'Textbooks will need to be updated': Jupiter is smaller and flatter than we thought, Juno spacecraft reveals

View of one-half of Jupiter. Swirling rings of gold, brown, green, and blue dust and gas clouds wrap vertically around the sphere of the planet. A large dark spot is seen toward the bottom left and smaller spots of white dot the surface, breaking up the rings of swirling clouds and dust.

Jupiter imaged by the Juno spacecraft, with the shadow of the massive moon Ganymede to the left. Data from Juno suggests that Jupiter is flatter than previously thought, according to a new study. (Image credit: Image data: NASA/JPL-Caltech/SwRI/MSSS Image processing by Thomas Thomopoulos © CC BY)

Jupiter is slightly smaller and flatter than scientists thought for decades, a new study finds.

Researchers used radio data from the Juno spacecraft to refine measurements of the solar system's largest planet. Although the differences between the current and previous measurements are small, they are improving models of Jupiter's interior and of other gas giants like it outside the solar system, the team reported Feb. 2 in the journal Nature Astronomy.

Until now, scientists' understanding of Jupiter's size and shape have been based on six measurements performed by the Voyager 1 and 2 and Pioneer 10 and 11 missions. Those measurements, which have since been adopted as standard, were performed around 50 years ago using radio beams, according to the statement.

But the Juno mission, which has been gathering data on Jupiter and its moons since it arrived at the gas giant in 2016, has collected much more of this radio data in the past two years. With that additional data, researchers have now refined measurements of Jupiter's size down to about 1,300 feet (400 meters) in each direction.

"Just by knowing the distance to Jupiter and watching how it rotates, it's possible to figure out its size and shape," Kaspi said. "But making really accurate measurements calls for more sophisticated methods."

Graphic showing Jupiter as a circle, with a line indicating the previous and current estimates of Jupiter's size. — An illustration of Jupiter showing the discrepancy between Juno’s observations and those of the Voyager and Pioneer probes. (Image credit: Weizmann Institute of Science)

Bending light

In the new study, the scientists tracked how the radio signals from Juno back to Earth bent as they passed through Jupiter's atmosphere, before cutting out when the planet blocked the signal entirely. Those measurements allowed the team to account for Jupiter's winds, which slightly alter the shape of the gaseous planet. Then, they used that information to make precise calculations of the planet's shape and size.

With the new data, the team calculated that the planet's radius from its pole to its center is 41,534 miles (66,842 km) — 7.5 miles (12 km) smaller than previous measurements. The newly calculated radius at the equator is 44,421 miles (71,488 km) — 2.5 miles (4 km) smaller than previously thought.

"These few kilometers matter," study co-author Eli Galanti, an expert on gas giants at the Weizmann Institute of Science, said in the statement. "Shifting the radius by just a little lets our models of Jupiter's interior fit both the gravity data and atmospheric measurements much better."