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

摘要

We study the masses and radii of 65 exoplanets smaller than 4R_? with orbital periods shorter than 100 days. We calculate the weighted mean densities of planets in bins of 0.5R_? and identify a density maximum of 7.6 g cm~(-3) at 1.4R_?. On average, planets with radii up to RP = 1.5R_? increase in density with increasing radius. Above 1.5R_?, 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_?, we find ρ_P = 2.43 + 3.39 (R_P/R_?) g cm~(-3) for RP < 1.5R_?, 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 4R_? is 4.3M_? with reduced χ~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.