The study, published in Nature Astronomy and led by Dr. Eli Galanti under the direction of Prof. Yohai Kaspi of Weizmann's Earth and Planetary Sciences Department, draws on 26 radio occultation measurements collected by NASA's Juno spacecraft -- far more than the six measurements that had underpinned the previous standard values since the 1970s.
"Jupiter's shape, as understood until now, was derived by researchers from just six measurements made almost five decades ago by NASA's Voyager and Pioneer missions," Galanti said. "Those missions provided a foundation, but now we got the rare opportunity to spearhead the analysis of as many as 26 new measurements made by NASA's Juno spacecraft."
Juno, launched in 2011, has been in orbit around Jupiter since 2016. When NASA extended the mission in 2021, the spacecraft's new orbital path brought it into occultation geometry -- passing behind Jupiter as seen from Earth -- for the first time. In that configuration, Juno's radio signal is blocked and bent by Jupiter's atmosphere before reaching ground stations, and the degree of bending encodes precise information about the planet's atmospheric structure and outer boundary.
PhD student Maria Smirnova developed the data processing technique used to convert those bent signals into detailed maps of Jupiter's temperature and density profiles. The result is a sharper picture of the planet's actual dimensions.
"Textbooks will need to be updated," Kaspi said. "The size of Jupiter hasn't changed, of course, but the way we measure it has."
The revision is not merely geometric. Shifting Jupiter's radius by a few kilometers brings interior models into significantly better agreement with both gravitational and atmospheric observations. PhD student Maayan Ziv tested this directly using state-of-the-art models of Jupiter's interior density structure, finding that the refined shape reduces the gap between theoretical predictions and measured data.
The Weizmann team also incorporated Jupiter's extreme wind patterns into their analysis, a factor absent from earlier calculations. The planet's zonal winds and polar hurricanes influence its apparent shape, and excluding them had introduced systematic errors into prior measurements. "It's difficult to see what's happening beneath the clouds of Jupiter, but the radio data give us a window into the depth of Jupiter's zonal winds and powerful hurricanes," Kaspi said.
That wind analysis connects to a parallel study by Kaspi and former group member Dr. Nimrod Gavriel, published in PNAS, which used Juno measurements of polar cyclone motion to estimate how deeply those cyclones penetrate into the interior. That depth prediction has since been confirmed by Juno microwave measurements.
According to the updated figures, Jupiter's equatorial radius exceeds its polar radius by about 7 percent. Earth's equatorial bulge, by comparison, is just 0.33 percent -- meaning Jupiter is roughly 20 times more flattened than Earth, a consequence of its rapid rotation, complex internal structure, and powerful atmospheric winds.
"These few kilometers matter," Galanti said. "Shifting the radius by just a little lets our models of Jupiter's interior fit both the gravity data and atmospheric measurements much better."
The findings carry implications beyond Jupiter. As the largest and best-studied gas giant in the solar system, Jupiter serves as the standard reference for modelling gas giants both here and around other stars. A more accurate description of its size and shape improves the baseline for that entire class of planetary science.
The techniques developed for this study will be applied to data from the European Space Agency's JUICE mission, launched in 2023 and en route to the Jovian system. JUICE carries a Weizmann-designed instrument designed to probe Jupiter's atmosphere to greater depth than previous missions.
The research was an international collaboration including scientists from the University of Bologna, NASA's Jet Propulsion Laboratory, the University of Arizona, the University of California Berkeley, the Observatoire de la Cote d'Azur, the University of Zurich, the Georgia Institute of Technology, and Boston University. Juno's Principal Investigator is Dr. Scott J. Bolton of Southwest Research Institute in San Antonio, Texas.
Research Report: The size and shape of Jupiter
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