Jerusalem, 2 February, 2026 (TPS-IL) — For more than 50 years, scientists believed they knew the size and shape of Jupiter, the solar system’s largest planet. Now, researchers at the Weizmann Institute of Science have revised that understanding using new data and advanced technology. The findings carry broader significance for planetary science — gas giants like Jupiter are used as benchmarks for understanding similar planets both in our solar system and around other stars.
The findings, published in the peer-reviewed Nature Astronomy, provide the most precise measurements yet of Jupiter’s dimensions and form.
“Just by knowing the distance to Jupiter and watching how it rotates, it’s possible to figure out its size and shape,” said Prof. Yohai Kaspi of Weizmann’s Earth and Planetary Sciences Department. “But making really accurate measurements calls for more sophisticated methods.”
Until now, Jupiter’s shape was determined from just six measurements taken nearly five decades ago by NASA’s Voyager and Pioneer missions, which sent radio beams from the spacecraft to Earth. “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,” explained Dr. Eli Galanti, a senior staff scientist who led the research in Kaspi’s team.
Launched in 2011 and orbiting Jupiter since 2016, Juno has continuously transmitted streams of raw data back to NASA. When the mission was extended in 2021, the spacecraft’s orbit was altered, allowing it to pass behind Jupiter from Earth’s perspective — a maneuver that its earlier orbit never allowed.
“Juno’s passing behind Jupiter provides an opportunity for new science objectives. When the spacecraft passes behind the planet, its radio communication signal is blocked and bent by Jupiter’s atmosphere. This enables an accurate measurement of Jupiter’s size,” says Juno’s Principal Investigator Dr. Scott J. Bolton of the Southwest Research Institute in San Antonio, Texas.
The Weizmann team seized this opportunity.
“We tracked how the radio signals bend as they pass through Jupiter’s atmosphere, which allowed us to translate this information into detailed maps of Jupiter’s temperature and density, producing the clearest picture yet of the giant planet’s size and shape,” says Maria Smirnova, a PhD student in Kaspi’s group, who developed a special technique to process Juno’s new data.
The new measurements reveal that Jupiter is slightly smaller than previously thought—about 8 kilometers less wide at the equator and 24 kilometers flatter at the poles. Its equatorial radius is now estimated to be roughly 7 percent greater than its polar radius, making it about 20 times flatter than Earth, whose equatorial radius exceeds its polar radius by just 0.33 percent. “Textbooks will need to be updated,” Kaspi notes. “The size of Jupiter hasn’t changed, of course, but the way we measure it has.”
“These few kilometers matter,” Galanti added. “Shifting the radius by just a little lets our models of Jupiter’s interior fit both the gravity data and atmospheric measurements much better.”
Maayan Ziv, another PhD student in Kaspi’s group, said the updated shape helps refine models of the planet’s internal density. “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.”
The study also accounts for Jupiter’s powerful winds, which had been overlooked in earlier calculations. “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 explained. This work builds on a recent study by Kaspi and Dr. Nimrod Gavriel, a graduate of his group, on the planet’s vast polar cyclones, showing how deep they extend into the interior.
“By refining Jupiter’s size and interior models, we improve our understanding of how gas giants form and evolve—insights that apply to planets far beyond our solar system,” Kaspi said.
Jupiter also provides a window into the early solar system. Likely the first planet to form, its internal structure and atmospheric dynamics reveal conditions that shaped the development of Earth and other planets. “By studying what’s happening inside it, we get closer to understanding how the solar system, and planets like ours, came to be,” Kaspi explained.
The techniques developed in this study also set a precedent for future missions. The same methods will be applied to data from the European Space Agency’s JUICE spacecraft, launched in 2023, which carries a Weizmann-designed instrument to probe gas giant atmospheres in unprecedented detail.
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