Uranus and Neptune

Beyond Saturn the Sun shrinks to a brilliant star and the light grows feeble. Out here, twenty and thirty times farther from the Sun than Earth, orbit two planets that no one saw until the age of telescopes. They are not quite like the gas giants closer in. Uranus and Neptune are ice giants, and they may be the most common type of planet in the galaxy, yet we know less about them than about any other worlds in the solar system.

Different inside

Uranus and Neptune are each about four times Earth's diameter, a third the size of Jupiter. Their outer atmospheres are hydrogen and helium, like the gas giants, but the resemblance ends there. Beneath the clouds, most of their bulk is a hot, dense fluid of water, ammonia and methane. Astronomers call these compounds ices because they would freeze in the outer solar system, but inside the planets they are a searing slush under crushing pressure. Deeper still is likely a rocky core. Laboratory experiments suggest that the pressure may squeeze carbon from methane into diamonds that rain slowly down toward the centre.

Methane in the upper atmosphere absorbs red light, which is why both planets appear blue. Neptune is a deeper shade, for reasons that are still debated.

Uranus: the planet on its side

Uranus was discovered in 1781 by William Herschel, the first planet found with a telescope. Its oddest feature is its tilt. Every other planet spins more or less upright relative to its orbit, like a top; Uranus spins on its side, tipped by 98 degrees, as if something enormous knocked it over. Its moons and rings are tilted with it. The consequence is bizarre seasons: as Uranus makes its 84-year orbit, each pole spends 42 years in continuous sunlight and then 42 years in darkness.

When Voyager 2 flew past in January 1986, Uranus appeared as a nearly blank pale ball, the least eventful planet in the solar system. Later telescope images caught storms and bright clouds as its seasons changed, so the blandness was partly bad timing. It is the coldest planet, colder even than more distant Neptune, apparently because it leaks almost no heat from its interior. Uranus has 13 faint, dark rings and 28 known moons, named for characters from Shakespeare and Pope rather than mythology.

Neptune: found by arithmetic

After Uranus was discovered, astronomers noticed it was not quite following the orbit Newton's laws predicted. Something unseen was tugging it. In 1846 the French mathematician Urbain Le Verrier calculated where that something must be, and a German astronomer pointed his telescope at the spot and found Neptune the same night. It was the first planet discovered by prediction rather than by chance.

Neptune is a far livelier world than Uranus. Despite receiving a thousand times less sunlight than Earth, its winds are the fastest known, screaming around the planet at up to 2,100 kilometres per hour. Voyager 2 photographed a Great Dark Spot the size of Earth, which had disappeared when Hubble looked a few years later; new dark storms have appeared and faded since. Neptune radiates more than twice the energy it receives from the Sun, driven by heat left over from its formation, and that internal engine appears to power the weather.

Its largest moon, Triton, is as strange as its parent. It orbits backwards, opposite to Neptune's rotation, which means it did not form alongside the planet but was captured, probably from the belt of icy bodies beyond. It is one of the coldest places measured in the solar system at minus 235 degrees, yet it is geologically active, with geysers of nitrogen gas blasting dark material eight kilometres into its thin atmosphere. Triton is slowly spiralling inward and will one day be torn apart by Neptune's gravity, forming a ring.

The unexplored frontier

A single spacecraft, Voyager 2, has visited the ice giants, spending only hours in the vicinity of each on its way out of the solar system. Everything else we know comes from telescopes. No orbiter has ever studied them, no probe has entered their atmospheres. Planetary scientists have made a dedicated Uranus mission one of their top priorities for the 2030s; a journey of a decade or more would be needed just to arrive.

Why it matters

Among the thousands of planets discovered around other stars, worlds of Neptune's size are far more common than worlds of Jupiter's or Earth's. Uranus and Neptune are our only nearby examples of this most typical kind of planet, and we have barely glanced at them. Understanding how they formed, why one is tipped over and why both are so different from the gas giants, is a key to understanding planets everywhere.