2014ApJ...783L...6W

We study the masses and radii of 65 exoplanets smaller than 4 R_⊕ with orbital periods shorter than 100 days. We calculate the weighted mean densities of planets in bins of 0.5 R_⊕and identify a density maximum of 7.6 g/cm3 at 1.4 R_⊕. On average, planets with radii up to R_P= 1.5 R_⊕increase in density with increasing radius. Above 1.5 R_⊕, 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_⊕, we find ρ_P= 2.43 + 3.39(R_P/R_⊕) g/cm3 for R_P< 1.5 R_⊕, which is consistent with rocky compositions. For 1.5 ≤ R_P/R_⊕< 4, we find M_P/M_⊕= 2.69(R_P/R_⊕)^0.93. The rms of planet masses to the fit between 1.5 and 4 R_⊕is 4.3 M_⊕with reduced χ² = 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.