Jupiter is smaller and more squashed than we thought

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.

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, especially around its poles, which also makes the planet more squashed.

“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 four kilometres smaller,” Galanti told the Europlanet Science Congress (EPSC) in Helsinki, Finland on 11 September.

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Galanti and his team took 13 different radio occultations with Juno, compared to the six for Voyager and Pioneer combined, and then used known measurements of Jupiter’s wind speeds to calculate its diameter. For gas at 1 bar, which is the pressure of Earth’s atmosphere at sea level, the planet measured 142,976 kilometres across at its poles and 133,684 kilometres across at its equator.

“It’s not about just knowing exactly where the radius is, but it’s really about understanding its internal workings,” says Oded Aharonson at the Weizmann Institute of Science, who wasn’t involved in the analysis. “The planet’s interior is mysterious and otherwise hard to probe, so having these new constraints could help us build more accurate interior models.”