Hugo's Solar System
Planet
Neptune
The deep blue ice giant at the edge of the planets, home to the fastest winds we know
"Windy and wavy, I'm deep ocean blue.
I'm nearly the farthest, with stories for you."From Hugo's Journey Through Space
Key facts
- Diameter
- 30,800 miles / 49,528 km at the equator, about four times wider than Earth
- Distance from the Sun
- About 30 AU (about 2.8 billion miles / 4.5 billion km) on average
- One orbit
- About 165 Earth years (60,190 Earth days)
- One day (rotation)
- About 16 hours
- Average temperature
- About -201 °C at the one-bar level of the atmosphere
- Known moons
- 16 (as of September 2026)
- Rings
- At least five main rings (Galle, Le Verrier, Lassell, Arago, Adams) and four ring arcs
- Named after
- Neptune, the Roman god of the sea
- Discovered
- 23 September 1846 by Johann Gottfried Galle in Berlin, using Urbain Le Verrier's calculations
- Axis tilt
- About 28 degrees, so Neptune has seasons
- Visited by
- Voyager 2 only, a flyby on 25 August 1989
About Neptune
Neptune is a big blue planet. It is far, far away. It is the last planet from the Sun. Sunlight takes four hours to get there. Neptune is very cold. It is very windy too. Its winds are the fastest of all. Neptune is made of gas and ice. You could not stand on it. It has sixteen moons. The biggest moon is called Triton. Neptune is named after a sea god. One space probe has seen it up close. It flew past a long time ago. It sent home the first close-up pictures.
Neptune is the eighth planet and the farthest from the Sun. It sits about 4.5 billion kilometres away, which is 30 times Earth's distance. Sunlight takes about four hours to reach it. Because it is so far out, one orbit takes about 165 Earth years.
Neptune rotates quickly, though, so a day lasts only about 16 hours. Its axis is tilted by about 28 degrees, so it has seasons like Earth. Neptune is a huge ice giant, not a rocky world like Earth. It is about four times wider than Earth, with a diameter of 49,528 kilometres.
There is no solid ground to stand on anywhere. Instead there is a deep, hot fluid of water, methane and ammonia. Its atmosphere is mostly hydrogen and helium, with a little methane. That methane helps give Neptune its deep blue colour.
Its winds are the fastest known, blowing at around 2,000 kilometres per hour. Neptune has faint rings and 16 known moons, as of August 2026. The largest moon, Triton, has icy geysers and is bitterly cold. Only one spacecraft has ever visited Neptune up close.
Voyager 2 flew past in August 1989 and took the first close-up pictures. Neptune was also the first planet found by maths before anyone saw it.
Neptune is the eighth and outermost planet, orbiting the Sun at an average distance of about 4.5 billion km (30 AU). Sunlight takes about four hours to reach it, and one orbit takes about 165 Earth years (60,190 days). Despite its distance, it rotates quickly: a Neptune day lasts about 16 hours. Its rotation axis is tilted about 28 degrees, so it experiences seasons, each lasting roughly 40 Earth years.
With an equatorial diameter of 49,528 km, Neptune is about four times wider than Earth and about 17 times as massive. It is an ice giant rather than a gas giant: beneath an atmosphere of mostly hydrogen and helium with a little methane lies a hot, dense fluid of water, methane and ammonia ices. The temperature at the level where the atmospheric pressure equals Earth's sea level is about minus 201 °C. Methane absorbs red light, which contributes to Neptune's blue colour. Its winds are the fastest measured on any planet, exceeding 1,200 mph (about 2,000 km/h), and its magnetic field is tipped about 47 degrees from the rotation axis.
Neptune was the first planet found by mathematics. Urbain Le Verrier in France (and, independently, John Couch Adams in England) calculated where an unseen planet must be from irregularities in the orbit of Uranus. Johann Gottfried Galle at the Berlin Observatory found it on 23 September 1846, on his first night of searching. Galileo had actually recorded Neptune in 1612 and 1613 but took it for a star. Just 17 days after the discovery, William Lassell found the largest moon, Triton.
Neptune has at least five main rings, named Galle, Le Verrier, Lassell, Arago and Adams, plus four bright ring arcs. It has 16 known moons as of August 2026. Triton, at about minus 235 °C, has geysers that throw icy material more than 8 km high.
Only one spacecraft has ever visited. Voyager 2 passed 4,800 km above the cloud tops on 25 August 1989, discovering six moons, four rings and the Great Dark Spot, an Earth-sized storm that has since vanished. No orbiter has followed, so much of what we know comes from that single flyby, supplemented by the Hubble and James Webb space telescopes.
Did you know?
- Neptune was the first planet found by maths: astronomers worked out where it must be before anyone had knowingly seen it.
- Galileo drew Neptune in his notebook in 1612 and 1613 but thought it was a star.
- Voyager 2 found a dark storm as big as Earth in 1989. By the time Hubble looked a few years later, it had gone.
Moons of Neptune
Neptune has 16 known moons as of September 2026. The largest are listed here; the other 2 are smaller moons, most only a few kilometres across. How many moons does Neptune have?
- Triton
- Proteus
- Nereid
- Larissa
- Galatea
- Despina
- Thalassa
- Halimede
- Neso
- Naiad
- Sao
- Laomedeia
- Psamathe
- Hippocamp
Questions children ask
Why is Neptune blue?
Neptune's atmosphere is mostly hydrogen and helium with a little methane. Methane soaks up red light and lets blue light bounce back out into space, so the planet looks deep blue to us.
Could you stand on Neptune?
No. Neptune has no solid surface. Below its cloudy atmosphere is a deep, hot fluid of water, methane and ammonia. Anything trying to land would just sink deeper and deeper into it.
How long would it take to get to Neptune?
A very long time. Neptune is about 4.5 billion kilometres from the Sun, so even sunlight takes about four hours to arrive. Voyager 2 launched in August 1977 and did not reach Neptune until August 1989, twelve years later.
How was Neptune discovered without anyone seeing it first?
Uranus was not moving quite as expected, so Urbain Le Verrier calculated that an unseen planet must be tugging on it. He sent his prediction to Berlin, and Johann Galle found Neptune on 23 September 1846, on his first night of looking.
Quick quiz
Three questions for space cadets. Nothing is recorded.
- How many known moons does Neptune have, as of August 2026?
- Which spacecraft is the only one to have visited Neptune up close?
- About how long does Neptune take to orbit the Sun once?
Uranus
Triton
Voyager 2