  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.
