Today , with the large number of detected exoplanets and improved measurements , we can reach the next step of planetary characterization .
Classifying different populations of planets is not only important for our understanding of the demographics of various planetary types in the galaxy , but also for our understanding of planet formation .
We explore the nature of two regimes in the planetary mass-radius ( M-R ) relation .
We suggest that the transition between the two regimes of ” small ” and ” large ” planets , occurs at a mass of 124 \pm 7 , M _ { \oplus } and a radius of 12.1 \pm 0.5 , R _ { \oplus } .
Furthermore , the M-R relation is R \propto M ^ { 0.55 \pm 0.02 } and R \propto M ^ { 0.01 \pm 0.02 } for small and large planets , respectively .
We suggest that the location of the breakpoint is linked to the onset of electron degeneracy in hydrogen , and therefore , to the planetary bulk composition .
Specifically , it is the characteristic minimal mass of a planet which consists of mostly hydrogen and helium , and therefore its M-R relation is determined by the equation of state of these materials .
We compare the M-R relation from observational data with the one derived by population synthesis calculations and show that there is a good qualitative agreement between the two samples .