Split Peas in a Pod: Intra-System Uniformity of Super-Earths and Sub-Neptunes

Abstract

The planets within compact multi-planet systems tend to have similar sizes, masses, and orbital period ratios, like "peas in a pod". This pattern was detected when considering planets with radii between 1 and 4 $R_\oplus$. However, these same planets show a bimodal radius distribution, with few planets between 1.5 and 2 $R_{\oplus}$. The smaller "super-Earths" are consistent with being stripped rocky cores, while the larger "sub-Neptunes" likely have gaseous H/He envelopes. Given these distinct structures, it is worthwhile to test for intra-system uniformity separately within each category of planets. Here, we find that the tendency for intra-system uniformity is twice as strong when considering planets within the same size category than it is when combining all planets together. The sub-Neptunes tend to be $1.7^{+0.6}_{-0.3}$ times larger than the super-Earths in the same system, corresponding to an envelope mass fraction of about 2.6% for a $5\,M_{\oplus}$ planet. For the sub-Neptunes, the low-metallicity stars are found to have planets with more equal sizes, with modest statistical significance ($p\sim 0.005$). There is also a modest $(\sim$2-$\sigma)$ tendency for wider-orbiting planets to be larger, even within the same size category.