Jupiter has the fastest rotation of all the planets in the Solar System, completing one rotation on its axis every 9.9 hours. It sounds like a simple question: what’s the rotation of Jupiter? But finding out the answer was surprisingly complicated.
Why was it so difficult to figure out Jupiter’s rotation? Unlike the inner terrestrial planets, Jupiter is a ball of almost entirely hydrogen and helium. Unlike Mars or Mercury, Jupiter has no surface features that you track to measure the rotation speed; there are no craters or mountains that rotate into view after a specific amount of time.
Jupiter has the fastest rotation of all the planets in the Solar System. This is quite a feat when you consider that Jupiter is also the largest planet in the Solar System; it’s turning a lot of mass very quickly. The rapid rotation causes the planet’s equator to bulge out. Instead of being a perfect sphere, Jupiter looks more like a squashed ball. The bulge at the equator is even visible in small, backyard telescopes.
This bulge dramatically effects the diameter of Jupiter, depending on whether you measure it from the center of Jupiter to the equator or to the poles. The polar radius of Jupiter is 66,800 km, while the equatorial radius is 71,500 km. In other words, points along Jupiter’s equator are actually 4,700 km more distant from the planet’s center.
Jupiter is a ball of gas, and so it actually experiences differential rotation. The rotation takes different amounts of time depending on where you are on the planet. The rotation of Jupiter at its poles takes about 5 minutes longer than the rotation of Jupiter at its equator. So the commonly quoted 9.9 hours is actually an average amount for the entire planet.
Scientists actually use three different systems to calculate the rotation of Jupiter. System 1 is for latitudes 10 degrees north and south of Jupiter’s equator – the rotation is 9 hours 50 minutes. System II is for latitudes north and south of this region, and the rotation rate is 9 hours, 55 minutes. These rates are measured by how long it takes for specific storms to come back into view. The final system, System III, measures the rotation speed of Jupiter’s magnetosphere and is usually considered the official rotation rate.
We have written many articles about Jupiter for Universe Today. Here’s an article about how Jupiter has Van Allen Belts, just like Earth. And here’s an article about how Jupiter is buffeted by the Solar wind.
We have recorded a podcast just about Jupiter for Astronomy Cast. Click here and listen to Episode 56: Jupiter.