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For over 50 years, we thought we knew the size and shape of Jupiter, the solar system鈥檚 largest planet. Now, Weizmann Institute of Science researchers have revised that knowledge using new data and technology.  

In a new study in Nature Astronomy, Weizmann scientists, who led an international team from Italy, the US, France and Switzerland, provide the most precise determination yet of Jupiter鈥檚 size and shape.

鈥淛ust by knowing the distance to Jupiter and watching how it rotates, it鈥檚 possible to figure out its size and shape,鈥� says Prof. Yohai Kaspi of Weizmann鈥檚 Earth and Planetary Sciences Department. 鈥淏ut making really accurate measurements calls for more sophisticated methods.鈥�

鈥淛upiter鈥檚 shape, as understood until now, was derived by researchers from just six measurements made almost five decades ago by NASA鈥檚 Voyager and Pioneer missions, which sent radio beams from the spacecraft to Earth,鈥� explains Dr. Eli Galanti, a senior staff scientist who led the research in Kaspi鈥檚 team. 鈥淭hose 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鈥檚 Juno spacecraft.鈥�

“”Textbooks will need to be updated,” Kaspi says. “The size of Jupiter hasn鈥檛 changed, of course, but the way we measure it has”

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鈥檚 new expanded path placed the spacecraft in an orbit that allowed it to pass behind Jupiter from Earth鈥檚 point of view, something its earlier orbit never did. 鈥淛uno鈥檚 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鈥檚 atmosphere. This enables an accurate measurement of Jupiter鈥檚 size,鈥� says Juno鈥檚 Principal Investigator Dr. Scott J. Bolton of Southwest Research Institute in San Antonio, Texas.

The Juno team at Weizmann seized this new opportunity. 鈥淲e tracked how the radio signals bend as they pass through Jupiter鈥檚 atmosphere, which allowed us to translate this information into detailed maps of Jupiter鈥檚 temperature and density, producing the clearest picture yet of the giant planet鈥檚 size and shape鈥�, says Maria Smirnova, a PhD student in Kaspi鈥檚 group, who developed a special technique to process Juno鈥檚 new data.

The new findings show that Jupiter is slightly smaller than previously estimated 鈥� it鈥檚 about 8 km less wide at the equator and 24 km flatter at the poles. In other words, it鈥檚 more flattened compared to previous assessments. 鈥淭extbooks will need to be updated,鈥� Kaspi says. 鈥淭he size of Jupiter hasn鈥檛 changed, of course, but the way we measure it has.鈥�

鈥淭hese few kilometers matter,鈥� Galanti explains. 鈥淪hifting the radius by just a little lets our models of Jupiter鈥檚 interior fit both the gravity data and atmospheric measurements much better.鈥� This implication was tested by another PhD student in Kapsi鈥檚 group, Maayan Ziv. 鈥淲e 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,鈥� Ziv says. This study also has broader implications for understanding the structure of gas planets in general, since Jupiter serves as a standard reference for the study of gas giants within the solar system and beyond.

Kaspi notes also that earlier measurements didn鈥檛 account for Jupiter鈥檚 powerful winds. By including these extreme winds in their calculations, the Weizmann team cleared up long-standing discrepancies in earlier measurements. 鈥淚t鈥檚 difficult to see what鈥檚 happening beneath the clouds of Jupiter, but the radio data give us a window into the depth of Jupiter鈥檚 zonal winds and powerful hurricanes,鈥� Kaspi explains.

The work on the winds ties into , a graduate of Kaspi鈥檚 group, on Jupiter鈥檚 vast polar cyclones. That study, published in PNAS, used Juno measurements of these cyclones鈥� motion to predict how deep into the interior they extend. Overall, an improved understanding of Jupiter鈥檚 winds enables scientists to elucidate the relation between the planet鈥檚 atmosphere and its deep interior.  Their prediction was recently confirmed by microwave measurements made by the Juno spacecraft.

鈥淭his research helps us understand how planets form and evolve,鈥� Kaspi says. 鈥淛upiter was likely the first planet to form in the solar system, and by studying what鈥檚 happening inside it, we get closer to understanding how the solar system, and planets like ours, came to be.鈥�

Looking into the future, the techniques developed in these studies will serve the team during their analysis of data from the European Space Agency鈥檚 unmanned spacecraft JUICE, launched in 2023. The mission carries that will allow a deeper view into the planet鈥檚 atmosphere.