Origin of compact exoplanetary systems via early accretion during stellar infall

Compact multi-planet systems of Earth to super-Earth sized planets have nearly circular, coplanar orbits located at distances of only ∼0.01 - 0.1 AU, a region devoid of planets in our Solar System. Although compact systems comprise a large fraction of known systems, their origin remains debated.

Common to prior models of compact system origin is the assumption that infall to the stellar disk ends before planets form. However, there is growing observational, theoretical, and meteoritical evidence of the early growth of mm-sized “pebbles” during the infall phase. Once pebbles are present and for appropriate conditions, concentrating mechanisms could rapidly produce large planetesimals. Pebbles and planetesimals formed during infall would decouple from the gas, potentially leading to a build-up of solids in the inner disk regions supplied by the infall. Planets in compact systems may then accrete in regions where the gas-to-solids ratio is small in comparison to the stellar metallicity, allowing for more favorable conditions for the growth and survival of planets against inward gas-driven orbital migration. Here we are developing the first models of compact system origin during the infall phase.

Accretion within infall-supplied disks has been studied in the context of gas planet satellite origin. Formation models predict that the total mass of the satellite system during this evolution maintains a nearly constant mass ratio ∼10⁻⁴ compared to the host planet’s mass. The maximum mass ratio of compact exoplanetary systems compared to the stellar mass are similar to those of the giant satellite system, suggesting that accretion of compact systems may be more similar to regular satellite formation than to the accretion of our terrestrial planets that was completed long after the Sun’s nebula had dispersed.

We find that for probable disk conditions, planets formed during the infall phase may survive until the nebula dissipates even for full strength Type I migration. The total planetary system mass is regulated by the balance between two processes: 1) planetary accretion, supplied by the infalling material; and 2) planetary loss, driven by Type I migration due to interaction with the nebular gas. Over a broad parameter regime, we find that this balance regulates the total compact system mass to ~ 10^-5 to 10^-4 times the stellar mass, consistent with the observed total mass ratio of compact systems, and creates broadly similar-mass planets within in each system (peas-in-the-pod architecture).