Spin separates giant planets from ‘failed stars’

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Spin separates giant planets from 'failed stars'

Clearest evidence yet that giant planets spin faster than their cosmic lookalikes

EMBARGOED UNTIL 10 A.M. EDT (U.S.) ON WEDNESDAY, MARCH 18, 2026

• Astronomers conducted the largest survey yet of spin measurements of giant planets and brown dwarfs, or so-called 'failed stars'
• Young giant planets and brown dwarfs often have similar brightness, temperatures and atmospheric fingerprints, making them difficult to distinguish
• New study finds giant planets spin significantly faster than brown dwarfs, suggesting spin measurements could help classify the objects
• Findings also indicate that giant planets and brown dwarfs form and evolve through distinct paths

EVANSTON, Ill. --- 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 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 Hawaiʻi, 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.

Animations of spin

Please credit all videos to W. M. Keck Observatory/Adam Makarenko

Chih-Chun "Dino" Hsu

Corresponding author

Postdoctoral researcher at CIERA

• [email protected]

Jason Wang

Senior author

Assistant professor of physics and astronomy

• [email protected]