Predicting the Dominant Formation Mechanism of Multi-Planetary Systems

Abstract

Most, if not all, sun-like stars host one or more planets, making multi-planetary systems commonplace in our galaxy. We utilize hundreds of multi-planet simulations to explore the origin of such systems, focusing on their orbital architecture. The first set of simulations assumes in-situ assembly of planetary embryos, while the second explores planetary migration. After applying observational biases to the simulations, we compare them to 250+ observed multi-planetary systems, including 13 systems with planets in the habitable zone. For all of the systems, we calculate two of the so-called statistical measures: the mass concentration ($S_c$) and orbital spacing ($S_s$). After analytic and empirical analyses, we find that the measures are related to first-order with a power law: $S_c\sim S_s^{\beta }$. The in-situ systems exhibit steeper power-law relations relative to the migration systems. We show that different formation scenarios cover different regions in the $S_s-S_c$ diagram with some overlap. Furthermore, we discover that observed systems with $S_s<30$ are likely dominated by the migration scenario, while those with $S_s\ge 30$ are likely dominated by the in-situ scenario. We apply these criteria to determine that a majority (62%) of observed multi-planetary systems formed via migration, whereas most systems with currently observed habitable planets formed via in-situ assembly. This work provides methods of leveraging the statistical measures ($S_s$ and $S_c$) to disentangle the formation history of observed multi-planetary systems based on their present-day architectures.

Accepted for publication in ApJ Letters

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