Key to rapid planet formation

In young planetary systems, gas giants form more efficiently and faster than previously assumed as shown by new computer simulations.

Ring-shaped perturbations in disks of gas and dust orbiting young stars can trigger the formation of several gas giants, as researchers from the ORIGINS Cluster of Excellence, Ludwig-Maximilians-Universität in Munich (Germany) and the Max Planck Institute for Solar System Research (MPS) in Göttingen (Germany) report in the current issue of the journal Astronomy and Astrophysics. The team has developed a model that for the first time combines all the necessary physical processes that play a role in planet formation. According to this model, giant planets can form more efficiently and faster than previously assumed. The results are consistent with recent observations.

Our solar system is our immediate cosmic neighborhood. We know it well: the Sun at the center; the rocky planets Mercury, Venus, Earth, and Mars; and then the asteroid belt followed by the gas giants Jupiter and Saturn; then the ice giants Uranus and Neptune; and finally the Kuiper belt with its comets. But how well do we really know our home? Previous theories have assumed that giant planets are formed by collisions and accumulations of asteroid-like bodies, so-called planetesimals, and the subsequent accretion of gas over the course of millions of years. However, these models explain neither the existence of gas giants located far from their stars nor the formation of Uranus and Neptune.

From grains of dust to giant planet

In their new model, the astrophysicists from the ORIGINS Cluster, LMU and MPS take into account for the first time all the processes that are decisive for planet formation. “This is the first time a simulation has traced the process whereby fine dust grows into giant planets,” observes Tommy Chi Ho Lau, lead author of the study and doctoral candidate at LMU.

With their model, the researchers demonstrate how millimeter-sized dust particles accumulate aerodynamically in the turbulent gas disk, and how this initial perturbation in the disk traps dust and prevents it from disappearing off in the direction of the star. This accumulation makes the growth of planets very efficient, as suddenly a lot of “building material” is available within a compact area and the right conditions for planet formation are present. “When a planet gets large enough to influence the gas disk, this leads to renewed dust enrichment farther out in the disk,” explains Til Birnstiel, Professor of Theoretical Astrophysics at LMU and member of the ORIGINS Cluster of Excellence. “In the process, the planet drives the dust – like a sheepdog chasing its herd – into the area outside its own orbit.” The process begins anew, from inside to outside, and another giant planet can form.

Variety of gas giants in our and other solar systems

In our solar system, the gas giants are situated at a distance of around 5 astronomical units (au) (in the case of Jupiter) to 30 au (Neptune) from the Sun. For comparison, the Earth is some 150 million kilometers from the Sun, which is equivalent to 1 au. "In other planetary systems a perturbation in the protoplanetary disk could trigger the process of planet formation at a much greater distances," says Dr. Joanna Drążkowska from the MPS. 

Such systems have been observed frequently in recent years by the ALMA radio observatory, which has found gas giants in young disks at a distance beyond 200 au. However, the model also explains why our solar system apparently stopped forming additional planets after Neptune: the building material was simply used up.

The results of the study match current observations of young planetary systems that have pronounced substructures in their disks. These substructures play a decisive role in planet formation. The study indicates that the formation of giant planets and gas giants proceeds with greater efficiency and speed than previously assumed. These new insights could refine our understanding of the origin and development of the giant planets in our solar system and explain the diversity of observed planetary systems.