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We study the masses and radii of 65 exoplanets smaller than 4 Earth radiiwith orbital periods shorter than 100 days. We calculate the weighted meandensities of planets in bins of 0.5 Earth radii and identify a density maximumof 7.6 g/cc at 1.4 Earth radii. Planets with radii up to R = 1.5 Earth radiiincrease in density with increasing radius. Above 1.5 Earth radii, planetdensity rapidly decreases with increasing radius, indicating that these planetshave a large fraction of volatiles by volume overlying a rocky core. Includingthe solar system terrestrial planets with the exoplanets below 1.5 Earth radii,we find \rho = 2.43 + 3.39 (R/R_E) g cm^-3 for R < 1.5 Earth radii, which isconsistent with rocky compositions. For exoplanets between 1.5 and 4 Earthradii, we find M/M_E = 2.69 (R/R_E)^0.93. The RMS of planet masses to the fitbetween 1.5 and 4 Earth radii is 4.3 M_E with reduced \chi^2 = 6.2. The largescatter indicates a diversity in planet composition at a given radius. Thecompositional diversity can be due to planets of a given volume (as determinedby their large H/He envelopes) containing rocky cores of different masses orcompositions.

The Mass-Radius Relation for 65 Exoplanets Smaller than 4 Earth Radii