Statistical properties of exoplanets - I. The period distribution: Constraints for the migration scenario

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(2) sin i greater than or equal to 2 M-Jup) on short-period orbits (P less than or equal to 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(2) sin i less than or equal to 0.75 M-Jup) on orbits with periods larger than similar to100 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.