Planet Jupiter is slightly smaller than previously estimated, researchers at the Weizmann Institute of Science in Rehovot say in a study published on Monday.

Jupiter is about eight kilometers (five miles) narrower at the equator and 24 kilometers (15 miles) flatter at the poles, the research published in Nature Astronomy says.

“The size of Jupiter hasn’t changed, of course, but the way we measure it has. Textbooks will need to be updated,” said Professor Yohai Kaspi of Weizmann’s Earth and Planetary Sciences Department.

“These few kilometers matter. Shifting the radius by just a little lets our models of Jupiter’s interior fit both the gravity data and atmospheric measurements much better,” Eli Galanti, a senior staff scientist who led the research in Kaspi’s team, was quoted by the institute as saying.

Galanti continued, “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, 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.”

Launched in 2011 and orbiting Jupiter since 2016, Juno has been sending back to NASA streams of raw data. When NASA extended the mission in 2021 so the spacecraft could keep studying Jupiter and its moons more closely, Juno’s new expanded path placed the spacecraft in an orbit that allowed it to pass behind Jupiter from Earth’s point of view, something its earlier orbit never did, according to the Weizmann Institute of Science.

“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,” said Juno’s Principal Investigator Scott J. Bolton of Southwest Research Institute in San Antonio, Texas.

The Weizmann scientists were assisted in their research by an international team from Italy, the U.S., France and Switzerland.

“Just by knowing the distance to Jupiter and watching how it rotates, it’s possible to figure out its size and shape,” Kaspi was quoted as saying. “But making really accurate measurements calls for more sophisticated methods.”

Kaspi said that earlier measurements did not account for Jupiter’s powerful winds. By including these extreme winds in their calculations, the Weizmann team cleared up long-standing discrepancies in earlier 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,” he said.

“This research helps us understand how planets form and evolve,” Kaspi continued. “Jupiter was likely the first planet to form in the solar system, and by studying what’s happening inside it, we get closer to understanding how the solar system, and planets like ours, came to be.”

Looking to the future, the techniques developed in these studies will serve the team during its analysis of data from the European Space Agency’s unmanned spacecraft JUICE, launched in 2023. The mission carries a Weizmann-designed instrument that will allow a deeper view into Jupiter’s atmosphere.

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