Neptune, the outermost planet in the Solar System, cannot be seen from Earth without a telescope or binoculars. It is covered by icy, bright blue methane clouds that whip around the globe at speeds of more than 700mph (1,100km/h).
Though its diameter is four times that of the Earth's and it is 17 times as massive, it is less dense and doesn't have a solid surface. Eleven moons orbit Neptune, the largest of which is Triton.
Photo: View of Neptune composed of images taken by the Voyager 2 probe (NASA)
Bright blue clouds speed around the outermost planet.
The early Solar System was a shooting gallery.
Professor Brian Cox explains how the orbiting gas giants may have caused an enormous asteroid and comet bombardment in the inner Solar System 3.6 billion years ago. Earth and the other planets were peppered by asteroids and comets.
Learn how to remember all the planets of the Solar System in order.
Here’s an animated guide to remembering all the planets of the Solar System in order, using an easy mnemonic trick.
It is predicted that Saturn will lose its rings.
In one hundred million years it is predicted that Saturn's rings will disappear. In one billion years, Neptune may develop its own rings when its moon Triton spirals from orbit and disintegrates into countless fragments.
Prof Brian Cox demonstrates gravity’s force on other planets.
Prof Brian Cox simulates the strength of gravity on other planets through a human centrifuge in Holland.
Enjoy Astronomer Marek Kukula's guide to the Solar System.
If Jupiter were much larger it would be a star in its own right! Enjoy Astronomer Marek Kukula's eloquent guide to the Sun, the planets and the outer reaches of the Solar System.
Neptune is the eighth and farthest planet from the Sun in the Solar System. It is the fourth-largest planet by diameter and the third-largest by mass. Among the gaseous planets in the Solar System, Neptune is the most dense. Neptune is 17 times the mass of Earth and is slightly more massive than its near-twin Uranus, which is 15 times the mass of Earth, and not as dense as Neptune.[c] Neptune orbits the Sun at an average distance of 30.1 astronomical units (4.50×109 km). Named after the Roman god of the sea, its astronomical symbol is ♆, a stylised version of the god Neptune's trident.
Neptune is not visible to the unaided eye and was the first and only planet found by mathematical prediction rather than by empirical observation. Unexpected changes in the orbit of Uranus led Alexis Bouvard to deduce that its orbit was subject to gravitational perturbation by an unknown planet. Neptune was subsequently observed with a telescope on 23 September 1846 by Johann Galle within a degree of the position predicted by Urbain Le Verrier. Its largest moon, Triton, was discovered shortly thereafter, though none of the planet's remaining 13 moons were located telescopically until the 20th century. The planet's distance from Earth gives it a very small apparent size, making it challenging to study it with Earth-based telescopes. Neptune was visited by Voyager 2, when it flew by the planet on 25 August 1989. The advent of Hubble Space Telescope and large ground-based telescopes with adaptive optics has allowed for more-detailed observations.
Neptune is similar in composition to Uranus, and both have compositions that differ from those of the larger gas giants, Jupiter and Saturn. Neptune's atmosphere, like Jupiter's and Saturn's, is composed primarily of hydrogen and helium, along with traces of hydrocarbons and possibly nitrogen; it contains a higher proportion of "ices" such as water, ammonia, and methane. Astronomers sometimes categorise Uranus and Neptune as "ice giants" to emphasise this distinction. The interior of Neptune, like that of Uranus, is primarily composed of ices and rock. Perhaps the core has a solid surface, but the temperature would be thousands of degrees and the atmospheric pressure crushing. Traces of methane in the outermost regions in part account for the planet's blue appearance.
In contrast to the hazy, relatively featureless atmosphere of Uranus, Neptune's atmosphere has active and visible weather patterns. For example, at the time of the 1989 Voyager 2 flyby, the planet's southern hemisphere had a Great Dark Spot comparable to the Great Red Spot on Jupiter. These weather patterns are driven by the strongest sustained winds of any planet in the Solar System, with recorded wind speeds as high as 2,100 kilometres per hour (580 m/s; 1,300 mph). Because of its great distance from the Sun, Neptune's outer atmosphere is one of the coldest places in the Solar System, with temperatures at its cloud tops approaching 55 K (−218 °C). Temperatures at the planet's centre are approximately 5,400 K (5,100 °C). Neptune has a faint and fragmented ring system (labelled "arcs"), which may have been detected during the 1960s but was indisputably confirmed only in 1989 by Voyager 2.