Neptune
What is Neptune?
Planets· Diameter 49,244 km. The windiest planet, with winds up to 2,100 km/h, and the first planet whose position was worked out by mathematics before anyone saw it.
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Orbit and size
| Diameter | 49,244 km |
|---|---|
| Distance | 30.1 AU |
| Year | 165 yr |
| Winds | up to 2,100 km/h |
Figures are representative approximations. Source: compiled from NASA and IAU public data.
Moons of Neptune
Related events
Neptune's first full orbit since its discovery
Neptune was discovered on 23 September 1846. It takes 164.8 years to circle the Sun, so it first returned to the spot where it was discovered on 12 July 2011. People have known about Neptune for only a little more than one Neptunian year. In that single orbit, humanity went from seeing the planet through telescopes to sending a spacecraft past it.
Neptune turns up in records long before its discovery. In December 1612 and January 1613 Galileo, while watching Jupiter's moons, drew Neptune right beside them. He took it for a star and moved on. Had he kept watching, the discovery might have come 234 years earlier. Setting the sky view to 28 December 1612 shows Neptune next to Jupiter.
Neptune's weather can change a lot within a few years. In 1989 Voyager 2 photographed the Great Dark Spot, a storm about the size of Earth, but when the Hubble Space Telescope looked in 1994 it was gone. Since then dark spots have appeared and faded every few years. A study published in 2023, using nearly thirty years of observations, found that the amount of cloud on Neptune rises and falls with the Sun's activity cycle.
The planet in depth
Neptune was found by calculation first. In the 1800s astronomers noticed that Uranus kept drifting slightly off its expected path. The French mathematician Urbain Le Verrier reasoned that an unseen planet farther out was pulling on it, and he worked out where that planet should be. On the night of 23 September 1846, Johann Galle at the Berlin Observatory found Neptune less than one degree from the predicted spot.
Neptune is about 30 AU from the Sun, thirty times farther than Earth, so it receives only one nine-hundredth as much sunlight. Even so, it has the fastest winds in the solar system. Sunlight alone cannot explain them. Neptune gives off 2.6 times as much energy as it receives from the Sun, and scientists think this inner heat stirs its atmosphere.
Neptune's largest moon, Triton, orbits in the opposite direction to the planet's spin. A moon born with Neptune would circle the same way, so Triton is thought to be an object captured from outside. Voyager 2, in 1989, is the only spacecraft to have seen Neptune up close. Neptune takes 165 years to go around the Sun, so in the orbit view it moves far more slowly than the other planets.
Frequently asked
- How big is Neptune?
- The diameter of Neptune is 49,244 km.
- How far is Neptune from Earth?
- The distance of Neptune is 30.1 AU.
- What are the main moons of Neptune?
- The main moons of Neptune are Triton.
Related
Figures are representative approximations. Simulated positions use J2000 mean elements.