Neptune
Neptune is the eighth and furthest planet from the Sun. It's a deep blue ice giant with the fastest winds in the solar system and the unusual claim of being the only planet to have been discovered using mathematics before it was actually seen. Neptune is so far from the Sun that one Neptunian year lasts 165 Earth years. Since its discovery in 1846, it has only just completed one full orbit.
- Position 8th planet furthest from the Sun
- Distance from Sun 4.5 billion km 30 AU
- Diameter 49,244 km 3.9 times wider than Earth
- Day length 16h 6m fast spin like other gas giants
- Year length 165 Earth years longest year of any planet
- Moons 14 including weird Triton
Where Neptune sits
Neptune is so far from the Sun that sunlight there is 900 times dimmer than on Earth. The Sun would look like a very bright star rather than the warm disc we see. At 4.5 billion km from the Sun, Neptune is so distant that even light, which travels at 300,000 km per second, takes over four hours to reach it. That's why radio messages to and from spacecraft visiting Neptune take so long.
Neptune vs Earth
The most distant planet compared to home
Neptune is 3.9 times wider than Earth. Despite being smaller than Uranus, it's more massive because it has a denser composition.
Neptune is 30 times further from the Sun than Earth. At that distance, the Sun's heat is 900 times weaker.
Neptune's gravity is similar to Earth's. A 50 kg person would weigh approx. 57 kg "on" Neptune (though there's no solid surface to stand on).
One year on Neptune is 165 Earth years. Neptune has only completed one orbit since it was discovered in 1846.
Neptune has the fastest winds in the solar system, five times faster than the strongest hurricanes Earth has ever seen.
Neptune has 14 known moons. The largest, Triton, is bigger than the dwarf planet Pluto.
The planet found by mathematics
Neptune is the only planet that was discovered using maths before it was seen with a telescope. In the early 1800s, astronomers noticed something odd about Uranus's orbit. It wasn't quite where it should be. The simplest explanation was that an undiscovered planet's gravity was tugging on Uranus from further out.
Two mathematicians, the British scientist John Couch Adams and the French scientist Urbain Le Verrier, independently calculated where this mystery planet must be. On 23 September 1846, astronomers at the Berlin Observatory pointed their telescope at the spot Le Verrier had predicted and found Neptune within an hour.
The fastest winds in the solar system
Despite being so far from the Sun, Neptune is one of the windiest places anywhere. Storms in its atmosphere can reach speeds of 2,100 km/h, around five times faster than the strongest hurricanes on Earth.
When NASA's Voyager 2 flew past in 1989, it photographed a huge dark storm called the Great Dark Spot, similar in size to Jupiter's Great Red Spot. By the time the Hubble Space Telescope looked again a few years later, the storm had vanished. New dark spots have appeared and disappeared since. Neptune's storms are nowhere near as long-lasting as Jupiter's.
Why does Neptune glow blue?
Neptune is a deeper, richer blue than Uranus, even though both planets get their colour from the same chemical (methane). Scientists aren't quite sure why. There might be some other unknown chemical in Neptune's atmosphere that adds to the blue, or it might just be that Neptune's atmosphere is clearer than Uranus's.
Triton: the backwards moon
Triton is by far the largest of Neptune's moons. It's almost 2,700 km across, bigger than Pluto. Triton is one of the most interesting moons in the solar system for several reasons:
- It orbits Neptune backwards, in the opposite direction to Neptune's rotation. It's the only large moon in the solar system to do this.
- It has active geysers that shoot nitrogen gas into the sky.
- It has a thin atmosphere of its own, made mostly of nitrogen.
- It's so cold that even nitrogen freezes on its surface.
Triton's backwards orbit is a strong clue that it didn't form alongside Neptune. Instead, it was probably a captured dwarf planet from the Kuiper Belt (the icy region beyond Neptune where Pluto lives). Some time in the distant past, Neptune's gravity caught Triton as it wandered past.
Triton is doomed
Just like Phobos around Mars, Triton is slowly spiralling inwards towards Neptune. In approx. 3.6 billion years, it will get so close that Neptune's tidal forces will rip it apart. The debris will probably form a magnificent ring system around Neptune, possibly rivalling Saturn's. Unfortunately, by then the Sun will have started to expand and life on Earth will be long gone.
Neptune's faint rings
Neptune has five thin rings, made of dust and small chunks of ice. They're much fainter than Saturn's rings and were only discovered in 1989 when Voyager 2 flew past. The rings are named after astronomers who contributed to Neptune's discovery: Galle, Le Verrier, Lassell, Arago and Adams.
The only visit: Voyager 2
Only one spacecraft has ever visited Neptune. NASA's Voyager 2 flew past on 25 August 1989, twelve years after launching from Earth. The flyby lasted only a few hours, but Voyager 2 sent back the first close-up photos of Neptune, discovered six of its moons, and revealed the rings and the Great Dark Spot.
Voyager 2 is still flying. It has now passed beyond the edge of our solar system into interstellar space, but it continues to send signals back to Earth almost half a century after launch.
Pluto, the demoted dwarf
For many years, Neptune was the second-furthest known planet, with Pluto beyond it. Pluto was officially classified as the ninth planet from 1930 to 2006. But in 2006, the International Astronomical Union reclassified Pluto as a "dwarf planet" because it shares its orbit with thousands of other similar icy objects in the Kuiper Belt. Since then, Neptune has been the eighth and final planet in the solar system.