Weizmann-led team remeasures Jupiter and finds it slightly smaller and flatter

An international team led by the Weizmann Institute of Science in Rehovot, Israel, has produced the most precise measurement yet of Jupiter's size and shape. The study, published in Nature Astronomy on 2 February 2026, found that the planet is about 8 kilometres narrower at the equator and about 24 kilometres flatter at the poles than the values used in textbooks for nearly fifty years. NASA highlighted the result on its science news pages two days later.

The old figures came from just six radio measurements made by the Pioneer and Voyager probes in the 1970s. The new work, led by Prof. Yohai Kaspi with the analysis directed by Dr. Eli Galanti, drew on a much larger set of radio occultations from NASA's Juno spacecraft, which has orbited Jupiter since 2016. As Juno passes behind the planet as seen from Earth, its radio signal travels through Jupiter's atmosphere on the way to NASA's Deep Space Network antennas. The way the signal bends and fades reveals the temperature and density of the atmosphere at different heights, which in turn pins down where the planet's edge lies.

A key step was accounting for Jupiter's powerful east-west winds, which slightly distort its shape and had caused disagreements between earlier data sets. With those winds included, the measurements fit together cleanly, with an uncertainty of well under a kilometre. Weizmann doctoral students Maria Smirnova and Maayan Ziv worked on the data processing and interior modelling, and the collaboration included researchers from NASA's Jet Propulsion Laboratory and universities in Italy, Switzerland, France and the United States.

The researchers stress that Jupiter has not changed; only the precision of the measurement has. But because Jupiter's radius is used as a reference unit for describing giant planets around other stars, a more accurate value feeds directly into the study of exoplanets. The refined shape also helps scientists build interior models that agree with both Juno's gravity data and its atmospheric readings. The team plans to apply the same techniques to data from the European Space Agency's JUICE mission, which carries an instrument designed at Weizmann.