Giant Planets Spin Faster Than More Massive Brown Dwarfs

Astronomers have found that giant planets can spin faster than more massive brown dwarfs, challenging previous assumptions about mass and rotation. Researchers used the W. M. Keck Observatory in Hawai'i to measure the spins of dozens of giant planets and brown dwarfs orbiting distant stars. The findings suggest that magnetic fields and formation processes significantly influence how fast celestial bodies rotate.

The study included 32 gas giants and brown dwarf companions in other star systems. Six of these were planets larger than Jupiter, and 25 were brown dwarf companions. The team also incorporated prior spin measurements from other studies. This created a dataset of 43 stellar or substellar companions and giant planets, along with 54 free-floating brown dwarfs and planetary-mass objects.

Many of the examined planets orbit their stars at distances ranging from tens to hundreds of Astronomical Units (AU). One AU is the distance between Earth and the Sun. Scientists are still determining how these distant worlds form. Some may emerge from disks of gas and dust around young stars. Others could form through a process similar to star formation.

To measure spin, researchers used the Keck Planet Imager and Characterizer (KPIC). This instrument isolates light directly from distant worlds. As a planet rotates, atmospheric features cause subtle broadening in its spectrum. Measuring these changes reveals the object's rotation speed. Lead author Dino Chih-Chun Hsu noted that spin provides a "fossil record" of a planet's formation.

An example from the HR 8799 system shows a gas giant, seven times Jupiter's mass, rotating six times faster than a brown dwarf companion, 24 times Jupiter's mass. This difference may stem from magnetic interactions early in the objects' histories. A stronger magnetic field can interact more intensely with a circumplanetary disk, slowing rotation. The more massive brown dwarf likely lost more original spin due to its stronger magnetic field.

The findings help scientists understand distant planetary systems and the origins of our own Solar System. The way angular momentum is distributed among planets influences a planetary system's overall architecture. Future studies will investigate the rotation of free-floating planets and the chemical makeup of their atmospheres. New technology, like the Keck Observatory's HISPEC instrument, will enable measurements of smaller, more distant worlds.