Distinguishing between failed stars, called brown dwarfs, and giant planets has long confused astronomers, but a clear separating factor has finally been identified.
Larger than planets but too small to produce nuclear fusion, brown dwarfs exhibit brightness, temperatures, and atmospheric characteristics remarkably similar to those of giant planets when observed across vast cosmic distances. Now, researchers report in a new paper published in The Astronomical Journal that a simple way to distinguish these large bodies lies in their rotational speeds.
The Slow Spin of Brown Dwarfs
In a new study led by Northwestern University researchers, the team found that giant planets spin much more rapidly than brown dwarfs. This finding suggests that the two types of bodies evolve very differently after forming through entirely separate processes.
“Spin is a fossil record of how a planet formed,” said lead author Chih-Chun “Dino” Hsu. “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.”
In most cases, spectral data, temperature, and luminosity provide astronomers with the clues needed to distinguish a planet from a star. However, the enormous size of giant planets and the relatively small size of brown dwarfs compared to other stars have long blurred that distinction. Even in terms of size and mass, the two categories overlap, and limited nuclear fusion in brown dwarfs produces a faint glow that can resemble that of a planet.
Considering Rotation Speed
To identify a clearer distinguishing parameter, the team turned to rotational data. They analyzed observations from Hawaii’s W. M. Keck Observatory, including 25 brown dwarfs and 6 giant planets. The Keck Planet Imager and Characterizer (KPIC) instrument provides astronomers with spectroscopic data on distant planets. Spectroscopic analysis allows researchers not only to interpret atmospheric conditions but also to determine rotation speeds, as spectral lines broaden with motion.
“With KPIC, we can detect these tiny signals that reveal a planet’s rotation around other nearby stars,” Hsu said.
After measuring rotational speeds, the team combined their results with spin data from earlier studies. Analysis of the expanded dataset revealed a clear trend: giant planets rotate more quickly than brown dwarfs.
Brown Dwarf and Giant Planet Differences
The team hypothesizes that these differences in rotation speed trace back to distinct formation mechanisms. Giant planets are generally believed to form within the accretion disks of young stars, where interactions with surrounding material significantly influence their spin.
Brown dwarfs, by contrast, form more like stars—through the collapse of gas clouds—but lack sufficient mass to sustain nuclear fusion. In these objects, strong magnetic fields can act as a braking mechanism, slowing their rotation as they interact with surrounding gas.
The researchers point to two objects that clearly illustrate this contrast. In the HR 8799 system, a giant planet roughly three times the mass of Jupiter spins unusually fast. Meanwhile, a nearby brown dwarf rotates at only one-sixth that speed, despite being three times more massive. The team suggests that the brown dwarf’s stronger magnetic field has caused it to lose angular momentum more rapidly.
Additionally, the researchers found that brown dwarfs orbiting stars tend to spin even more slowly, indicating that formation environment also plays a significant role.
Beyond Brown Dwarfs, Onto Rogue Exoplanets
“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.”
The team plans to extend their work by examining the rotational speeds of free-floating planetary-mass objects. Spectroscopic analysis will also allow researchers to compare the atmospheric compositions of these rogue worlds.
“We’re just beginning to explore what planetary spin can tell us,” Hsu concluded. “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 paper, “Distinct Rotational Evolution of Giant Planets and Brown Dwarf Companions,” appeared in The Astronomical Journal on March 18, 2026.
Ryan Whalen covers science and technology for The Debrief. He holds an MA in History and a Master of Library and Information Science with a certificate in Data Science. He can be contacted at ryan@thedebrief.org, and follow him on Twitter @mdntwvlf.