ALMA constraints on assembly of Core Accretion planets

TL;DR

This paper uses ALMA observations to examine the rapid growth and inward migration of young giant planets in protoplanetary discs, revealing challenges to current core accretion models and proposing suppression of accretion and migration as solutions.

Contribution

It demonstrates that the runaway growth phase of planets is very short and incompatible with observed disc features unless accretion and migration are suppressed.

Findings

Runaway planetary growth phase lasts less than 0.1 Myr.

Rapid inward migration conflicts with observed disc structures.

Suppression of accretion and migration can reconcile models with observations.

Abstract

Resolved dust continuum and CO line ALMA imaging, and in some cases detection of H$\alpha$ emission, hint that young massive planets are abundant at wide separations in protoplanetary discs. Here, we show how these observations can probe the runaway phase of planetary growth in the Core Accretion theory. Planets in this phase have the right range of masses to account for the predominantly moderate contrast gaps and rings seen in ALMA observations. However, we find that these planets gain mass and migrate inward very rapidly. As a result, the phase when they could produce gaps with properties similar to those observed is very short, i.e., $t_{\rm gap} \lesssim 0.1$~Myr, independently of the disc viscosity parameter. This would require many tens to hundreds of gas giant planets to be born per ALMA system, violating the available mass budget of solids in realistic discs. This also predicts preponderance of discs with very wide gaps or complete inner disc holes, which is not observed. We show that suppression of both planet accretion and migration by a factor of at least ten is a possible solution to these serious problems. Future population synthesis models of planet formation should aim to address both exoplanetary data of older discless planetary systems and ALMA discs with embedded planets in one framework.