Jupiter is smaller and more squashed than we thought

The gas giant has been measured for the first time in decades, trimming 8 kilometres from its diameter

By Alex Wilkins

12 September 2025

An artist’s impression of the Juno spacecraft over Jupiter’s South Pole

NASA/JPL-Caltech

Jupiter is not quite as large as astronomers thought, according to the first measurements of its radius taken in more than 40 years.

Jupiter is a gas giant and doesn’t have a solid outer surface like Earth. But astronomers can still assess its shape by measuring how the height of its gas, for a certain pressure level, fluctuates around the planet, similar to measuring where sea level lies on Earth.

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Our best measurements of this gas pressure level were taken by NASA’s Voyager and Pioneer spacecraft more than 40 years ago. The probes sent radio waves through Jupiter’s atmosphere towards Earth, where the data was then measured and the radio waves’ properties used to calculate the gas’ pressure at a given height, a technique called radio occultation.

Now, Eli Galanti at the Weizmann Institute of Science in Israel and his colleagues have measured Jupiter’s radius using radio occultation measurements from NASA’s Juno spacecraft, which has been in orbit around the gas giant since 2016. They found Jupiter is around 8 kilometres smaller on average than we thought, with the reduction more pronounced around its poles, which also makes the planet more squashed, or oblate.

“Based on the Juno radio occultations, we find that the size of Jupiter is smaller, more oblate, because at the equator, the change is about 4 kilometres smaller,” Galanti told the Europlanet Science Congress (EPSC) in Helsinki, Finland on 11 September.