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 \pm 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 \pm 0.8 days ) .
These two clusters follow strikingly different distributions in the period-radius parameter plane .
The branch of sub-Jupiter mass exoplanets is censored by the orbital period at the large-radius end : 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 .