Inner rocky super-Earth formation: distinguishing the formation pathways in viscously heated and passive discs

TL;DR

This paper compares super-Earth formation in viscously heated versus passive protoplanetary discs, revealing how disc structure influences planetary growth, resonance trapping, and system architecture, with implications for interpreting Kepler observations.

Contribution

It introduces a comparative analysis of super-Earth formation pathways in different disc heating regimes, linking disc structure to observed planetary system configurations.

Findings

Viscously heated discs produce larger pebble isolation masses matching Kepler planet masses.

Resonant chains formed early in viscously heated discs require mergers to match observed period ratios.

Passive discs lead to smaller planets, implying mergers are necessary for observed super-Earth systems.

Abstract

The formation of super-Earths is strongly linked to the structure of the protoplanetary disc, which determines growth and migration. In the pebble accretion scenario, planets grow to the pebble isolation mass, at which the planet carves a small gap in the gas disc halting the pebble flux and thus its growth. The pebble isolation mass scales with the disc's aspect ratio, which directly depends on the disc structure. I compare the growth of super-Earths in viscously heated discs and discs purely heated by the central star with super-Earth observations. This allows two formation pathways of super-Earths to be distinguished in the inner systems. Planets growing within 1 Myr in the viscously heated inner disc reach pebble isolation masses that correspond directly to the inferred masses of the Kepler observations for systems that feature planets in resonance or not in resonance. However, to explain the period ratio distribution of Kepler planets -- where most Kepler planet pairs are not in mean motion resonance configurations -- a fraction of these resonant chains has to be broken. In case the planets are born early in a viscously heated disc, these resonant chains thus have to be broken without planetary mergers. If super-Earths form either late or in purely passive discs, the pebble isolation mass is too small to explain the Kepler observations, implying that planetary mergers are important for the final system architecture. Resonant planetary systems thus have to experience mergers already during the gas disc phase, so the planets can get trapped in resonance after reaching 5-10 Earth masses. In case instabilities are dominating the system architecture, the systems should not be flat with mutually inclined orbits. This implies that future observations of planetary systems with RV and transits could distinguish between these two formation channels of super-Earth. (abridged)