Jupiter-like Gas Giants or Brown Dwarfs – How Does One Tell Them Apart?
Brown Dwarfs are more massive than planets but less massive than stars. But they have similar diameters to planets such as Jupiter. Brown dwarfs are a class of objects halfway between giant planets, like Jupiter, and stars. All objects with a mass greater than about 75 times that of Jupiter (7% of the Sun’s mass) are stars, and mainly burn hydrogen for their entire lifetimes. During this phase of nuclear combustion, their luminosity remains quite stable. (Image Credit: W.M Keck Observatory, Adam Makarenko)
Jupiter-like Gas Giants or Brown Dwarfs – How Does One Tell Them Apart?
For decades, astronomers have struggled to differentiate giant planets from brown dwarfs, a class of objects more massive than planets but too small to ignite nuclear fusion like true stars. Through a telescope, these cosmic look alikes can have overlapping brightness, temperatures, and even atmospheric fingerprints. The striking similarity leaves astronomers unsure if they have observed an over-sized planet or an undersized star.
In our Solar System, Jupiter is the largest planet, being about 318 times as massive as the Earth and lying about five times farther from the Sun than does the Earth. Brown dwarfs are similar in many ways to Jupiter-like gas giants, but range from 13 to 90 times the mass of Jupiter… And while they can be up to a tenth the mass of the Sun, they lack the nuclear fusion in their core to burn as a star, so they lie somewhere between a diminutive star and a super-planet.
Now, a Northwestern University-led team has uncovered a crucial clue that separates the two: how fast they spin.
In a new study, astrophysicists found the clearest evidence yet that giant planets spin significantly faster than their brown dwarf counterparts. The new results suggest rotation measurements may provide a powerful new diagnostic for classifying these indistinguishable populations and suggest that these two objects evolve differently, perhaps even forming through distinct processes.
The study will be published in The Astronomical Journal. It marks the largest survey of spin measurements of directly imaged extrasolar planets and brown dwarfs to date.
“Spin is a fossil record of how a planet formed,” said Northwestern’s Chih-Chun “Dino” Hsu, who led the study. “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.”
An expert on exoplanets and brown dwarfs, Hsu is a postdoctoral researcher at Northwestern’s Center for Interdisciplinary Exploration and Research in Astrophysics (CIERA), where he is advised by study coauthor Jason Wang. Wang is an assistant professor of physics and astronomy at Northwestern’s Weinberg College of Arts and Sciences and a member of CIERA.
For more information:
https://news.northwestern.edu/stories/2026/03/spin-separates-giant-planets-from-failed-stars?fj=1
https://ciera.northwestern.edu/2026/03/18/spin-separates-giant-planets-from-failed-stars/
https://exoplanetes.umontreal.ca/en/research/naines-brunes-planetes-errantes/
https://www.astromart.com/news/show/giant-gas-planet-or-brown-dwarf-where-does-one-draw-the-line
https://www.astromart.com/news/show/yes-virginia-pluto-really-is-a-planet
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