Growing the gas-giant planets by the gradual accumulation of pebbles

TL;DR

This paper proposes that slow formation of pebbles allows gravitational interactions among planetesimals to limit their growth, enabling the formation of a few gas giants similar to those in our Solar System.

Contribution

It introduces a new model where slow pebble formation and planetesimal interactions produce realistic gas giant systems, resolving previous overgrowth issues.

Findings

Formation of 1-4 gas giants between 5 and 15 AU.

Slow pebble formation prevents excessive planetesimal growth.

Model aligns with Solar System structure.

Abstract

It is widely held that the first step in forming the gas giant planets, such as Jupiter and Saturn, is to form solid `cores' of roughly 10 M$_\oplus$. Getting the cores to form before the solar nebula dissipates ($\sim\!1-10\,$Myr) has been a major challenge for planet formation models. Recently models have emerged in which `pebbles' (centimeter- to meter-size objects) are first concentrated by aerodynamic drag and then gravitationally collapse to form 100 --- 1000 km objects. These `planetesimals' can then efficiently accrete leftover pebbles and directly form the cores of giant planets. This model known as `pebble accretion', theoretically, can produce 10 M$_\oplus$ cores in only a few thousand years. Unfortunately, full simulations of this process show that, rather than creating a few 10 M$_\oplus$ cores, it produces a population of hundreds of Earth-mass objects that are inconsistent with the structure of the Solar System. Here we report that this difficulty can be overcome if pebbles form slowly enough to allow the planetesimals to gravitationally interact with one another. In this situation the largest planetesimals have time to scatter their smaller siblings out of the disk of pebbles, thereby stifling their growth. Our models show that, for a large, and physically reasonable region of parameter space, this typically leads to the formation of one to four gas giants between 5 and 15 AU in agreement with the observed structure of the Solar System.