How young galaxies grew magnetic fields faster than expected
Tata Institute of Fundamental Research
image:
Lower panel shows a collapsing plasma cloud with uniform magnetic field (red).Top Right: Compression alone amplifies the field. Bottom Right: Collapse-driven turbulence accelerates dynamo amplification (also generating a horizontal component (blue)), producing magnetic fields stronger than compression alone.
view moreCredit: Pallavi Bhat, Anvar Shukurov, Muhammed Irshad and Kandaswamy Subramanian. NASA; SOFIA; HAWC+; A. S. Borlaff/NASA; JPL-Caltech; ESA; Hubble.
How fast can a galaxy build ordered magnetic fields spanning thousands of light-years? Existing theories say several billion years, but observations of galaxies in our universe imply shorter timescales.
In a study published in the Physical Review Letters and highlighted in the Physics magazine, scientists propose an explanation that resolves this contradiction. They say that the collapse of plasma clouds during the formation of galaxies could significantly accelerate the growth of these magnetic fields.
Almost all visible matter in our universe is in the form of plasma, which can be stirred by forces related to gravity, temperature gradients and rotation. If these lead to turbulent flow, the dynamo theory predicts that the existing magnetic fields in the plasma are amplified. The dynamo theory is our primary framework for understanding the origin of cosmic magnetic fields.
“However, dynamo theory has its limitations”, says Pallavi, an assistant professor at the International Centre for Theoretical Sciences (ICTS) and an author of the study. “In particular, it struggles to explain observations of young galaxies with robust magnetic fields across thousands of light-years”.
The new study explores how dynamos might operate differently during galaxy formation. It considers an ionized gas cloud collapsing under gravity - the stage during which galaxies assemble. “When the galaxy is forming, gravity itself can stir the plasma, which can amplify magnetic fields”, says Irshad, a graduate student at ICTS and the lead author of the paper.
Through analytic calculations, the team was able to show that such stirring of plasma that occurs during the collapse can accelerate the formation of magnetic fields. As a result, the observed magnetic fields could be established significantly faster than was earlier thought.
The reason for this, they say, is how the turbulent flow of plasma is altered during the collapse. One characteristic of turbulent flows are eddies, not unlike the swirls we see in water streams. How fast the magnetic field grows depends on the ‘turnover rate’ of these eddies — how fast they swirl.
The team found that this turnover rate increases as the cloud collapses, leading to an accelerated ‘super exponential’ growth in magnetic fields, explaining how magnetic fields could have formed faster in the younger galaxies. Through numerical analysis, the team also showed that the field thus formed is stronger than one would expect from the standard dynamo theory.
In their study, the team uses a mathematical framework called ‘supercomoving coordinates’. In cosmology, this is used to absorb the expansion of the universe. “These coordinates essentially make the equations of a collapsing galaxy the same as a static galaxy, making the calculations very straightforward”, says Irshad. “This works well for a uniformly collapsing spherical system, but we would need to extend this study for more realistic cases”.
There’s still much to learn in this “zeroth-order question you ask about the timescale” too, says Pallavi. For example, there have been efforts to create computational models for the structure formation in the universe. This study can predict how fast the magnetic fields are set up in the universe, enabling scientists to test and refine their models accordingly, she says.
Although magnetic forces are typically much weaker than gravity in shaping cosmic structures, the new study suggests that strong, ordered magnetic fields may have appeared earlier, slowly nudging the evolution of our universe for longer than we thought.
Journal
Physical Review Letters
Method of Research
Computational simulation/modeling
Article Title
Turbulent Dynamos in a Collapsing Cloud
Spin separates giant planets from ‘failed stars’
Clearest evidence yet that giant planets spin faster than their cosmic lookalikes
For decades, astronomers have struggled to differentiate giant planets from brown dwarfs, a class of objects more massive than planets but too small to ignite nuclear fusion like true stars.
Through a telescope, these cosmic lookalikes can have the overlapping brightness, temperatures and even atmospheric fingerprints. The striking similarity leaves astronomers unsure if they have observed an oversized planet or an undersized star.
Now, a Northwestern University-led team has uncovered a crucial clue that separates the two: how fast they spin.
In a new study, astrophysicists found the clearest evidence yet that giant planets spin significantly faster than their brown dwarf counterparts. The new results suggest rotation measurements may provide a powerful new diagnostic for classifying these indistinguishable populations and suggest that these two objects evolve differently, perhaps even forming through distinct processes.
