Here’s a twist from deep space. Some giant planets appear to spin faster than much heavier brown dwarfs, according to observations from the W. M. Keck Observatory on Maunakea, Hawai’i.

Astronomers studied 32 gas giants and brown dwarf companions in other star systems, including six planets larger than Jupiter and 25 brown dwarf companions. They found that, when mass, size and age are taken into account, giant gas planets tend to rotate faster than more massive brown dwarfs.

The team also added earlier spin measurements from other studies, building a dataset that included 43 stellar or substellar companions and giant planets, along with 54 free-floating brown dwarfs and planetary-mass objects. The findings were published in The Astronomical Journal.

The international team was led by scientists at Northwestern University’s Center for Interdisciplinary Exploration and Research in Astrophysics, or CIERA. Collaborators included researchers from the Center for Astrophysics and Space Sciences at UC San Diego, the Division of Geological and Planetary Sciences at Caltech, the W. M. Keck Observatory, the Steward Observatory, the James C. Wyant College of Optical Sciences, NASA’s Jet Propulsion Laboratory and other institutions.

To make the measurements, researchers used the Keck Planet Imager and Characterizer, or KPIC, which can isolate light coming directly from distant worlds. As a planet rotates, features in its atmosphere subtly broaden its spectrum, allowing astronomers to work out how fast it is spinning.

“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,” lead author Dino Chih-Chun Hsu, a researcher at CIERA, said in a W. M. Keck Observatory press release.

Many of the planets in the study orbit their stars at distances from tens to hundreds of astronomical units, with one astronomical unit defined in the source as the distance between Earth and the Sun. Scientists are still trying to determine how these faraway worlds form.

One example came from the HR 8799 system. There, a gas giant about seven times the mass of Jupiter rotates six times faster than a brown dwarf companion that is roughly 24 times Jupiter’s mass.

Researchers said the gap may be tied to magnetic interactions early in the objects’ histories. In this case, the more massive brown dwarf likely lost more of its original spin because of its stronger magnetic field interacting more intensely with the surrounding circumplanetary disk.

Hsu said the findings also help scientists study the origins of our own Solar System.

“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,” he said.

The team plans to extend the work to free-floating planets, often called rogue planets, and to study the chemical makeup of these worlds’ atmospheres. Future observations are also expected to use the Keck Observatory’s High-resolution Infrared Spectrograph for Exoplanet Characterization, or HISPEC, which is scheduled to begin operations in 2027.

“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,” Jason Wang, an assistant professor at Northwestern University and a co-author of the study, said.

“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.”

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