Laboratory swing reveals how planets form stars

Scientists used a rotating cylinder filled with liquid metal to recreate cosmic planet formation in a laboratory tank to confirm that the plasma rings that swing around the stars could cause matter to drift inward and condense into planets.

Experiments from Princeton Plasma Physics Laboratory show that these cosmic sways, known as magnetization-type instability, are growing more easily than previously thought, to explain why planetary systems are so common throughout the universe.

The discovery focuses on an unexpected mechanism: the oscillation generated in the free shear layer, two plasma flows flowing at different velocities. This discovery suggests that planet formation may be more frequent throughout the universe than astronomers once thought.

Star imitation in metal cylinders

“This finding suggests that swaying may be more than we expected, and may be the cause of the formation of more solar systems,” explained Yin Wang, a research physicist at Princeton’s Plasma Physics Laboratory.

These experiments used nested metal cylinders, each cylinder about one foot tall and 2 inches wide, and could rotate at different rates. The researchers filled the space between the cylinders with Galinstein, a liquid metal mixture of gold-plated, immature and tin, to mimic how different regions of the star-accumulated disk rotate at different speeds. When they apply the magnetic field, the settings faithfully reproduce the conditions around young stars formed by planets.

Liquid metal simulation perfectly captures the essential physics. Just like in an actual stellar disk, experimental swaying accelerates particles on the outer edge and can escape, while internal particles slow down and drift inward towards the central mass.

Shear layer surprise

Previous research has focused on swaying in the interaction between plasma and magnetic field in a gravitational environment. But Princeton found that swings might be more likely to appear in free shear layers—in which case fluids at different speeds meet and mix, similar to the turbulence that a plane flies through the clouds.

Computer simulations using programs called SFEMAN and DEDALUS confirm the experimental results and reveal the underlying mechanisms. “These computer simulations confirm our previous experimental analyses, but they also open up different boundaries to help us understand what this data means,” noted Fatima Ebrahimi, principal research physicist and study co-author of PPPL.

Simulations show that these axisymmetric magnetic instability represent a magnetic hydraulic dynamic turbulence with increased complexity of magnetic fields similar to what happens on the solar surface and on the Earth’s magnetosphere.

Universal meaning

The study is based on an experiment in 2022 that first demonstrates the magnetization instability of laboratory creation. The new work reveals that these cosmic sways can develop through magnetic field lines wrapped in patterns interleaved in shear layers, thus producing different magnetic intensities in different directions.

This mechanism has profound astronomical implications because in such cases:

• The boundary between the stellar disk and the central star
• The solar speed ladder that meets the solar radiation and convection zone
• Planets carved on gap edges on proto-star field disk
• In the area around the black hole, matter spirals inward

This discovery helps solve what Ebrahimi calls the “long-standing mystery of astrophysical mystery” involving migration inwards in the star disk to form planets. Previous models have worked to explain the rapid time scales formed by planetary systems.

From laboratory bench to cosmic scale

Experimental methods show how laboratory plasma physics illuminates billions of times larger cosmic processes. Princeton created conditions where the conditions of the magnetic Reynolds number (a measure of how a magnetic field interacts with a moving plasma) make the critical threshold of instability much lower than the traditional model predicted.

“The simulations show that when two fluids with different velocities meet and mix, a free shear layer is generated, large-scale non-axially symmetric MRI can grow, which makes the entire disk swing,” Ebrahimi explains. This swaying motion redistributes angular momentum, causing the matter to spiral inward and accumulate.

The results show that the planetary formation mechanisms operate more generally than before. Rather than requiring specific rare conditions, it can occur anywhere the shear layer exists, rather than an unlikely coincidence that planetary systems bring natural consequences of the evolution of stellar disks.

The research team includes Erik Gilson, head of plasma science at PPPL; Distinguished Researcher and Princeton University professor Hantao ji; Jeremy Goodman, professor of astrophysical sciences in Princeton; and Summer Intern Long Lu. Their work was supported by the Department of Energy, NASA and the National Science Foundation.

Future experiments will explore how the mechanisms confirmed by these labs extend to huge disks surrounding young stars, potentially revealing new details about how cosmic dust and gas stand out from cosmic dust and gas.

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