NASA’s latest mission redefines Jupiter as we’ve previously known it

When Juno passed behind Jupiter from Earth's perspective, its radio signal traveled through the planet's atmosphere before reaching Earth,

Jupiter, the colossal gas giant of our solar system, has been found to be smaller than scientists previously believed, according to new, highly precise data from NASA’s robotic Juno spacecraft.

While still undeniably the largest planet, these refined measurements offer a deeper understanding of its complex interior structure.

The latest observations from Juno reveal Jupiter’s equatorial diameter to be 88,841 miles (142,976 km), a reduction of approximately 5 miles (8 km) compared to earlier estimates. Similarly, its diameter from the north to south pole now stands at 83,067 miles (133,684 km), about 15 miles (24 km) less than previously thought.

Like Earth, Jupiter is not a perfect sphere but rather flattened at its poles. The new data indicates it is even more oblate than previously known, with its equator being about 7 per cent larger than its polar diameter.

For context, Earth’s equator is only 0.33 per cent larger than its diameter at the poles. Previous measurements of Jupiter relied on data collected by NASA’s Voyager and Pioneer spacecraft in the late 1970s. Juno, launched in 2011, has been orbiting Jupiter since 2016, continuously transmitting raw data back to Earth.

Illustration of the main asteroid belt, orbiting the Sun between Mars and Jupiter (NASA)

The mission was extended in 2021, enabling scientists to conduct the specific observations required to fine-tune these measurements, including tracking Juno as it passed behind Jupiter from Earth’s vantage point.

"When Juno passed behind Jupiter from Earth's perspective, its radio signal traveled through the planet's atmosphere before reaching Earth," explained planetary scientist Eli Galanti of the Weizmann Institute of Science in Israel, lead author of the study published this week in the journal Nature Astronomy.

"Measuring how the signal changed due to Jupiter's atmospheric composition, density and temperature allowed us to probe the atmosphere and determine the planet's size and shape with high precision. Interestingly, this geometrical configuration did not occur during Juno's prime mission, so these experiments were not originally planned."

Jupiter, the fifth planet from the sun, is so immense that it could contain all the other planets in the solar system, including more than 1,300 Earths. Composed primarily of hydrogen and helium, with traces of other gases, its vibrant, striped appearance is dominated by powerful winds and vast storms. Beyond its size and shape, Juno has been gathering crucial data on Jupiter's atmosphere, internal structure, magnetic field, and magnetosphere.

These precise new measurements are invaluable to scientists because Jupiter’s radius – half its diameter – serves as a fundamental reference point in models of the planet’s interior and atmospheric structure.

"Jupiter is the largest planet in the solar system and contains most of its planetary mass, so understanding its composition and internal structure is central to understanding how the solar system formed and evolved," Galanti stated. "Jupiter likely formed early, and strongly influenced the distribution of material, the growth of other planets and the delivery of volatiles to the inner solar system, including Earth."

Volatiles, such as water, carbon dioxide, and ammonia, evaporate easily. Their delivery to the inner solar system, home to the four rocky planets, was crucial, as Galanti noted, they "supplied Earth with water and key ingredients for its atmosphere and for life."