EVANSTON, Ill. — In a groundbreaking study set to be published in The Astronomical Journal on March 18, 2026, astronomers have made significant strides in differentiating between giant planets and brown dwarfs, often referred to as “failed stars.” This discovery hinges on a critical factor: the speed at which these celestial bodies spin.
For decades, the astronomical community has grappled with the challenge of distinguishing between these two types of objects. Both giant planets and brown dwarfs can exhibit similar brightness, temperatures, and atmospheric characteristics, making it difficult to tell them apart. However, a team led by Northwestern University has found that giant planets spin significantly faster than brown dwarfs, providing a new method for classification.
Spin: A Key to Cosmic Classification
The study represents the largest survey of spin measurements of directly imaged extrasolar planets and brown dwarfs to date. According to Chih-Chun “Dino” Hsu, the study’s lead author and a postdoctoral researcher at Northwestern’s Center for Interdisciplinary Exploration and Research in Astrophysics (CIERA), “Spin is a fossil record of how a planet formed. 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.”
Hsu and his team utilized the W.M. Keck Observatory on Maunakea in Hawaiʻi to analyze six giant exoplanets and 25 brown dwarfs. The high-resolution spectroscopy from the Keck Planet Imager and Characterizer Instrument (KPIC) allowed the team to measure the spin rates by examining the broadening of spectral features, akin to the Doppler effect for sound.
A Cosmic Identity Crisis
Typically, astronomers have relied on a combination of brightness, temperatures, and spectral information to distinguish planets from stars. However, giant planets and brown dwarfs occupy a nebulous zone in this classification system. The largest planets and the smallest brown dwarfs overlap in size and mass, and brown dwarfs, lacking sustained nuclear fusion, emit a faint glow similar to giant planets.
The Northwestern team hypothesized that spin rates could serve as a distinguishing factor. Their findings revealed that giant planets rotate at a larger fraction of their theoretical maximum speed—known as their “breakup velocity”—compared to brown dwarfs, which rotate more slowly.
A New Spin on Formation
This difference in spin rates likely originates from the objects’ formation processes. Giant planets are thought to form within disks of gas and dust surrounding young stars, where interactions with the disk influence their angular momentum. In contrast, brown dwarfs can form like stars, through the collapse of gas clouds, or like planets. The strong magnetic fields of brown dwarfs act as a cosmic brake, causing them to lose angular momentum.
“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 explained. “That helps us narrow down the physics of how these systems form.”
One notable example from the study is a giant planet in the HR 8799 exoplanet system, which is about seven times the mass of Jupiter and spins unusually fast. In contrast, a nearby brown dwarf, despite being three times more massive, rotates six times slower. This illustrates the significant impact of magnetic fields and formation environments on spin rates.
Looking Forward: Expanding the Research
With these findings, the research team plans to further explore the spins of free-floating planetary-mass objects—rogue worlds that drift through space without a host star. They also aim to investigate 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, titled “Distinct rotational evolution of giant planets and brown dwarf companions,” received support from NASA, the National Science Foundation, and the Heising-Simons Foundation.