The study will be published on Wednesday (March 18) in The Astronomical Journal. It marks the largest survey of spin measurements of directly imaged extrasolar planets and brown dwarfs to date.
“Spin is a fossil record of how a planet formed,” said Northwestern’s Chih-Chun “Dino” Hsu, who led the study. “By measuring how quickly these worlds rotate, we can start to piece together the physical processes that shaped them tens to hundreds of millions of years ago.”
An expert on exoplanets and brown dwarfs, Hsu is a postdoctoral researcher at Northwestern’s Center for Interdisciplinary Exploration and Research in Astrophysics (CIERA), where he is advised by study coauthor Jason Wang. Wang is an assistant professor of physics and astronomy at Northwestern’s Weinberg College of Arts and Sciences and a member of CIERA.
A cosmic identity crisis
Typically, astronomers can distinguish planets from stars based on a combination of brightness, temperatures and spectral information. But giant planets and brown dwarfs, which are often called “failed stars,” sit right in the blurry middle of this classification system. The size and mass of the largest planets overlap with the size and mass of the smallest brown dwarfs. And because brown dwarfs lack sustained nuclear fusion, they emit a faint glow like giant planets.
The Northwestern team wondered if the objects’ spins could provide a differentiating factor. Using Northwestern’s institutional access to W.M. Keck Observatory on Maunakea in HawaiÊ»i, the astrophysicists analyzed six giant exoplanets and 25 brown dwarfs.
“We were only able to conduct a spectroscopic survey of this scale because Northwestern is a Keck Observatory partner,” Wang said. “That allowed us to access Keck’s telescopes for many nights to make this survey a reality.”
With high-resolution spectroscopy from the Keck Planet Imager and Characterizer Instrument (KPIC), the team isolated light from the faint objects to measure fine details in their atmospheres. As these distant worlds rotate, features in their spectra broaden, much like the Doppler effect for sound. By analyzing those broadened features, scientists can determine how quickly a planet is spinning.
“With KPIC, we can detect these tiny signals that reveal a planet’s rotation around other nearby stars,” Hsu said.
After measuring the spins of the exoplanets and brown dwarfs, the team combined those new measurements with spin measurements from previous studies. This enabled the team to build a larger curated sample of planets, brown dwarfs and related objects for comparison.
When Hsu and his collaborators compared rotation rates across the full sample, a clear pattern emerged. Giant planets tend to rotate at a larger fraction of their theoretical maximum speed — known as their “breakup velocity,” or the point at which an object would tear itself apart from centrifugal force. By contrast, brown dwarfs rotate more slowly.
A new spin on formation
According to the researchers, this difference likely traces back to the objects’ masses and how their mass compares to that of their host stars. Astronomers have long thought that giant planets form within disks of gas and dust surrounding young stars. During formation, interactions with the disk can influence how much angular momentum — or amount of spin — the planet retains.
Brown dwarfs, on the other hand, can form like stars — through the collapse of gas clouds — or like planets. Interactions between the brown dwarf’s strong magnetic field and the surrounding gas act like a cosmic brake, causing the object to lose angular momentum.
One exoplanet and one brown dwarf in Hsu’s study highlight this difference. A giant planet in the HR 8799 exoplanet system is about seven times the mass of Jupiter and spins unusually fast. But a nearby brown dwarf is roughly three times more massive than the giant exoplanet yet rotates six times slower.
While both objects lost angular momentum during their formation, the spin of the more massive brown dwarf lost significantly more momentum likely due to its stronger magnetic field. The study also found that brown dwarfs orbiting stars rotate even more slowly than isolated brown dwarfs, drifting through space. This possibly reflects different formation environments.
“Our results suggest that both the planet’s mass and the ratio between the planet’s mass and its star’s mass influence how fast the planet ultimately spins,” Hsu said. “That helps us narrow down the physics of how these systems form.”
Next, the research team plans to expand their studies by examining the spins of free-floating planetary-mass objects — rogue worlds that drift through space without a host star — and by investigating the chemical composition of planetary atmospheres across the population.
“We’re just beginning to explore what planetary spin can tell us,” Hsu said. “With future instruments and larger telescopes, we’ll be able to measure spins for even more worlds and connect rotation, chemistry and formation history across entire planetary systems.”
The study, “Distinct rotational evolution of giant planets and brown dwarf companions,” was supported by NASA, the National Science Foundation and the Heising-Simons Foundation.
Journal
The Astronomical Journal
Method of Research
Observational study
Article Title
Distinct Rotational Evolution of Giant Planets and Brown Dwarf Companions
Article Publication Date
18-Mar-2026
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