Jupiter is Smaller and More ‘Squashed’ than Previously Believed, New Juno Data Reveal
Using high-precision radio occultation measurements from NASA’s Juno mission and incorporating the effects of zonal winds, planetary scientists derived the shape of Jupiter with an order of magnitude reduction in uncertainty, finding smaller polar, equatorial, and mean radii than previous estimates made with NASA’s Pioneer and Voyager missions.
This visible light image of Jupiter was created from data captured on January 11, 2017 using Hubble’s Wide Field Camera 3. Near the summit, a long brown formation called a “brown barge” extends 72,000 km (nearly 45,000 miles) in an east-west direction. The Great Red Spot stands out prominently in the lower left, while the smaller feature dubbed Red Spot Jr. (known to Jovian scientists as Oval BA) appears in the lower right. Image credit: NASA / ESA / NOIRLab / NSF / AURA / Wong and others. / of Pater and others. /Mr. Zamani.
“Jupiter, the largest planet in the solar system, is approximately an oblate spheroid (ellipsoid of revolution), meaning it is slightly flattened at the poles and bulged at the equator due to its rapid rotation period of 9:55:29,” said Dr. Eli Galanti of the Weizmann Institute of Science and colleagues.
“This shape results from the balance between gravitational forces pulling inward in the radial direction and centrifugal forces pushing outward from the axis of rotation, resulting, in the case of Jupiter, in an equatorial radius about 7% larger than its polar radius.”
“For a constant density body, the shape is an exact ellipsoid. However, Jupiter’s interior density profile varies considerably from cloud level to around 1 bar, where the density is less than 1 kg/m.3down to deep levels, where the density reaches thousands of kg/m3. “
“This leads to variations in the shape of the planet from an ellipsoid of the order of several tens of kilometers, which are expressed by latitudinal variations of the gravity field.”
“Additional variations in Jupiter’s shape arise from the strong zonal winds observed at cloud level.”
“These modify centrifugal forces to create variations of the order of 10 km, mainly at low latitudes.”
Previously, Jupiter’s physical dimensions were based on data from six radio occultation experiments carried out by NASA’s Pioneer and Voyager missions in the 1970s.
In a new study, the authors analyzed radio occultation data obtained by Juno during 13 flybys of Jupiter and incorporated the effects of zonal winds.
“Radio occultation is used to ‘see’ through the dense, opaque clouds of Jupiter’s atmosphere to understand its internal structure,” they explained.
“In an occultation experiment, Juno sends radio signals back to NASA’s Deep Space Network on Earth.”
“As these signals pass through the charged upper layer of Jupiter’s atmosphere, called the ionosphere, the gases bend and delay the signals.”
“By measuring the change in frequency caused by this curvature, we can calculate the temperature, pressure and electron density of Jupiter’s atmosphere at different depths.”
The team’s results show that Jupiter is about 8 km narrower at the equator and 24 km flatter at the poles.
“By integrating the effects of zonal winds, we obtain the shape of Jupiter with an order of magnitude reduction in uncertainty,” the researchers said.
“At the 1 bar pressure level, we find a polar radius of 66,842 km, an equatorial radius of 71,488 km, and a mean radius of 69,886 km, respectively 12 km, 4 km, and 8 km smaller than previous estimates.”
The results were published this week in the journal Natural astronomy.
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E. Galanti and others. The size and shape of Jupiter. Nat Astronpublished online February 2, 2026; doi: 10.1038/s41550-026-02777-x
