We study the masses and radii of 65 exoplanets smaller than 4 R _ { \earth } with orbital periods shorter than 100 days .
We calculate the weighted mean densities of planets in bins of 0.5 R _ { \earth } and identify a density maximum of 7.6 g cm ^ { -3 } at 1.4 R _ { \earth } .
On average , planets with radii up to R _ { P } = 1.5 R _ { \earth } increase in density with increasing radius .
Above 1.5 R _ { \earth } , the average planet density rapidly decreases with increasing radius , indicating that these planets have a large fraction of volatiles by volume overlying a rocky core .
Including the solar system terrestrial planets with the exoplanets below 1.5 R _ { \earth } , we find \rho _ { P } = 2.43 + 3.39 \left ( R _ { P } / R _ { \earth } \right ) g cm ^ { -3 } for R _ { P } < 1.5 R _ { \earth } , which is consistent with rocky compositions .
For 1.5 \leq R _ { P } / R _ { \earth } < 4 , we find M _ { P } / M _ { \earth } = 2.69 \left ( R _ { P } / R _ { \earth } \right ) ^ { 0.93 } .
The RMS of planet masses to the fit between 1.5 and 4 R _ { \earth } is 4.3 M _ { \earth } with reduced \chi ^ { 2 } = 6.2 .
The large scatter indicates a diversity in planet composition at a given radius .
The compositional diversity can be due to planets of a given volume ( as determined by their large H/He envelopes ) containing rocky cores of different masses or compositions .