Computer Simulations Shed New Light on Gas Giant Formation

Solar system’s gas giants, such as Jupiter, Saturn, and massive exoplanets, were formed via the gas accretion onto the solid cores, each with a mass of roughly 10 Earth masses. However, rapid migration due to disk-planet interaction prevents the formation of such massive cores via accretion of planetesimals. Comparably rapid core growth via pebble accretion requires very massive protoplanetary disks because most pebbles fall into the central star. In a new paper, astrophysicists from Nagoya University and Tohoku University report the result of computer simulations for collisional evolution from dust to gas giants in a whole protoplanetary disk.

An artist’s impression of the super-Neptune exoplanet TOI-1728b. Image credit: Sci-News.com.

“We already know quite a bit about how planets are made,” said Dr. Hiroshi Kobayashi, a researcher in the Department of Physics at Nagoya University.

“Dust lying within the far-reaching protoplanetary disks surrounding newly formed stars collides and coagulates to make celestial bodies called planetesimals. These then amass together to form planets.”

“Despite everything we know, the formation of gas giants, like Jupiter and Saturn, has long baffled scientists.”

“This is a problem, because gas giants play huge roles in the formation of potentially habitable planets within planetary systems.”

“For gas giants to form, they must first develop solid cores that have enough mass, about 10 times that of Earth, to pull in the huge amounts of gas for which they are named.”

“Scientists have long struggled to understand how these cores grow.”

The problem is two-fold: (i) core growth from the simple amassing of nearby planetesimals would take longer than several million years during which the dust-containing protoplanetary disks survive; (ii) forming planetary cores interact with the protoplanetary disk, causing them to migrate inward towards the central star; this makes conditions impossible for gas accumulation.

To tackle this problem, Dr. Kobayashi and Dr. Hidekazu Tanaka from the Tohoku University’s Astronomical Institute used state-of-the-art computer technologies to develop simulations that can model how dust lying within the protoplanetary disk can collide and grow to form the solid core necessary for gas accumulation.

“The new program is able to handle celestial bodies of all sizes and simulate their evolution via collisions,” Dr. Kobayashi said.

The simulations showed that pebbles from the outer parts of the protoplanetary disk drift inwards to grow into icy planetesimals at about 10 AU from the central star.

A single astronomical unit represents the mean distance between the Earth and the Sun. Jupiter and Saturn are about 5.2 and 9.5 AU away from the Sun, respectively.

Pebble growth into icy planetesimals increases their numbers in the region of the developing planetary system that is about 6-9 AU from the central star.

This encourages high core growth rates, resulting in the formation of solid cores massive enough to accumulate gas and develop into gas giants in a period of about 200,000 years.

“We expect our research will help lead to the full elucidation of the origin of habitable planets, not only in the Solar System, but also in other planetary systems around stars,” Dr. Kobayashi said.

The paper was published in the Astrophysical Journal.

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Hiroshi Kobayashi & Hidekazu Tanaka. 2022. Rapid Formation of Gas-giant Planets via Collisional Coagulation from Dust Grains to Planetary Cores. ApJ 922, 16; doi: 10.3847/1538-4357/ac289c