Predicting the Dominant Formation Mechanism of Multiplanetary Systems

Abstract

Most, if not all, Sun-like stars host one or more planets, making multiplanetary systems commonplace in our Galaxy. We utilize hundreds of multiplanet 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 multiplanetary 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 ≥ 30 are likely dominated by the in situ scenario. We apply these criteria to determine that a majority (62%) of observed multiplanetary 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 multiplanetary systems based on their present-day architectures.

Keywords

• Exoplanet dynamics
• Planet formation
• Planetary migration
• Exoplanet evolution
• Protoplanetary disks