A second symbol, an L-V monogram ⯉ for ‘Le Verrier’, analogous to the H monogram ♅ for Uranus, was never much used outside of France and is now archaic. It is named after the Roman god of the sea and has the astronomical symbol ♆, representing Neptune’s trident. The planet orbits the Sun once every 164.8 years at an average distance of 30.1 AU (4.5 billion km 2.8 billion mi). Neptune is denser and physically smaller than Uranus because its greater mass causes more gravitational compression of its atmosphere. It is 17 times the mass of Earth, slightly more massive than its near-twin Uranus. In the Solar System, it is the fourth-largest planet by diameter, the third-most-massive planet, and the densest giant planet. Neptune is the eighth and farthest-known Solar planet from the Sun. Wind speeds can reach 250 metres per second (900 km/h 560 mph). Observations from Earth have shown seasonal change and increased weather activity as Uranus approached its equinox in 2007. Voyager 2 remains the only spacecraft to visit the planet. In 1986, images from Voyager 2 showed Uranus as an almost featureless planet in visible light, without the cloud bands or storms associated with the other giant planets. Its north and south poles, therefore, lie where most other planets have their equators. The Uranian system has a unique configuration because its axis of rotation is tilted sideways, nearly into the plane of its solar orbit. Like the other giant planets, Uranus has a ring system, a magnetosphere, and numerous moons. The interior of Uranus is mainly composed of ices and rock. It has the coldest planetary atmosphere in the Solar System, with a minimum temperature of 49 K (−224 ☌ −371 ☏), and has a complex, layered cloud structure with water thought to make up the lowest clouds and methane the uppermost layer of clouds. Uranus’s atmosphere is similar to Jupiter’s and Saturn’s in its primary composition of hydrogen and helium, but it contains more “ices” such as water, ammonia, and methane, along with traces of other hydrocarbons.

For this reason, scientists often classify Uranus and Neptune as “ice giants” to distinguish them from the other giant planets. Uranus is similar in composition to Neptune, and both have bulk chemical compositions which differ from that of the larger gas giants Jupiter and Saturn. It has the third-largest planetary radius and fourth-largest planetary mass in the Solar System. Its name is a reference to the Greek god of the sky, Uranus, who, according to Greek mythology, was the great-grandfather of Ares (Mars), grandfather of Zeus (Jupiter) and father of Cronus (Saturn). Uranus is the seventh planet from the Sun.

Neptune planet composition