  In this paper we present phenomenological evidence for the validity of an
exponential distance relation (also known as generalized Titius-Bode law) in
the 32 planetary systems (31 extra solar plus our Solar System) containing at
least 5 planets or more. We produce the semi-log fittings of the data, and we
check them against the statistical indicators of $R^2$ and $Median$. Then we
compare them with the data of 4000 artificial planetary systems created at
random. In this way, a possible origin by chance of the Titius-Bode (TB) law is
reasonably ruled out. We also point out that in some systems the fittings can
be definitely improved by the insertion of new planets into specific positions.
We discuss the Harmonic Resonances method and fittings, and compare them with
the Titius-Bode fittings. Moreover, for some specific systems, we compare the
TB fitting against a polynomial fitting ($r\sim n^2$). This analysis allows us
to conclude that an exponential distance relation can reasonably be considered
as ``valid'', or strongly corroborated, also in extra solar planetary systems.
Further, it results to be the most economical (in terms of free parameters) and
best fitting law for the description of spacing among planetary orbits.
