The gas giants

Beyond Mars and the asteroid belt the solar system changes its face. The four outer planets are enormous, consist mainly of gas and ice, and have no solid surface to land on. Anyone trying to fly into Jupiter would simply sink deeper and deeper while the pressure grows beyond measure. These four giants also carry almost all the moons of the solar system.

Jupiter, king of the planets

Jupiter is the largest planet: you could line up about 11 Earths across it, and it weighs more than all the other planets combined. Like the Sun it consists mainly of hydrogen and helium. Its most famous feature is the Great Red Spot, a storm wider than Earth that has been raging for centuries. Despite its mass Jupiter spins incredibly fast: a Jupiter day lasts only about 10 hours, and no other planet turns faster.

Saturn and its rings

Saturn is almost as large as Jupiter but has only just under a third of its mass: its average density is even lower than that of water. It is famous for its rings. They consist of countless chunks of water ice and rock, from dust grains to the size of a house, circling the planet like cars on a giant racetrack. From one edge to the other the ring system measures around 280,000 kilometres, yet it is mostly only a few dozen metres thick, wafer-thin by comparison. Jupiter, Uranus and Neptune have rings too, but only Saturn's are this magnificent.

The ice giants Uranus and Neptune

Uranus and Neptune are smaller than Jupiter and Saturn and contain, besides hydrogen and helium, a lot of frozen water, ammonia and methane. That is why they are called ice giants. The methane in their atmospheres swallows red light and makes both shine blue-green to deep blue. Uranus has a curious quirk: its rotation axis is tilted by about 98 degrees, so it effectively rolls around the Sun lying on its side. Neptune is the outermost planet, and of all places it has the fastest winds in the solar system, blowing at around 2,000 kilometres per hour.

Moons like worlds of their own

The most exciting places in the outer solar system may not be the planets at all, but their moons. Jupiter's moon Europa probably hides an ocean of liquid water under an ice crust kilometres thick, a possible place for life. Ganymede, also at Jupiter, is the largest moon in the solar system and has a larger diameter than the planet Mercury. Saturn's moon Titan is the only moon with a dense atmosphere, where methane rains from the sky and collects in lakes. Space probes are set to explore these worlds more closely in the coming decades.