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Jupiter is the fifth planet from the Sun, and the largest in the solar system, by far — more than twice as massive as the other planets combined.

Jupiter is the largest and oldest planet in our solar system.

If Jupiter was a hollow shell, 1,000 Earths could fit inside. But the "King of Planets" is no lumbering giant — Jupiter has the shortest day in the solar system, taking about 9.9 hours to spin around once on its axis.

The first detailed observations of Jupiter were made by Galileo Galilei in 1610 with a small, homemade telescope.

More recently, this planet has been studied by orbiters, probes, and spacecraft passing by on their way to other worlds. NASA’s Juno spacecraft currently is studying the giant planet from orbit. Europa Clipper launched on Oct. 14, 2024, to study Jupiter's icy moon, Europa.

Jupiter has 95 moons that have been officially recognized by the International Astronomical Union.

Jupiter shines bright in the night sky, so people have known about it since ancient times. The first detailed observations of this planet were made by Galileo Galilei in 1610 with a small, homemade telescope. More recently, this planet has been visited by orbiters, probes, and by spacecraft passing by on their way to other worlds. NASA’s Juno spacecraft has been studying Jupiter from orbit since July 2016. Europa Clipper launched Oct. 14, 2024, to study Jupiter's icy moon, Europa.

NASA's first spacecraft to visit the outer planets — Pioneer 10 — was launched in March 1972 on a 21-month mission to Jupiter. It made its closest approach to Jupiter on Dec. 4, 1973. After successfully completing its mission at Jupiter, the spacecraft was put on a trajectory that will eventually take it out of the solar system. Pioneer 10 sent its last signal to Earth in January 2003 from a distance of 7.6 billion miles.

The sibling spacecraft to Pioneer 10, Pioneer 11 flew even closer to Jupiter in 1974, passing by on its way to Saturn. Pioneer 11 also is on a path that will eventually take it out of the solar system. Like its sibling, it carries a plaque with a message for any intelligent beings that may encounter it.

As it flew by Jupiter in March 1979, Voyager 1 discovered a thin ring around the planet, two new moons, and active volcanoes on the volatile moon Io, before continuing on to Saturn and interstellar space.

Voyager 2 began transmitting images of Jupiter April 24, 1979, for time-lapse movies of atmospheric circulation. Unlike Voyager 1, Voyager 2 made close passes of the Jovian moons on its way into the system, with scientists especially interested in Europa and Io.

The Galileo mission consisted of two spacecraft: an orbiter and an atmospheric probe. Launched on Oct. 18, 1989, from the cargo bay of Space Shuttle Atlantis, the spacecraft orbited Jupiter for almost eight years, and made close passes by all the planet's major moons. Galileo even carried a small probe that it deployed and sent deep into the atmosphere of Jupiter. When the main spacecraft plunged into Jupiter's crushing atmosphere on Sept. 21, 2003, it was being deliberately destroyed to protect one of its key discoveries - a possible ocean beneath the icy crust of the moon Europa.

Jupiter is a world of extremes. It's the largest planet in our solar system – if it were a hollow shell, 1,000 Earths could fit inside. It's also the oldest planet, forming from the dust and gases left over from the Sun's formation 4.6 billion years ago. But it has the shortest day in the solar system, taking about 9.9 hours to spin around once on its axis.

Jupiter's signature stripes and swirls are actually cold, windy clouds of ammonia and water, floating in an atmosphere of hydrogen and helium. The dark orange stripes are called belts, while the lighter bands are called zones, and they flow east and west in opposite directions. Jupiter’s iconic Great Red Spot is a giant storm bigger than Earth that has raged for hundreds of years.

The king of planets was named for Jupiter, king of the gods in Roman mythology. Most of its moons are also named for mythological characters, figures associated with Jupiter or his Greek counterpart, Zeus.

Jupiter, being the biggest planet, gets its name from the king of the ancient Roman gods.

Jupiter’s environment is probably not conducive to life as we know it. The temperatures, pressures, and materials that characterize this planet are most likely too extreme and volatile for organisms to adapt to.

While planet Jupiter is an unlikely place for living things to take hold, the same is not true of some of its many moons. Europa is one of the likeliest places to find life elsewhere in our solar system. There is evidence of a vast ocean just beneath its icy crust, where life could possibly be supported.

Jupiter has 101 moons that have been officially recognized by the International Astronomical Union, as of March 2026. But the number doesn't capture the complexity of the Jovian system of moons, rings and asteroids. The giant planet has thousands of small objects in its orbit. Scientists are getting so good at spotting tiny moons orbiting distant, giant planets that the International Astronomical Union has decided the smallest will no longer be given mythological names unless they are of “significant” scientific interest.

Jupiter's four largest moons were the first moons discovered beyond Earth. They are called the Galilean satellites after Italian astronomer Galileo Galilei, who is credited with their discovery in 1610. German astronomer Simon Marius observed them around the same time, but is largely forgotten because he published his findings after Galileo. But the names Marius proposed for the moons in 1614 (mythological characters associated with Jupiter, suggested to him by fellow astronomer Johannes Kepler) are the ones we use today — Ganymede, Callisto, Io, and Europa. Here they are in order of size:

Ganymede is Jupiter's largest moon, and the largest moon in our solar system. It's even bigger than Mercury, and Pluto.

Callisto is Jupiter’s second largest moon and the third largest moon in our solar system.

Io is Jupiter's third largest moon, and the most volcanically active world in our solar system.

Europa is Jupiter's fourth largest moon. It's about 90% the size of Earth’s Moon.