On the origin of wide-orbit ALMA planets: giant protoplanets disrupted by their cores

S Nayakshin,J Humphries

doi:10.1093/mnras/stz2497

Abstract

ABSTRACT Recent ALMA observations may indicate a surprising abundance of sub-Jovian planets on very wide orbits in protoplanetary discs that are only a few million years old. These planets are too young and distant to have been formed via the core accretion (CA) scenario, and are much less massive than the gas clumps born in the classical gravitational instability (GI) theory. It was recently suggested that such planets may form by the partial destruction of GI protoplanets: energy output due to the growth of a massive core may unbind all or most of the surrounding pre-collapse protoplanet. Here we present the first 3D global disc simulations that simultaneously resolve grain dynamics in the disc and within the protoplanet. We confirm that massive GI protoplanets may self-destruct at arbitrarily large separations from the host star provided that solid cores of mass ∼10–20 M⊕ are able to grow inside them during their pre-collapse phase. In addition, we find that the heating force recently analysed by Masset & Velasco Romero (2017) perturbs these cores away from the centre of their gaseous protoplanets. This leads to very complicated dust dynamics in the protoplanet centre, potentially resulting in the formation of multiple cores, planetary satellites, and other debris such as planetesimals within the same protoplanet. A unique prediction of this planet formation scenario is the presence of sub-Jovian planets at wide orbits in Class 0/I protoplanetary discs.

Highlights

It is widely believed that the gaps in the ∼ 1 mm dust discs observed by the Atacama Large Millimeter Array (ALMA) are the signatures of young planets (ALMA Partnership et al 2015; Isella et al 2016; Long et al 2018; Andrews et al 2018; Zhang et al 2018)
We examine whether grains remain coupled to this new atmosphere or sediment inside the radius that contains the initial mass of the protoplanet (rP (M0)), which we plot as a dotted line in the middle panel of Figure 4
Similar conclusions were already reached in Nayakshin (2016); in this study we have modelled the disruption of Gravitational Instability (GI) protoplanets by the core feedback for the first time in 3D smoothed particle hydrodynamics (SPH) simulations

Summary

Introduction

It is widely believed that the gaps in the ∼ 1 mm dust discs observed by the Atacama Large Millimeter Array (ALMA) are the signatures of young planets (ALMA Partnership et al 2015; Isella et al 2016; Long et al 2018; Andrews et al 2018; Zhang et al 2018). Modelling suggests that these planets are often wide-orbit Saturn analogues (Dipierro et al 2016; Clarke et al 2018; Lodato et al 2019). This is an order of magnitude larger than the typical masses inferred for the ALMA gap opening planets (Nayakshin et al 2019)

Methods

Findings

Discussion

Conclusion