Why this is the case, and how you actually measure “spin” on a gas giant like Jupiter, is an interesting question, something that scientists have only recently been able to grapple thanks to results from a NASA spacecraft orbiting Jupiter, called Juno.
Rotation, rotation, rotation
Measuring spin on a gas giant like Jupiter compared to a rocky planet like Earth is a little tricky. “It’s not like you can look at the surface,” said Ravit Helled at the University of Zurich in Switzerland. Jupiter is almost entirely composed of hydrogen and helium, a giant gaseous ball spinning in space. So how do we know how fast it’s actually spinning?
We calculate this by studying the planet’s magnetic field. Measuring the planet’s radio emissions, we can work out the spin of its internal magnetic field, in turn giving us the spin of the planet. “We’re finding periodicities in the magnetic fields,” said Helled. This same technique allows us to measure the spins of the other gas giants, too.
Juno, which entered orbit around Jupiter in 2016, also used gravitational measurements to determine how Jupiter spins. The spacecraft has shown that the winds in the planet’s outer atmosphere extend down about 3,000 kilometers (1,900 miles) into the planet. “For decades it was not known if the winds penetrate very deep into the interior,” said Helled, a member of the Juno science team. “Thanks to Juno we could constrain the depth. Now we believe below 3,000 kilometers, the planet reaches uniform rotation.”
Spinning up
Where Jupiter gets this rotation rate from is another important question, and the answer might again lie in the planet’s magnetic fields. When the planet first formed 4.6 billion years ago, it accumulated mass from the disk of material around the young Sun. As it grew in size it would have spun faster and faster as its angular momentum increased, like a spinning ice skater drawing in their arms.