3D simulations add evidence to incomplete planet-formation theory

February 9, 2021
Planet formation may have been decoded. (Pixabay/JCK5D)

Planet formation may have been decoded. (Pixabay/JCK5D)

An Iowa State University astrophysicist led the first successful 3D simulations that demonstrate the formation of planetesimals through dust particle build-up catalyzed by pressure bumps, possibly confirming a long-standing theory of how larger planets, including those in our solar system, came to be.

Most physicists believe that planetesimals, which are like starter planets, either produce or originate from the dust particles found in rings that surround stars. However, they have yet to prove which is the case and why. 

In a study published Jan. 29 in The Astronomical Journal, postdoctoral researcher Daniel Carrera and his team isolated a portion of one of those rings, called protoplanetary disks, and ran simulations of it with dust particles varying in concentration and size. 

The results showed the first concrete evidence to support the origin story by filling a historically huge gap in planetesimal formation theory: how the process of particle-clumping begins.

“Many people have made simulations of only the disk. What I am doing is a simulation where actual particles move through the disk and collect in the bump,” Carrera explained, referring to a pressure bump, the foundation for his simulations’ success. “Eventually, there is enough density for them to collapse gravitationally and make particle clumps.”

Particle-clumping is a product of what physicists call streaming instability, a hypothetical mechanism of planetary formation. While this mechanism is thought to be triggered by high concentrations of particles, the question of why there would be a high concentration in the first place has continued to elude researchers.

Carrera’s work offers an answer: Particle build-up could occur through pressure bumps that inherently exist in protoplanetary rings.

“The streaming instability only works if you first concentrate particles somehow," Carrera said. "The idea that maybe pressure bumps can do that is not mine; people have thought about it before, but it was just an idea. These are the first simulations that took that idea and can say, 'Look, it works.'" 

As per the study, pressure bumps slow down the dust particles that are constantly in motion around the star. This slowing of particles creates junctures in the ring with a high density.

From here, through the process of streaming instability, the high concentration triggers the particles to come together. Over time, according to the study, this could be how planets are created. 

“If you think about how a traffic jam works, typically the cars don't stop, there's just a region in the highway where the cars are going slower," Carrera said. "But, that region of slow-moving cars creates a pattern; you can actually see a region of the highway where there are more cars than anywhere else. It could be that the rings are like that.”

Carrera's work is built on a groundbreaking image of a young sun-like star taken by the Alma Observatory, in Chile, six years ago. The observatory’s array of radio telescopes was able to photograph a million-year-old star called HL Tau, located about 450 light-years from Earth in the Taurus constellation. This image clearly depicts the star’s protoplanetary, planet-forming disk. 

“This is the inspiration, and the inspiration for a lot of people doing similar work,” remarked Carrera. “Since then, we have taken images of several other disks, and while not all of them have rings, a very, very large number of them do; the majority of them have rings. This is a key piece.”

He explained how the image of HL Tau, and all subsequent images of protoplanetary disks, provided physicists with evidence that the disks are made of rings, and that the rings are made of dust. Because gas is transparent, visibility of the rings indicates that dust particles are present. 

Carrera stressed that while these 3D simulations are the first of their kind to take steps in confirming the theory of planetesimal, and therefore planet, formation, there is a multitude of other discoveries to be made in this field. 

He said the next steps would be to test these simulations with a greater variety of scenarios, for example, by exposing the particles to turbulence, a force that tends to heavily affect space matter. 

“Science is a process. It's very rare that any one paper will be the end of a field,” Carrera emphasized. “This is an important step, but there will be others.”

The paper, “Protoplanetary Disk Rings as Sites for Planetesimal Formation,” was published Jan. 29 in The Astronomical Journal. The authors were Daniel Carrera and Jacob B. Simon, Iowa State University; Rixin Li, University of Arizona; Katherine A. Kretke, Southwest Research Institute; and Hubert Klahr, Max-Planck-Institut für Astrononomie.

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