Neptune is the outermost planet of the Solar system, one of the four gas giants. It’s smaller, but heavier than its near-twin Uranus, making it the fourth largest, after Jupiter, Saturn and Uranus, and the third heaviest planet, after Jupiter and Saturn.
Neptune orbits the Sun at 30.1 astronomical units (AUs) i.e. it is about 30 times farther away from the Sun than Earth, making it invisible to the naked eye. It’s distance is the reason Neptune was discovered late compared to other planets - on September 23, 1846. It is the only planet discovered by mathematical prediction and not by empirical observation. Urbain Le Verrier predicted its position based on changes in orbit of Uranus and Johann Galle first observed it where Le Verrier calculated it would be.
Named after Roman god of the sea, Neptune is 17 times heavier than Earth and the densest of the four gas giant planets. Its atmosphere is made of hydrogen, helium and methane gas. Neptune has the strongest winds in Solar system, with wind speeds reaching 600 m/s (2,200 km/h). Methane gas in the highest regions of the atmosphere absorbs the red part of the sunlight and is, together with some unknown element, what gives the planet its sea-blue colour.
The only thing “tainting” the nice blue colour are anti-cyclonic storms, dark spots that last for several years. First one, observed by Voyager 2 in 1989, was the Great Dark Spot, an Earth-sized spot in upper atmosphere, similar to the Great Red Spot on Jupiter. Now it seems to have disappeared, but another, Northern Great Dark Spot was observed by Hubble mid-1990s and it lasted for several years.
Neptunian Great Dark Spot
After Jupiter, Neptune has the highest surface pressure of all planets. Under the layer of atmosphere, there’s a mantle consisting of fluid made of water, ammonia and methane. This fluid is very hot because of the said pressure. It is possible that because of this pressure, at depths of 7000 kilometres, methane is turned into diamond crystals that sink even further, creating a diamond rain.
Under this mantle is planet’s core made of iron, nickel and silicates, with temperatures reaching 5400 degrees Kelvin.
It takes 165 Earth years for Neptune to orbit the Sun. Since its axis is tilted 28.3 degrees, it has similar seasonal changes as Earth (Earth’s axis is tilted 23 degrees), with each season lasting about forty Earth years.
A day on Neptune lasts 16.1 hours, but because it’s not a solid planet, it takes more time for regions around equator to make a full rotation (18 hours), while polar regions have rotational period of 12 hours.
On its way around the Sun, Neptune and Pluto sometimes switch places. Between 1979 and 1999 Pluto was nearer the Sun than Neptune. These two will never collide though, because for every two revolutions Pluto makes around Sun, Neptune makes exactly three, never putting little Pluto in harm’s way.
Orbits of outer bodies in Solar system shows Neptune (blue) and Pluto (violet) orbits crossing paths
Neptune has 14 moons, largest of which is Triton, as well as three rings. Only human-made spacecraft that visited and observed Neptune was Voyager 2 in 1989 and it confirmed that Neptune indeed has three rings, first of which was discovered in 1968. These rings are unstable and thought to be short-lived.
Image showing two of three Neptune's rings.
Neptune is theorised to have been formed in inner regions of the Solar system and has since then migrated to where it is now. The reason for this hypothesis is the probable lack of material to create gas giants in outer regions.
All images credited to NASA