NASA refines Jupiter’s true size

The dimensions of the giant planet Jupiter are not as large as previously believed, according to new measurements made by NASA’s Juno probe and published on February 2 in the journal Nature Astronomy, Space.com reported, CE Report quotes AGERPRES.

“Textbooks will have to be updated,” Yohai Kaspi of Israel’s Weizmann Institute said in a statement. “Jupiter’s size hasn’t changed, of course, but the way we measure it has.”

Jupiter is a massive gas giant and the largest planet in the Solar System. Current textbooks state that Jupiter has an equatorial diameter of 142,984 kilometers and a pole-to-pole diameter of 133,708 kilometers. In other words, Jupiter is flatter at the poles than at the equator — a result of its rapid rotation (just under 10 hours), which creates an equatorial bulge. However, the new measurements made by the Juno spacecraft, currently orbiting the gas giant, reduce the equatorial diameter by about 8 km and the pole-to-pole diameter by about 24 km.

Previous measurements came from NASA’s Pioneer 10 and 11 and Voyager 1 and 2 missions, totaling only six data points, while Juno has added 26 new measurements.

Juno has been orbiting Jupiter since 2016, but after its mission was extended in 2021, it switched to a different trajectory that allows it to closely fly past Jupiter’s Galilean moons and pass behind Jupiter as seen from Earth — something it had never done before.

To measure the size of a planet like Jupiter, the spacecraft must pass behind it from Earth’s perspective while transmitting radio signals back to Earth. Where the signals bend due to Jupiter’s upper atmosphere or are completely blocked by the planet’s body — only to reappear on the other side — reveals the planet’s dimensions.

“We tracked how radio signals bend as they pass through Jupiter’s atmosphere, allowing us to translate this information into detailed maps of Jupiter’s temperature and density, producing the clearest picture yet of the giant planet’s shape and size,” said Maria Smirnova of the Weizmann Institute, who developed the techniques needed to process Juno’s raw data.

Although a difference of just a few kilometers may not seem significant given Jupiter’s immense size, it actually highlights an important issue in accurately modeling the planet’s interior.

“These few kilometers matter,” said Eli Galanti of the Weizmann Institute, who led the research. “Shifting the radius even slightly allows our models of Jupiter’s interior to match both gravitational data and atmospheric measurements much better.”

Indeed, by adjusting Jupiter’s dimensions to those measured by Juno, Galanti’s team found that current state-of-the-art models describing Jupiter’s internal density structure performed even better. And since Jupiter is the gold standard for understanding gas giant planets, the better scientists understand Jupiter, the better they can understand other planets of this class.