Jupiter

Jupiter is the fifth planet outward from the Sun and the most massive planet in the Solar System. It formed early in the Solar System’s history from material around the young Sun. Its rapid rotation gives it a day of roughly ten hours, while a complete orbit takes nearly 12 Earth years.

Jupiter is composed primarily of hydrogen and helium. Unlike Earth, it has no solid surface on which a spacecraft could land: the outer atmosphere gradually thickens into fluid at great depth. The visible cloud tops form alternating light zones and darker belts, shaped by strong east–west atmospheric currents. The Great Red Spot is a vast, persistent storm system, large enough to exceed Earth in size. Jupiter also has a powerful magnetic field that creates a broad magnetosphere and traps energetic charged particles.

Jupiter has a diverse family of natural satellites and a faint ring system. Its four largest moons—Io, Europa, Ganymede and Callisto—were observed by Galileo Galilei in 1610 and are known as the Galilean moons. They are not interchangeable: Io is intensely volcanic, while Europa, Ganymede and Callisto are icy worlds that spacecraft missions investigate for evidence of subsurface oceans and other clues to their histories. Jupiter’s rings are much fainter than Saturn’s; observations indicate that fine dust, including material knocked from small inner moons by impacts, contributes to them.

Telescopic observations established Jupiter as a changing world, but spacecraft greatly expanded knowledge of its atmosphere, interior and moons. NASA’s Galileo became the first spacecraft to orbit Jupiter and released an atmospheric probe, collecting measurements unavailable from distant observation. NASA’s Juno mission, which entered orbit in 2016, used measurements of gravity and magnetic fields, together with close imaging, to study the planet’s interior, atmosphere and polar storms. The European Space Agency’s Juice mission is designed to examine Jupiter and, in particular, its icy moons Ganymede, Europa and Callisto.

Jupiter’s composition and early formation make it an important clue to how the Solar System developed. Its moons provide a contrasting set of environments: volcanic activity, ancient cratered terrain, ice and possible hidden oceans all occur within one planetary system. Research on Jupiter therefore connects planetary formation with atmospheric science, magnetism and the search for potentially habitable environments beyond Earth. The evidence for subsurface oceans is a reason for investigation, not evidence that life has been found there.