New measurements from NASA’s Juno spacecraft reveal that Jupiter is slightly smaller and more flattened than scientists have believed for decades, prompting researchers to say that even astronomy textbooks may need updating.
For nearly 50 years, the accepted figures for Jupiter’s size came from flybys by Voyager 1, Voyager 2, Pioneer 10, and Pioneer 11. Those missions used radio beams in the 1970s to estimate the planet’s shape, and their measurements became the standard reference for generations of researchers.
But Juno, which has been orbiting Jupiter since 2016, has gathered far more detailed radio data. Scientists tracked how signals sent between the spacecraft and Earth bent as they passed through the planet’s thick atmosphere and disappeared when Jupiter blocked them. Those tiny changes allowed the team to refine the planet’s dimensions with much higher precision.
The differences are small but significant. Researchers calculated that Jupiter’s polar radius is about 12 kilometers smaller than previously thought, while its equatorial radius is about 4 kilometers smaller. That makes the gas giant slightly more compact and a bit flatter than older models suggested, the team reported Feb. 2 in the journal Nature Astronomy.
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)
While a few kilometers might seem trivial for a planet tens of thousands of miles wide, scientists say these adjustments matter. Jupiter spins very quickly, which causes it to bulge at the equator and flatten at the poles. Even minor corrections help researchers better understand how mass is distributed inside the planet and how its powerful winds affect its shape.
The improved measurements also sharpen models of Jupiter’s interior, including its core structure and gravity field. Because Jupiter likely formed early in the solar system’s history and influenced the formation of other planets, understanding its makeup provides clues about how planetary systems develop more broadly, according to Live Science.
Researchers from the Weizmann Institute of Science say the updated figures allow their simulations to match both gravity and atmospheric data more accurately than before. That could also help scientists interpret observations of gas giants orbiting distant stars.
In short, Jupiter itself has not changed. Our tools have simply gotten better. But that improvement is enough to revise some of the most basic numbers astronomers have relied on for half a century.