A new way to wobble: Scientists uncover mechanism that causes formation of planets

Instead of a tempest in a teapot, imagine the cosmos in a canister. Scientists have performed experiments using nested, spinning cylinders to confirm that an uneven wobble in a ring of electrically conductive fluid like liquid metal or plasma causes particles on the inside of the ring to drift inward. Since revolving rings of plasma also occur around stars and black holes, these new findings imply that the wobbles can cause matter in those rings to fall toward the central mass and form planets. 

The scientists found that the wobble could grow in a new, unexpected way. Researchers already knew that wobbles could grow from the interaction between plasma and magnetic fields in a gravitational field. But these new results show that wobbles can more easily arise in a region between two jets of fluid with different velocities, an area known as a free shear layer. 

“This finding shows that the wobble might occur more often throughout the universe than we expected, potentially being responsible for the formation of more solar systems than once thought,” said Yin Wang, a staff research physicist at the U.S. Department of Energy’s (DOE) Princeton Plasma Physics Laboratory (PPPL) and lead author of the paper reporting the results in Physical Review Letters. “It’s an important insight into the formation of planets throughout the cosmos.”

The findings follow up on previous results from 2022 that focused on a simpler picture of fluid behavior. Together, the two findings strengthen the evidence that a type of plasma wobble known as the magnetorotational instability (MRI) can cause the formation of planets from so-called accretion disks of matter circling stars. 

Creating a stellar accretion disk in a lab

The original experiment occurred in 2022 and involved PPPL’s MRI Experiment, a device consisting of two nested metal cylinders, each around 1 foot high and 2 inches wide, that can spin at different rates. The scientists created regions of galinstan — a fluid mixture of the elements gallium, iridium and tin — that mimicked how different parts of an accretion disk move at varying speeds. The scientists then applied a magnetic field.

Using computer programs to analyze the 2022 results, the scientists confirmed that they had created a form of the MRI in which magnetic field lines do not have the same orientation around and through the plasma. Instead, they wound around in a twisting shape, interlacing through the free shear layer and developing different strengths in different orientations. 

Just as in the 2022 result, the wobble causes particles on the outside of the plasma to move more quickly and those on the inside to move more slowly. While the quick particles can gain so much speed that they fly off into space, the slow particles can fall inward and coalesce into bodies, including planets. 

Using computer codes to interpret observations 

The scientists confirmed the findings using the computer programs SFEMaNS and Dedalus to create plasma simulations based on data from the earlier 2022 experiments. “Those computer simulations confirmed our previous experimental analyses, but they also opened up different frontiers to help us understand what that data meant,” said Fatima Ebrahimi, a principal research physicist at PPPL and one of the paper’s co-authors. 

The new simulations showed the researchers that this uneven wobble, or nonaxisymmetric MRI, is a type of magnetohydrodynamic instability. It resembles turbulence caused by the meeting of fluids of different velocities — like the swirls caused by an airplane flying through a cloud — but with added complexity caused by a magnetic field. Similar turbulence occurs on the sun’s surface and in the region of space influenced by Earth’s magnetic field. 

Uncovering a longstanding enigma

“The simulations showed that in situations when two fluids with different velocities meet and mix, creating a free shear layer, a large-scale nonaxisymmetric MRI can grow, which makes the whole disk wobble,” Ebrahimi said. “This new understanding has led to new physics that helps solve a long-standing astrophysical mystery.”

Collaborators included Erik Gilson, head of PPPL’s discovery plasma science; Hantao Ji, a PPPL distinguished research fellow and professor of astrophysical sciences at Princeton University; Jeremy Goodman, a professor of astrophysical sciences at Princeton University; and Hongke Lu, a summer intern.

This research was supported by DOE under contract number DE-AC02-09CH11466, NASA under grant number NNH15AB25I, the National Science Foundation under grant number AST-2108871 and the Max-Planck-Princeton Center for Fusion and Astro Plasma Physics.

PPPL is mastering the art of using plasma — the fourth state of matter — to solve some of the world’s toughest science and technology challenges. Nestled on Princeton University’s Forrestal Campus in Plainsboro, New Jersey, our research ignites innovation in a range of applications, including fusion energy, nanoscale fabrication, quantum materials and devices, and sustainability science. The University manages the Laboratory for the U.S. Department of Energy’s Office of Science, which is the nation’s single largest supporter of basic research in the physical sciences. Feel the heat at https://energy.gov/science and http://www.pppl.gov.