Astronomers have long suspected that a planet's mass and its rotation speed are linked. In our own Solar System, Jupiter and Saturn provide striking examples. Despite their enormous size, both complete a full rotation in about 10 hours and account for a large share of the Solar System's total rotational energy.
To test whether this relationship extends beyond our cosmic neighborhood, researchers used the W. M. Keck Observatory on Maunakea, Hawai'i, to study a large sample of distant giant worlds. Their survey included 32 gas giants and brown dwarf companions in other star systems, including 6 planets larger than Jupiter and 25 brown dwarf companions.
The observations revealed an intriguing trend. When factors such as mass, size, and age are considered, giant gas planets tend to rotate faster than more massive brown dwarfs. To strengthen their analysis, the researchers also incorporated previous spin measurements from other studies, creating a carefully selected dataset that included 43 stellar/substellar companions and giant planets, along with 54 free-floating brown dwarfs and planetary-mass objects.
The international team was led by scientists at Northwestern University's Center for Interdisciplinary Exploration and Research in Astrophysics (CIERA). Collaborators included researchers from the Center for Astrophysics and Space Sciences (CASS) at UC San Diego, the Division of Geological & Planetary Sciences (GPS) at Caltech, the W. M. Keck Observatory, the Steward Observatory, the James C. Wyant College of Optical Sciences, NASA's Jet Propulsion Laboratory, and several other institutions. Their findings were published in The Astronomical Journal.
Measuring the Spin of Distant Worlds
Many of the planets examined orbit their stars at distances ranging from tens to hundreds of Astronomical Units (AUs), the distance between Earth and the Sun. Scientists are still trying to determine how these faraway worlds form. Some may gradually emerge within disks of gas and dust surrounding young stars, while others could form through a process more similar to the collapse that creates stars themselves.
To investigate, the researchers used the Keck Planet Imager and Characterizer (KPIC), a specialized instrument capable of isolating light coming directly from these distant worlds. As a planet rotates, features in its atmosphere cause subtle broadening in its spectrum. By measuring these changes, astronomers can determine how quickly the object is spinning.
Lead author Dino Chih-Chun Hsu, a researcher at CIERA, explained the significance of these measurements in a W. M. Keck Observatory press release:
"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. With KPIC, we can detect these tiny signals that reveal a planet's rotation around other nearby stars. 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. That helps us narrow down the physics of how these systems form."
A Giant Planet Outspins a Much Larger Neighbor
One of the clearest examples comes from the HR 8799 system. There, a gas giant about 7 times the mass of Jupiter rotates six times faster than a brown dwarf companion that is roughly 24 times Jupiter's mass.
Researchers believe the difference may be linked to magnetic interactions early in the objects' histories. A stronger magnetic field can interact more intensely with the surrounding circumplanetary disk, slowing rotation over time. In this case, the more massive brown dwarf likely lost more of its original spin because of its stronger magnetic field.
The findings are helping scientists better understand not only distant planetary systems but also the origins of our own Solar System. Hsu said:
"The way that angular momentum is distributed among planets influences the overall architecture of a planetary system. Even Earth's rotation and magnetic field ultimately connect to how that spin budget was divided when the solar system formed. KPIC is the first instrument of its kind, opening an entirely new way to study exoplanets. It allowed us to measure properties like spin that were previously almost impossible to detect."
Future Studies of Rogue Planets and Exoplanet Atmospheres
The team plans to expand this work by studying the rotation of free-floating planets (FFPs), often called "Rogue Planets." Researchers also hope to investigate the chemical makeup of these worlds' atmospheres.
Future observations will benefit from new technology, including the Keck Observatory's upcoming HISPEC (High-resolution Infrared Spectrograph for Exoplanet Characterization), scheduled to begin operations in 2027. According to Hsu, the new instrument will make it possible to study smaller and more distant worlds than ever before.
Jason Wang, an Assistant Professor at Northwestern University and a co-author of the study, said:
"We took the lessons learned from KPIC, and put them into HISPEC, which will have better sensitivity, higher spectral resolution, and wider wavelength coverage. With HISPEC we will be able to drastically increase the number of planets that we can measure spins of, and in particular, we can study planets closer to our own Jupiter in nature to see if our own Jupiter is typical."
Researchers believe they are only beginning to uncover what planetary rotation can reveal.
"We're just beginning to explore what planetary spin can tell us," said Hsu. "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."
