Jupiter Turns Out To Be Smaller (and More Complex) Than We Thought

Jupiter now officially measures fewer kilometres than the textbooks say.

04 Feb 2026, 19:05 by Mihai Andrei · ZME Science

Jupiter didn’t lose any weight; we just see it better now. Image credits: Weizmann Institute of Science.

Jupiter is officially “smaller” than it was yesterday. To be clear: the planet itself didn’t physically contract overnight. There was no cosmic gym session. Instead, thanks to the Juno spacecraft’s daring dives, we have finally upgraded our measuring tape.

At the critical 1-bar pressure level, considered the “surface” of a gas giant, Jupiter’s polar radius is 12 kilometers shorter than we believed. Its equatorial bulge is 4 kilometers slimmer. Even its mean radius has been trimmed by 8 kilometers. For a planet with a radius of over 70,000 kilometers, it doesn’t seem like much. But for researchers working in planetary science, this can make the difference between a model that works and one that doesn’t.

“Textbooks will need to be updated,” mused Yohai Kaspi, the study’s senior author and an astronomer at the Weizmann Institute of Science, in a statement.

Why We Were Wrong

For nearly five decades, much of our understanding of Jupiter’s physical dimensions rested on a few measurements taken by the Voyager and Pioneer missions in the late 1970s. These “Old Guard” spacecraft essentially looked at how radio signals bent as they passed through Jupiter’s atmosphere.

It’s an established, reliable method. But they only had six data points to work with. You can’t map out the details properly. Even more importantly, the scientists of the 70s and 80s made a simplifying assumption: they ignored the winds.

Jupiter’s south pole as seen by NASA’s Juno spacecraft at a close pass. Photo courtesy of NASA.

Jupiter is the fastest-rotating planet in our neighborhood, spinning once every 9 hours and 55 minutes, while being much larger than the Earth. This rapid rotation creates a massive equatorial bulge, making the planet look like a basketball that someone is sitting on. But Jupiter is a roiling, screaming mess of atmospheric jets, which makes everything much more complicated. These zonal winds (the stripes that Jupiter has) move at hundreds of miles per hour.

This matters because the winds also generate their own centrifugal forces, pushing the atmosphere out and pulling it in at different latitudes. The previous estimates treated Jupiter like a static, solid object. The new study, led by Eli Galanti and Yohai Kaspi, used 24 high-precision measurements from Juno and finally accounted for the “dynamical height” created by those winds.

So How Big Is It?

Jupiter measures 71,488 kilometers at its bulging equator and 66,842 kilometers from pole to pole.

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The change from the old measurements is less than the distance from Manhattan to Brooklyn. In the grand scheme of things, it seems like nothing. But these few kilometers matter, Galanti explains.

“Shifting the radius by just a little lets our models of Jupiter’s interior fit both the gravity data and atmospheric measurements much better.”

This implication was tested by another PhD student in Kapsi’s group, Maayan Ziv. By “shrinking” the equatorial radius by 14 km (when removing wind effects for static models), the math finally works.

“We were in a unique position to use our state-of-the-art models for the interior density structure of Jupiter to show that the refined shape helps bridge the gap between the models and the measurements,” Ziv says.

This isn’t just about Jupiter. Because Jupiter is used as the “gold standard” or calibration tool for modeling exoplanets in other star systems, this refined shape will help astronomers more accurately calculate the size and composition of distant worlds across the galaxy.

Jupiter is still the same swirling, beautiful behemoth it has always been. But now, we can look at it through a better lens. It was about time.

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