THE MASS-RADIUS RELATION FOR 65 EXOPLANETS SMALLER THAN 4 EARTH RADII

We study the masses and radii of 65 exoplanets smaller than 4 R-circle plus. with orbital periods shorter than 100 days. We calculate the weighted mean densities of planets in bins of 0.5 R-circle plus and identify a density maximum of 7.6 g cm(-3) at 1.4 R-circle plus. On average, planets with radii up to R-P = 1.5R(circle plus) increase in density with increasing radius. Above 1.5 R-circle plus, 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 1.5R(circle plus), we find rho(P) = 2.43 + 3.39 (R-P/R-circle plus) g cm(-3) for R-P < 1.5R(circle plus), which is consistent with rocky compositions. For 1.5 <= R-P/R-circle plus < 4, we find M-P/M-circle plus = 2.69 (R-P/R-circle plus)(0.93). The rms of planet masses to the fit between 1.5 and 4 R-circle plus is 4.3 M-circle plus 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.