Jupiter

Jupiter is the fifth planet from the Sun and the largest planet in the Solar System, classified as a gas giant with a mass roughly 318 times that of Earth. Its immense gravitational influence has shaped the architecture of the Solar System, shepherding asteroids and affecting the orbits of other planets. Named after the king of the gods in Roman mythology, Jupiter has been observed with the naked eye since prehistoric times and studied telescopically since the early seventeenth century.

Physical characteristics

Jupiter has an equatorial diameter of approximately 142,984 kilometres, making it more than eleven times wider than Earth. Despite its size, it rotates faster than any other planet in the Solar System, completing one rotation in roughly ten hours, which causes a noticeable equatorial bulge.

Composition and structure

Jupiter is composed predominantly of hydrogen and helium, mirroring the composition of the Sun. Current models suggest the planet has a dense core of rock and possibly metallic material, surrounded by a layer of metallic hydrogen — a state in which hydrogen conducts electricity like a metal — produced by the extreme pressures in Jupiter's interior. Above that lies a thick layer of liquid hydrogen, transitioning gradually into the gaseous atmosphere with no clearly defined solid surface.

Atmosphere and weather

Jupiter's atmosphere is organised into alternating light zones and dark belts driven by powerful jet streams. The most iconic feature is the Great Red Spot, a persistent anticyclonic storm larger than Earth that has been observed continuously since at least the late seventeenth century, though it has been shrinking in size in recent decades. Lightning, ammonia ice clouds, and complex organic chemistry have all been detected in the Jovian atmosphere.

Magnetic field

Jupiter possesses the strongest planetary magnetic field of any Solar System planet, generated by convecting metallic hydrogen in its interior. The magnetosphere extends millions of kilometres into space and traps energetic particles in intense radiation belts.

Moons and ring system

As of the mid-2020s, Jupiter has the largest confirmed moon count of any planet in the Solar System, with well over ninety known moons (the precise number continues to rise as surveys improve).

Galilean moons

The four largest moons — Io, Europa, Ganymede, and Callisto — were discovered by Galileo Galilei in January 1610 and are collectively called the Galilean moons. Ganymede is the largest moon in the Solar System, exceeding the planet Mercury in diameter. Europa is of particular scientific interest because it harbours a subsurface liquid water ocean beneath its icy crust, making it a prime candidate in the search for extraterrestrial life.

Ring system

Jupiter has a faint ring system composed mainly of dust particles ejected from its inner moons by meteorite impacts. The rings were first detected by the Voyager 1 spacecraft in 1979 and confirmed by subsequent missions.

Exploration

Jupiter has been visited by multiple robotic spacecraft.

Flyby missions

Pioneer 10 (1973) and Pioneer 11 (1974) made the first close flybys of Jupiter, followed by Voyager 1 and Voyager 2 in 1979, which returned detailed images of the atmosphere, rings, and Galilean moons.

Orbital missions

The Galileo spacecraft entered orbit around Jupiter in December 1995 and operated until 2003, releasing an atmospheric probe and conducting extensive studies of the moons. The Juno spacecraft entered a polar orbit in July 2016 and, as of the mid-2020s, continues to study Jupiter's interior structure, gravity field, and aurora.

Future missions

The European Space Agency's JUICE (Jupiter Icy Moons Explorer) launched in April 2023 and is planned to arrive at the Jovian system in the early 2030s, with a focus on Ganymede, Callisto, and Europa. NASA's Europa Clipper launched in October 2024 and is targeted at detailed study of Europa's habitability.

Observation from Earth

Jupiter is typically the third or fourth brightest object in the night sky, after the Moon, Venus, and sometimes Mars. It is visible to the naked eye and through small telescopes, which can reveal its cloud bands and the four Galilean moons. Opposition, when Jupiter is closest to Earth and fully illuminated by the Sun, occurs roughly every thirteen months.

Role in the Solar System

Jupiter's gravity acts as a partial shield for the inner Solar System, deflecting or capturing some comets and asteroids that might otherwise strike the terrestrial planets, though the extent of this protective effect is debated among planetary scientists. The Shoemaker–Levy 9 comet famously broke apart and impacted Jupiter in July 1994, providing direct evidence of such interactions.

Frequently asked questions

Is Jupiter a failed star?

Jupiter is sometimes informally called a failed star because it shares a hydrogen-and-helium composition with stars, but it would need to be roughly 80 times more massive to sustain hydrogen fusion in its core. It is more accurately classified as a gas giant planet.

Is Jupiter solid or does it have a surface?

Jupiter has no solid surface in the conventional sense; its gaseous atmosphere transitions gradually into denser fluid layers at increasing depth. The planet likely has a solid or semi-solid core at its centre, but it is inaccessible to direct observation.

Is Europa's ocean on Jupiter's moon real?

Scientific consensus, supported by data from the Galileo mission and Hubble Space Telescope observations, strongly indicates that Europa harbours a global liquid water ocean beneath a shell of ice, kept liquid by tidal heating caused by Jupiter's gravity.

Is Jupiter visible tonight without a telescope?

Jupiter is visible to the naked eye for much of the year when it is above the horizon, appearing as a bright, steady white point of light. Its visibility depends on the current date and observer's location; astronomy apps and ephemeris tools provide precise visibility windows.

Is the Great Red Spot disappearing?

Measurements have shown that the Great Red Spot has been shrinking in size over the past century or more. Whether it will eventually dissipate entirely or stabilise at a smaller size remains an open question among atmospheric scientists.