Uranus and Neptune are known as the ice giants, huge blue planets located in the outer Solar System. They are similar to Jupiter and Saturn in that they are predominantly composed of gases and ice but differ in many key areas. While still dwarfing the inner planets the ice giants are smaller than the other gas giants. Due to their distance from Earth they were the last planets to be discovered, Uranus in 1781 and Neptune in 1846.



Uranus is the seventh planet from the Sun and orbits between 3,004,419,704 km and 2,700,938,461 km from the star. It has 27 known moons as well as a faint ring system. Uranus takes 17 hours to complete a rotation and 84 Earth years to complete one orbit of the Sun. The planet is named after the Greek God of the sky, the father of Saturn and grandfather of Jupiter.

What makes Uranus different from any other planet in the Solar System is that the axis is tilted onto its side. The tilt of Uranus is 97.7 degrees, meaning that the pole is facing the Sun, making its day/night cycles and seasons entirely different to any of the other planets. Each pole of Uranus gets 42 years of light followed by 42 years of darkness. What caused this extreme tilt is not known for sure but is thought to be due to a large planetary body striking the ice giant.

Uranus is composed of three layers, a rocky core, an icy mantle and an outer layer of gaseous hydrogen and helium. Uranus is the coldest planet in the Solar System, despite being closer to the Sun than Neptune. The lowest temperature ever recorded on Uranus was -224 degrees C, just 49 Kelvin. Why Uranus doesn't emit as much of its own internal heat as other planets is not known for sure, but it has been speculated that the internal structure of the planet blocks much of the heat from the core from reaching the surface.

Neptune is the second ice giant, the eighth planet from the Sun orbiting at a distance of around four billion km. Neptune takes 164 years to orbit the Sun and takes 16 hours to complete one rotation, giving the planet short days and extremely long years when compared to Earth. After Pluto's demotion from planet status Neptune has become the furthest planet from the Sun.

The structure of Neptune is similar to Uranus with its atmosphere making up around 10 percent of its mass, slowly becoming more solid as you reach the core where temperatures sit at around 5000 Kelvin. The atmosphere is scattered with clouds that form in the upper layers, believed to made of ammonia, ammonium sulphide, hydrogen sulfide and water. Neptune has a ring system, but it is much less prominent than Saturn's.

Neptune has a much more active weather system than Uranus with many clouds and storms. The planet exhibits many similar features to those found on Jupiter, including its own great dark spot. The winds are thought to even reach speeds of up to 600 m/s, near supersonic speeds. The great dark spot was discovered in 1989 by the Voyager 2 probe. There are many other similar but smaller storms present on Neptune.

The ice giant has many moons but only one is large enough to be spherical, Triton, which orbits in the opposite direction to Neptune's orbit, a unique property among the moons of the Solar System. Because of this it is believed that Triton is a captured body from the Kupier belt, a huge asteroid belt surrounding the Solar System. Pluto is one of the largest objects in the belt, bodies present in the Kuiper belt are known as Plutinos.

The ice giants are the least explored planets in the Solar System, Voyager 2 visited in the late 80s gathering images and scientific data. Neither planet is considered a high priority for human visitation or colonisation due to their immense distances from Earth. Their moons do not lend themselves well to settlements and the Saturnian and Jovian moons have much more to offer.