptic tori may be used as replacements of elliptic
keplerian orbits in Lagrange-Laplace theory. With this paper we support with
rigorous convergence estimates the semi-analytical work in our previous article
(2011), where an explicit calculation of an invariant torus for a planar model
of the Sun-Jupiter-Saturn-Uranus system has been made. With respect to previous
works on the same subject we exploit the characteristic of Lie series giving a
precise control of all terms generated by our algorithm. This allows us to
slightly relax the non-resonance conditions on the frequencies.
