Jupiter has long been described as a bloated, fast-spinning giant, but its exact dimensions are based on measurements over time. For decades, scientists relied on radio signals briefly collected by the Voyager and Pioneer spacecraft in the late 1970s. Those numbers have shaped textbooks, models, and even the way astronomers judge planets outside our solar system. New analyzes are now being conducted based on richer data. NASA’s Juno Mission offers a quiet but important fix. The researchers dramatically narrowed the uncertainty by using dozens of modern radio occultation measurements and taking Jupiter’s powerful winds into account. The planet is still huge and visibly flattened, but slightly smaller than previously thought. Although this change is modest in kilometers, it has implications for how Jupiter is understood and used as a reference world.

NASA‘s Juno mission will redefine Jupiter’s size and shape

Juno has been orbiting close to Jupiter for several years, sending radio signals back to Earth through Jupiter’s atmosphere. As these signals bend and slow, they reveal the shape of the planet at certain pressure levels. Juno provided more than 20 high-quality measurements compared to the six profiles available in previous missions. This dense coverage allows scientists to match observed radii to physical models with far fewer guesses. As a result, the uncertainty is reduced from approximately 4 kilometers to less than 0.5 kilometers.

Jupiter’s equatorial bulge confirmed, but slightly reduced

The new numbers confirm that Jupiter’s equator is bulging out due to its rapid rotation of just under 10 hours. At a pressure level of 1 bar near the visible cloud tops, the equatorial radius lies at approximately 71,488 kilometers. The polar radius is approximately 66,842 kilometers. Both values ​​are smaller than what has been accepted for many years: 4 kilometers at the equator and 12 kilometers at the poles. The average radius shifts downward as well, although the difference is small on a planetary scale.

Atmospheric winds shape the planet more than previously thought

Early studies treated Jupiter primarily as a smoothly rotating body. The new work takes these strong east-west winds into consideration. These winds add extra centrifugal force, changing the planet’s profile by several kilometers, especially near low latitudes. Juneau data suggest that winds above the cloud deck change little up to an altitude of about 100 millibars. This nearly barotropic behavior simplifies how the atmosphere and interior couple together and explains why the wind-corrected shape fits the data so closely.

The modified shape provides a harmonious interior model.

Slightly smaller Jupiter creates a ripple effect below the clouds. The internal model, which had struggled to match temperature measurements from the Galileo spacecraft, found more room for a cooler, more metal-rich outer layer. The revised shape alleviates long-standing tensions between different datasets, rather than creating new ones. It will also improve how gravity and pressure measurements are mapped to the actual depth inside the planet.

Jupiter remains an important benchmark for giant planets

Jupiter is often used as a yardstick when studying gas giant planets orbiting other stars as well as our solar system. A more accurate radius will be reflected directly in their comparison. As Juno continues and future missions like Juice expand in scope, the picture could become even clearer. For now, adjustments are small, measured, and based on better data. Jupiter hasn’t changed, but its outline has settled into a more defined shape.