Two populations of transiting planets?

A short-period censor of sub-Jupiter mass exoplanets with low density
http://arxiv.org/abs/1012.4791

Quote :

Despite the existence of many short-period hot Jupiters, there is not one hot Neptune with an orbital period less than 2.5 days. Here we discuss a cluster analysis of the currently known 106 transiting exoplanets to investigate a possible explanation for this observation. We find two distinct clusters in the mass-density space, one with hot Jupiters with a wide range of orbital periods (0.8--114 days) and a narrow range of planet radii (1.2 +- 0.2 R_J); and another one with a mixture of super-Earths, hot Neptunes and hot Jupiters, exhibiting a surprisingly narrow period distribution (3.7 +- 0.8 days). These two clusters follow different distributions in the period-radius parameter plane. The branch of sub-Jupiter mass exoplanets is censored by the orbital period at large-radii: no planets with mass between 0.02--0.8 M_J or with radius between 0.25--1.0 R_J are known with P_orb<2.5 days. This clustering is not predicted by current theories of planet formation and evolution that we also review briefly.