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An artist\u2019s impression of the Juno spacecraft over Jupiter\u2019s South Pole<\/p>\n
NASA\/JPL-Caltech<\/p>\n<\/div>\n<\/figcaption><\/figure>\n<\/p>\n
Jupiter is not quite as large as astronomers thought, according to the first measurements of its radius taken in more than 40 years.<\/p>\n
Jupiter is a gas giant and doesn\u2019t 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.<\/p>\n
Our best measurements of this gas pressure level were taken by NASA\u2019s Voyager and Pioneer spacecraft more than 40 years ago. The probes sent radio waves through Jupiter\u2019s atmosphere towards Earth, where the data was then measured and the radio waves\u2019 properties used to calculate the gas\u2019 pressure at a given height, a technique called radio occultation.<\/p>\n
<\/p>\n Now, Eli Galanti at the Weizmann Institute of Science in Israel and his colleagues have measured Jupiter\u2019s radius using radio occultation measurements from NASA\u2019s 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.<\/p>\n \u201cBased 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,\u201d Galanti told the Europlanet Science Congress (EPSC) in Helsinki, Finland on 11 September.<\/p>\n 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\u2019s wind speeds to calculate its diameter. For gas at 1 bar, which is the pressure of Earth\u2019s atmosphere at sea level, the planet measured 142,976 kilometres across at its poles and 133,684 kilometres across at its equator.<\/p>\n \u201cIt\u2019s not about just knowing exactly where the radius is, but it\u2019s really about understanding its internal workings,\u201d says Oded Aharonson at the Weizmann Institute of Science, who wasn\u2019t involved in the analysis. \u201cThe planet\u2019s interior is mysterious and otherwise hard to probe, so having these new constraints could help us build more accurate interior models.\u201d<\/p>\n Topics:<\/p>\n<\/section><\/div>\n