The origins of nearly coplanar, non-resonant systems of close-in super-Earths

Abstract

Some systems of close-in “super-Earths” contain five or more planets on non-resonant but compact and nearly coplanar orbits. The Kepler-11 system is an iconic representative of this class of system. It is challenging to explain their origins given that planet-disk interactions are thought to be essential to maintain such a high degree of coplanarity, yet these same interactions invariably cause planets to migrate into chains of mean motion resonances. Here we mine a large dataset of dynamical simulations of super-Earth formation by migration. These simulations match the observed period ratio distribution as long as the vast majority of planet pairs in resonance become dynamically unstable. When instabilities take place resonances are broken during a late phase of giant impacts, and typical surviving systems have planet pairs with significant mutual orbital inclinations. However, a subset of our unstable simulations matches the Kepler-11 system in terms of coplanarity, compactness, planet-multiplicity and non-resonant state. This subset have dynamical instability phases typically much shorter than ordinary systems. Unstable systems may keep a high degree of coplanarity post-instability if planets collide at very low orbital inclinations (≲ 1○) or if collisions promote efficient damping of orbital inclinations. If planetary scattering during the instability takes place at low orbital inclinations (i ≲ 1○), orbital inclinations are barely increased by encounters before planets collide.When planetary scattering pumps orbital inclinations to higher values (≳ 1○) planets tend to collide at higher mutual orbital inclinations, but depending on the geometry of collisions mergers’ orbital inclinations may be efficiently damped. Each of these formation pathways can produce analogues to the Kepler-11 system.

Architectures of Compact Super-Earth Systems Shaped by Instabilities

“…In the instability scenario, these may be the systems that remained stable and did not experience growth in eccentricity. Or, if they did encounter an instability, they may have experienced a smaller degree of scattering that left them unusually coplanar and circular (Esteves et al 2020).…”

The Nature and Origins of Sub‐Neptune Size Planets

“…The answer to this apparent contradiction is not immediately clear. Perhaps impacts often occur before the full dissipation of the gaseous disk (Esteves et al., 2020) such that a thin atmosphere can still be accreted (Lee & Chiang, 2015). Or perhaps resonant chains of planets are spread out by a different mechanism, such as by the magnetospheric rebound migration torque from the expanding disk cavity (Liu et al., 2017).…”

A Tidal Origin for a Three-body Resonance in Kepler-221

“…Within the context of "classical" models of planet formation, the present-day architecture of planetary systems was assumed to be inherited from the detailed structure of the protoplanetary disk from which the planets emerged (Cameron 1988). Recent theoretical progress on the origin of the exoplanetary period distribution, as well as the early evolution of the Solar System itself, however, indicates that violent, post-nebular dynamical instabilities may play an important role in sculpting the terminal outcome of the planet formation process (Tsiganis et al 2005;Morbidelli 2010;Izidoro et al 2017;Esteves et al 2020). Unfortunately, this markedly chaotic evolutionary framework renders a deterministic model that unambiguously connects the properties of the planets natal disk with their present-day (observable) attributes an impossibility.…”