The eighth and outermost planet in the Solar System is Neptune, at a mean distance of 4.498 billion km (2.795 billion miles) from the Sun. It was discovered in 1846, and like its inner neighbour Uranus, it is classed as an ice giant.
Neptune is 49,244 km (30,599 miles) wide at the equator and 17 times more massive than the Earth. It rotates in 16 hours 7 minutes and takes 165 years to orbit the Sun.
Neptune’s discovery was remarkable because it came as a result of mathematical calculation rather than a sweep of the heavens. Observations of Uranus had shown that it was not keeping to its expected track around the Sun. Astronomers blamed the pull of another yet unknown body.
Mathematicians in the UK and France independently performed calculation which allowed them to predict where this undiscovered world lay. But Frenchman Urbain Le Verrier just beat Englishman John Couch Adams to the line in locating it.
Though Adams identified,in 1845, where in the sky Neptune must lie, he was unable to muster sufficient interest among British astronomers to look for it. Meanwhile, Le Verrier came to a similar conclusion. He alerted astronomers at Berlin Observatory, and Johann Galle spotted Neptune on 23 Setpember, 1846, the same day he had received Le Verrier’s letter.
Interestingly, the noted Italian astronomer Galileo had spotted Neptune in 1612 and again in 1613,but he recorded it in his sketches as just another star.
Neptune is bluer than its ice twin Uranus. This is because it has more methane in its atmosphere, which also contains hydrogen, helium, ammonia and water. A rocky core the size of the Earth is thought to lie deep within the planet.
Neptune experiences seasons because its axis is tilted, by 28°, to the plane of its orbit. Weather features have been seen in its cloud tops, including bright clouds driven by ferocious winds. NASA’s Voyager 2 spacecraft imaged deep blue bands and a huge dark spot when it flew by in August 1989. It also discovered that Neptune has faint rings around it.
Neptune’s moons
Neptune has two main moons, Triton and Nereid, plus 11 much smaller satellites. Triton is the largest moon, with a diameter of 2,700 km (1,678 miles). It orbits the “wrong way” around Neptune, and is probably an icy body, like Pluto, captured from the Kuiper Belt.
Triton is slowly spiralling in towards Meptune and will eventually shatter to form another ring of debris.
Nereid is only 340 km (211 miles) across and has an extremely elongated orbit that takes it nearly seven times further from Neptune at some times than at others.
Related: Where you can find Neptune in the night sky
Observing Neptune
Neptune only reaches magnitude 7.8 at its best, so it is invisible to the naked eye. However, you will be able to see it with binoculars, or a small telescope, if you know where to look. Here is our guide to help you find Neptune.