Jupiter, the solar system’s largest planet, isn’t quite as large as previously thought. New measurements from NASA’s Juno mission reveal the gas giant is approximately 5 miles (8 kilometers) narrower at the equator and 15 miles (24 kilometers) flatter at the poles than earlier estimates suggested. The findings, published in , in Nature Astronomy, represent the most precise determination of Jupiter’s size and shape to date, refining data collected over half a century ago.
For decades, scientists relied on data from NASA’s Pioneer and Voyager missions – flybys conducted in the 1970s – to define Jupiter’s dimensions. These earlier measurements served as a crucial baseline for understanding not only our own solar system but also for modeling exoplanets – planets orbiting other stars. Accurately defining Jupiter’s size is critical because it acts as a calibration standard for interpreting data from planets observed passing in front of their host stars, a common method for characterizing distant worlds.
The new analysis leverages a technique called radio occultation, utilizing data gathered during 13 flybys of Jupiter by the Juno spacecraft. This method involves beaming radio signals from Juno back to NASA’s Deep Space Network on Earth. As these signals pass through Jupiter’s ionosphere – the charged upper layer of its atmosphere – the gases bend and delay them. By meticulously measuring these changes in frequency, scientists can calculate the temperature, pressure, and electron density at different depths within the atmosphere, effectively “seeing” through the dense cloud cover.
“With just knowing the distance to Jupiter and observing its rotation, it is possible to determine its size and shape,” explains Yohai Kaspi, a professor at the Weizmann Institute of Science in Israel and a co-author of the study. “But performing really precise measurements requires more sophisticated methods.”
The team, which included researchers from Italy, the United States, France, and Switzerland, benefited from Juno’s unique orbital path. After its primary mission concluded in , Juno’s trajectory was altered, allowing it to pass behind Jupiter from Earth’s perspective. This positioning enabled the radio occultation measurements, providing a fresh dataset for analysis.
The key to the improved accuracy lies in accounting for Jupiter’s powerful zonal winds. Previous models hadn’t fully incorporated the impact of these winds on the planet’s shape. “The shape of Jupiter, as understood until now, was derived from researchers based on only six measurements made almost five decades ago by NASA’s Voyager and Pioneer missions,” explains Dr. Eli Galanti, a senior scientist who led the research team at the Weizmann Institute. “These missions laid the foundations, but now we have the exceptional opportunity to lead the analysis of up to 26 new measurements made by NASA’s Juno spacecraft.”
The findings reveal that Jupiter’s equatorial radius is approximately 7% larger than its polar radius. For comparison, Earth’s equatorial radius is only 0.33% larger than its polar radius, making Jupiter significantly more oblate – flattened at the poles – due to its rapid rotation, complex internal structure, and atmospheric dynamics.
“Textbooks will need to be updated,” Kaspi stated. “The size of Jupiter hasn’t changed, of course, but the way we measure it has.”
This refined understanding of Jupiter’s shape isn’t merely an academic exercise. It has implications for modeling the planet’s interior and understanding the relationship between its atmosphere and deeper layers. The research also informs the development of more accurate models for other gas giants, both within our solar system and beyond.
The techniques developed during this study will also be valuable for analyzing data from the European Space Agency’s JUICE mission, launched in . JUICE carries an instrument designed by the Weizmann Institute that will provide further insights into Jupiter’s atmosphere.
The research team included researchers from the University of Bologna, Italy; the Jet Propulsion Laboratory at Caltech, USA; the University of Arizona, USA; the University of California, Berkeley, USA; the Côte d’Azur Observatory, France; the University of Zurich, Switzerland; the Georgia Institute of Technology, USA; and Boston University, USA.
