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 $1.7_{-0.3}^{+0.6}$ times larger than the super-Earths in the same system, corresponding to an envelope mass fraction of about 2.6% for a $5M_{\oplus }$ 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.

Accepted to ApJ Letters. 7 pages, 5 figures

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• Generalized Peas-in-a-Pod: Extending Intra-System Mass Uniformity to Non-TTV Systems via the Gini Index