  Interesting emerging observational properties of the period-mass distribution
of extra-solar planets are discussed. New recent detections confirm the already
emphasized lack of massive planets (m_2sini>=2M_Jup) on short-period orbits
(P<=100 days). Furthermore, we point out i) a shortage of planets in the
10--100 day period range as well as ii) a lack of light planets
(m_2sini<=0.75M_Jup) on orbits with periods larger than ~100 days. The latter
feature is shown not to be due to small-number statistics with Monte-Carlo
simulations. These observational period-related characteristics are discussed
in the context of the migration process of exoplanets. They are found to be in
agreement with recent simulations of planet interactions with viscous disks.
The observed valley at a few tens of days in the period distribution is
interpreted as a transition region between two categories of planets that
suffered different migration scenarios. The lack of light planets on
longer-period orbits and the corresponding intriguing sharp limit in mass is
tentatively explained by the runaway migration process recently studied by
Masset & Papaloizou (2003). The observed properties also have implications for
the observation strategies of the on-going surveys and of future
higher-precision searches.
