Jupiter

If the planets were gathered in one place, Jupiter would be impossible to miss. It outweighs all the others combined two and a half times over. Eleven Earths would fit across its face and more than 1,300 inside it. Ancient skywatchers who named it king of the gods had no idea how right they were, yet Jupiter is still only a thousandth the mass of the Sun. It is the biggest planet, not a failed star.

A planet without a surface

Jupiter is a gas giant. There is no ground to stand on. Its outer layer is an atmosphere of hydrogen and helium, the same ingredients as the Sun, with traces of methane, ammonia and water that colour the clouds. Descend and the gas grows denser and hotter until, thousands of kilometres down, it becomes a liquid. Deeper still the pressure is so extreme that hydrogen is squeezed into a metal, an ocean of liquid metallic hydrogen tens of thousands of kilometres deep. At the centre there is probably a core of rock and ice several times Earth's mass, though NASA's Juno spacecraft has found it may be oddly diffuse, as if a giant collision long ago stirred it into the surrounding layers.

The fastest spinner

Jupiter rotates in less than ten hours, faster than any other planet, and that speed shapes its weather. The spin stretches the clouds into the stripes you can see through any small telescope: pale zones where gas is rising, and darker belts where it sinks. Winds at the boundaries blow at up to 600 kilometres per hour, in opposite directions on either side.

Between the belts churn storms of staggering size. The Great Red Spot is an anticyclone about 16,000 kilometres across, wider than Earth, that has been observed continuously since 1831 and probably for far longer. It is slowly shrinking, and nobody knows whether it will eventually vanish. Juno has photographed clusters of cyclones at both poles, arranged in neat geometric patterns, each as large as a continent.

The magnetic monster

Jupiter's spinning metallic hydrogen generates the strongest magnetic field of any planet, twenty thousand times Earth's. The region it dominates, the magnetosphere, is the largest structure in the solar system apart from the Sun's own; if it glowed, it would appear several times bigger than the full Moon in our sky. It traps charged particles into radiation belts fierce enough to destroy unprotected electronics in hours, which is why spacecraft visiting Jupiter carry thick shielding and keep their visits brief.

Worlds in orbit

Jupiter has 95 known moons, but four dominate: Io, Europa, Ganymede and Callisto, discovered by Galileo in 1610 and so large that they would count as planets if they orbited the Sun. Ganymede is bigger than Mercury and the only moon with its own magnetic field.

Io is the most volcanic body in the solar system. Jupiter's gravity, tugged by the other moons, flexes it like a squeezed rubber ball, heating its interior until it erupts through hundreds of volcanoes, some throwing plumes 400 kilometres high.

Europa is the opposite: a smooth shell of ice criss-crossed with cracks. The same tidal flexing warms its interior, and beneath about 20 kilometres of ice lies a global ocean of salty liquid water, perhaps 100 kilometres deep, holding more water than all of Earth's oceans. Where water meets a rocky floor, chemistry can happen, and Europa is considered one of the most promising places to look for life beyond Earth. NASA's Europa Clipper, launched in 2024, arrives in 2030.

Visitors

Pioneer and Voyager flew past in the 1970s. Galileo orbited from 1995 to 2003 and dropped a probe into the atmosphere that survived for 58 minutes. Juno has circled since 2016, skimming the cloud tops on each orbit to map the interior. Europe's JUICE mission is on its way to study the icy moons.

Why it matters

Jupiter's gravity has shaped everything else. During the solar system's formation it stirred up the asteroids and comets, flinging many into the inner solar system and many more out of it entirely. It still deflects or captures objects that might otherwise strike Earth, though it also nudges some our way. In 1994 astronomers watched a comet, torn into fragments by Jupiter's gravity, slam into the planet piece by piece, leaving scars larger than Earth. Our neighbourhood would be a very different, and probably more dangerous, place without it.