Jupiter

Orbit and rotation

Jupiter orbits the Sun at an average distance of about 778 million km (5.20 AU), completing one sidereal orbit every 11.86 years — about 4,332.59 days, or roughly 12 Earth years. Despite its enormous size, it has the shortest day of any planet: it rotates once every 9 hours, 55 minutes, and 30 seconds, about 9.9 hours. This rapid spin is what flattens the planet into an oblate shape, with the equatorial radius about 7% larger than the polar radius.

The Great Red Spot

Jupiter's most famous feature is the Great Red Spot, a long-lived anticyclonic storm and the largest in the Solar System, centered near 22°S in the southern hemisphere. Its peripheral wind speeds reach about 400–500 km/h (250–310 mph), and Juno gravity and microwave data indicate the storm extends 200–500 km below the visible cloud tops — far deeper than terrestrial hurricanes.

The spot has shrunk dramatically over the past century. In the late 19th century it spanned roughly 40,000–48,000 km east–west, more than three times Earth's diameter. By the 1979 Voyager flybys it had shrunk to about 23,000 km, and by April 2017 its width was about 16,350 km. Hubble observations from late 2023 to early 2024 measured it at about 16,500 km (roughly 10,250 miles) across at its widest point — only somewhat larger than Earth. Since about 2012 the spot has become more circular, with its contraction accelerating to roughly 900 km per year in length.

Around 2019, observers saw "flaking" events in which fragments of cloud peeled off the edges and dissipated. High-resolution Hubble observations from December 2023 to March 2024 showed the spot wobbling, squeezing in and out, and fluctuating in size, with winds speeding up and slowing down over weeks to months. Some researchers suggest the visible spot could dissipate within one or two decades, while others argue the observed area mainly reflects cloud coverage and that the underlying vortex may persist for many decades or centuries. The consensus is that the storm is evolving but still strong, with an uncertain ultimate lifetime.

Moons

Jupiter has 101 confirmed moons recognized by the International Astronomical Union as of March 2026. The four largest are the Galilean moons — Io, Europa, Ganymede, and Callisto — first observed in 1610 by Galileo Galilei, making them the first moons ever discovered around another planet.

Ring system and magnetosphere

Jupiter possesses the largest magnetosphere in the Solar System, driven by an unusually strong magnetic field. The equatorial surface field is approximately 4.2–4.3 gauss, while the polar surface field reaches about 11 gauss at the south pole and 14 gauss at the north pole. Because Jupiter's field is not a perfect dipole, exact values vary depending on the measurement location and reference frame used.

Jupiter's faint dust ring system sits deep within this magnetic environment. The rings are shaped in part by electromagnetic forces from the magnetosphere — especially the halo ring — while charged particles from the Io torus reflect the coupling between the rings, plasma, and magnetic field.