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⊕ 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 cm−3 at 1.4 R⊕. On average, planets with radii up to RP = 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 ρP = 2.43 + 3.39(RP/R⊕) g cm−3 for RP < 1.5 R⊕, which is consistent with rocky compositions. For 1.5 ⩽ RP/R⊕ < 4, we find MP/M⊕ = 2.69(RP/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.

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