Neptune is the furthest planet from the Sun, orbiting at about thirty AU (one AU is the distance between the Earth and the Sun). It was discovered by German astronomer Johann Galle in 1846, following calculations made by French astronomer Alexis Bouvard and French mathematician Urbain Le Verrier. Its largest moon, Triton, was discovered two and a half weeks later by British astronomer William Lassell.
Uranus and Neptune
The Star Garden
Uranus' five largest moons, images from Voyager 2 (to scale) Image credit: NASA/JPL
Unlike Mercury, Venus, Mars, Jupiter, and Saturn, Uranus was not named after a Roman God. It is instead named after the Ancient Greek god of the sky, the father of the Greek God Kronos, which corresponds to the Roman God Saturn, and the grandfather of Zeus, known to the Roman's as Jupiter.
Uranus has a ring system similar to Saturn's, except that it did not form when the planet did and orbits at an angle of nearly ninety degrees. It has almost 30 other moons, the five largest are Miranda, Ariel, Umbriel, Titania, and Oberon. The largest of these is Titania, which is about half the diameter of the Moon. Uranus' moons are mostly composed of frozen water, ammonia, and carbon dioxide, as well as silicate rock.
Neptune's atmosphere is similar to Uranus', composed mostly of hydrogen and helium, with trace amounts of hydrocarbons, and frozen ammonia and methane. Neptune's core is mostly composed of ice and rock.
Neptune, image taken by Voyager 2 Image credit: NASA/JPL
Triton has a thin nitrogen atmosphere. It is the coldest atmosphere in the Solar System, at over -200 degrees Celsius.
Triton is thought to have once been a dwarf planet like Pluto, but was captured by Neptune's gravitational pull. It is composed of a core of rock and metal with an icy mantle and a crust of active volcanoes and frozen nitrogen, up to 35% of its mass is composed of frozen water.
Uranus is the seventh closest planet to the Sun and, unlike the first six planets, it was not discovered until the invention of the telescope. Uranus was discovered by German-British astronomer William Herschel in 1781. It is the third largest planet in the Solar System, after Jupiter and Saturn. It is larger than Neptune but less massive. It takes over 30,000 days for Uranus to orbit the Sun, this is over 80 years. One day on Uranus is just over seventeen hours long.
Uranus is similar to Jupiter and Saturn, with an atmosphere primarily composed of hydrogen and helium, but it also contains trace amounts of hydrocarbons, and large amounts of frozen water, ammonia, and methane. Uranus has the coldest planetary atmosphere in the Solar System and can reach over -200 degrees Celsius. The core is composed of ice and rock. Uranus has a magnetosphere but the whole planet is tilted on its side so that its magnetic poles are at the equator.
NASA's Voyager 2 came within 80,000 km of Uranus in 1986, travelling past its five largest moons and discovering 10 new ones. The Cassini spacecraft may pass Uranus about 20 years after it leaves Saturn.
Neptune is the third most massive planet in the Solar System after Jupiter and Saturn. It is the fourth largest planet, having a slightly smaller diameter than Uranus. It takes over 60,000 days for Neptune to orbit the Sun, this is over 150 years. One day on Neptune is just over sixteen hours long. Like Mercury, Venus, Mars, Jupiter, and Saturn, but unlike Uranus, Neptune is named after a Roman God, the god of the sea.
Neptune has a faint ring system and thirteen known moons. The largest of these is Triton, which contains over 90% of the mass of all the Neptunian moons, and is the only one that is spherical. Triton is the only moon in the Solar System to have a retrograde orbit. This mean it orbits in the opposite direction to Neptune's rotation.
NASA's Voyager 2 probe came within 4400 km of Neptune in 1989, after passing Triton and discovering six new moons. Many new missions to Neptune have been proposed but none are currently funded.