Universe Today
For some time, astronomers have theorized that there is a connection between planetary mass and rotation. In the Solar System, Jupiter and Saturn both rotate rapidly, completing a rotation in roughly ten hours, while accounting for a significant fraction of the Solar System's rotational energy. Using the W.M. Keck Observatory on Maunakea, Hawai'i, a team of astronomers tested this predicted relationship by studying 32 gas giants and brown dwarfs in distant star systems - 6 giant planets larger than Jupiter and 25 brown dwarf companions
The high-resolution spectroscopy they obtained with the Keck Planet Imager and Characterizer (KPIC) instrument showed that gas giant planets spin faster than their more massive counterparts when mass, size, and age are taken into account. They also consulted historical data on companions with spin measurements to create a curated sample of 43 stellar/substellar companions and giant planets, and 54 free-floating brown dwarfs and planetary-mass objects.
The team was led by researchers from the Center for Interdisciplinary Exploration and Research in Astrophysics (CIERA) at Northwestern University. They were by scientists 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 multiple universities. The study describing their findings is published in The Astronomical Journal.
*The gas giant exoplanet (left) and a more massive brown dwarf companion (right) in the HR 8799 system. Credit: W.M. Keck Observatory*
Many of the planets observed in this study orbit their stars at a distance of tens to hundreds of Astronomical Units (AUs), the distance between the Earth and the Sun. Astronomers are still debating how such distant worlds form, whether it's a gradual process within a circumstellar disk or a gravitational collapse similar to that of stars. To investigate, the team used the KPIC to isolate light from these rotating planets, which broadens the spectra of atmospheric features.
By analyzing these features, scientists can determine how rapidly a planet is spinning. Said lead author Dino Chih-Chun Hsu, a researcher at the CIERA at Northwestern University, 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.
This complex relationship is illustrated by one planet and one brown dwarf in particular. In the system HR 8799, there is a gas giant roughly 7 times the mass of Jupiter that spins six times more rapidly than a brown dwarf companion in the same system that is 24 times the mass of Jupiter. This can be explained by interactions between the planet's magnetic field in its infancy and the circumplanetary disk that caused it to lose rotational speed.
Basically, the spin of the more massive companion was slowed because it had a much stronger magnetic field. Understanding this relationship between size, mass, and spin is also helping scientists learn more about the history of our Solar System. Said Hsu:
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.
The research team plans to expand its studies by examining the spins of free-floating planets (FFPs), also known as "Rogue Planets." They also hope to investigate the composition of these planets' atmospheres. This will be assisted by next-generation instrumentation, such as the Keck Observatory’s upcoming HISPEC (High-resolution Infrared Spectrograph for Exoplanet Characterization), which will become operational in 2027. As Hsu explained, HISPEC will extend these measurements to even smaller and more distant worlds. Said Jason Wang, an Assistant Professor at Northwestern University and co-author of the study:
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.
"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."
Further Reading: Keck Observatory
