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

Abstract

Observations have revealed that super-Earths (planets up to 10 Earth masses) are the most abundant type of planets in the inner systems. Their formation 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 heating source of the protoplanetary disc. I compare the growth of super-Earths in viscously heated discs, where viscous heating dissipates within the first million years, and discs purely heated by the central star with super-Earth observations from the Kepler mission. This allows two formation pathways of super-Earths to be distinguished in the inner systems within this framework. 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 in a viscously heated disc, these resonant chains thus have to be broken without planetary mergers, for example through the magnetic rebound effect, and the final system architecture should feature low mutual inclinations. If super-Earths form either late or in purely passive discs, the pebble isolation mass is too small (around 2-3 Earth masses) to explain the Kepler observations, implying that planetary mergers have to play a significant role in determining 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, but feature mutually inclined orbits. This implies that future observations of planetary systems with radial velocities (RV) and transits (for example through the Transiting Exoplanet Survey Satellite (TESS) and its follow up RV surveys) could distinguish between these two formation channels of super-Earth and thus constrain planet formation theories.

The CARMENES search for exoplanets around M dwarfs

“…While the available data are insufficient to draw definite conclusions on the exact formation channel of these systems, the derived orbital and planetary parameters allow for some cautious conjectures. These planets are commonly thought to form by combined accretion of planetesimals and pebbles (e.g., Ormel & Klahr 2010;Lambrechts & Johansen 2012;Bitsch 2019).…”

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

“…Growing planets may eventually reach pebble isolation mass (Bitsch et al 2018), where the pebble isolation mass in the inner disc regions could explain the presumed masses of the Kepler planets (e.g. Wu 2019;Bitsch 2019). After the dissipation of the gaseous disk, each simulation was integrated for an-other 50 Myr taking only gravitational perturbations into account.…”

Formation of Giant Planet Satellites

“…The ensuing possibility that this similarity may point to a deeper analogy between conglomeration pathways of solar system satellites and short-period exoplanets has not eluded the literature (Kane et al 2013;Ronnet & Johansen 2020). Nevertheless, it is intriguing to notice that while the pursuit to quantify the formation of extrasolar super-Earths has received considerable attention over the course of the recent decades (see, e.g., the recent works of Bitsch 2019;Izidoro et al 2019;Liu et al 2019;Rosenthal & Murray-Clay 2019;Kuwahara & Kurokawa 2020;Poon et al 2020 and the references therein), a complete understanding of the formation of the solar system's giant planet satellites themselves remains incomplete (Canup & Ward 2009;Miguel & Ida 2016;Ronnet & Johansen 2020). Outlining a new theory for the their conglomeration is the primary purpose of this paper.…”