• Discovered: 1846 (proved mathematically 1845)
• Diameter: 49,500 km
• Temperature: -220 °C (at cloud tops)
• Distance from the Sun: Varies between 4.46 and 4.54 billion km
• Satellites: 13

Named after the Roman god of the sea, Neptune is about four times larger than Earth and is the fourth largest planet in our Solar System. Its beautiful blue colour results from the presence of methane in the atmosphere.

Now that Pluto has been officially demoted to a dwarf planet, Neptune is the only planet in our Solar System that cannot be seen with the naked eye. Although Pluto is further away from the Sun, every 248 years it moves inside Neptune’s orbit for around 20 years.

A day on Neptune lasts 17 hours but each year is the equivalent of 165 years on Earth. It has a ring system, which is not as pronounced as Saturn’s, and several moons. The largest moon is Triton (2,705 km in diameter).

Temperatures on Neptune reach several hundred degrees below zero with winds of over 1,900 km per hour close to the equator. There is also evidence of storms or vortices similar to those on Jupiter and Saturn.

Thick clouds cover the planet and scientists believe that Neptune is made up chiefly of hydrogen, helium, water and silicates. It does not have a solid surface and instead consists of compressed gases, like frozen methane and possibly hydrogen sulphide.

Neptune was first predicted mathematically when astronomers realised that Uranus’ orbit was being influenced by another, unseen planet.

The English astronomer and mathematician, John C Adams, began working on this new planet’s location in 1843. Two years later he had a solution and sent it to England’s Astronomer Royal, George B Airy. Unfortunately Airy did not have much faith in Adams’ work and a French mathematician, Urbain Joseph Le Verrier, presented his similar predictions to Johann Galle at the Urania Observatory in Berlin in 1846.

The Frenchman’s predictions were proved right by Galle and Neptune was located. Today both Le Verrier and Adams are credited with its discovery, as well as Galle.

Voyager 2 was the first mission to reach Neptune. In August 1989, it provided the first close-up views of the planet and most of its moons.

The spacecraft also discovered the planet's rings and discovered that they were narrower than Saturn’s rings and contained a greater proportion of fine dust particles, rather than lumps of ice and rock.