Jupiter

Jupiter

Jupiter's banded clouds and the Great Red Spot
Orbit
Distance from Sun	778 million km (5.2 AU)
Orbital period	11.86 years
Rotation period	9 h 56 min
Physical
Mean radius	69,911 km (11 Earths)
Mass	318 Earths
Composition	Mostly hydrogen and helium
Known moons	95 confirmed

Jupiter is the fifth planet from the Sun and by far the largest in the Solar System. Its mass is about 318 times Earth's and more than twice that of all the other planets combined. It is a gas giant, composed mainly of hydrogen and helium in roughly the proportions of the Sun, and has no solid surface: descending into it, the atmosphere simply grows denser until the distinction between gas and liquid stops meaning anything.

Jupiter has no crust. Below the visible cloud tops, pressure and temperature rise until hydrogen becomes a supercritical fluid, and deeper still, at around a million atmospheres, it is squeezed into metallic hydrogen, a state in which electrons are no longer bound to individual nuclei and the material conducts electricity like a metal. Currents in this layer, stirred by the planet's rapid rotation, generate a magnetic field about 20,000 times stronger than Earth's, with a magnetosphere that would appear larger than the full Moon from Earth if it could be seen.

Whether there is a rocky core is unsettled. Measurements by the Juno spacecraft suggest not a sharply defined core but a "fuzzy" one, tens of Earth masses of heavier elements gradually diluted into the hydrogen above, possibly the result of a giant impact early in the planet's history.

The familiar banding comes from alternating jet streams: light zones of rising gas and dark belts of sinking gas, driven by the planet's fast rotation. A day on Jupiter lasts under ten hours, and that speed both flattens the planet noticeably at the poles and stretches weather systems into bands running the full way around.

The Great Red Spot is an anticyclonic storm larger than the Earth, with winds of several hundred kilometres per hour. It has been observed continuously since 1830, and possibly since the 1660s. It is shrinking: nineteenth-century measurements put its width at about 40,000 km, against roughly 14,000 km today. Why it has persisted for centuries when terrestrial storms last days is not fully explained, though the absence of a surface to dissipate energy against is clearly part of it.

Jupiter has 95 confirmed moons. The four largest were seen by Galileo Galilei in January 1610 and are called the Galilean moons; that they visibly circled Jupiter rather than the Earth was among the first direct evidence against a geocentric cosmos.

Jupiter's gravity dominates the outer Solar System. It holds the asteroid belt in resonances that keep it from accreting into a planet, and it captures or deflects comets. The impact of Comet Shoemaker–Levy 9 in July 1994 was watched from Earth, leaving Earth-sized dark scars in the atmosphere for months. Whether this makes Jupiter a net shield for the inner planets or a net hazard is debated; it both removes long-period comets and perturbs objects onto Earth-crossing paths.

Pioneer 10 made the first flyby in 1973. Voyager 1 and 2 returned detailed images of the moons in 1979 and discovered Jupiter's faint ring system and Io's volcanoes. Galileo orbited from 1995 to 2003 and dropped a probe into the atmosphere. Juno, in a polar orbit since 2016, has mapped the gravitational and magnetic fields and shown that the belts and zones extend some 3,000 km deep.

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