Orbit of Neptune
The outermost planet takes 165 years to go round once, and sunlight needs about four hours to reach it.
Neptune orbits at about 30 AU, some 4.5 billion km from the Sun, so its orbit is about 9 billion km across. It is so slow that it completed its first full orbit since its discovery only in 2011.
It was found by mathematics before it was seen. Small irregularities in the motion of Uranus led the French mathematician Urbain Le Verrier to predict where an unseen planet ought to be, and on 23 September 1846 Johann Galle at the Berlin Observatory found Neptune within about a degree of the predicted spot.
Only one spacecraft has visited. Voyager 2 flew past in 1989, twelve years after launch, and its radio signals, travelling at the speed of light, took about four hours to reach Earth. From Neptune the Sun looks like a brilliant star, giving about 900 times less light than it does at Earth.
The solar system does not end here. Beyond lie the Kuiper Belt, Pluto and other dwarf planets, and much farther out the Oort Cloud of comets, thought to extend a light year or more. Even so, Proxima Centauri, the next stop, is about 4,500 times farther away than the width of Neptune's orbit.
- Size
- 9 × 10¹² m
- Orbital radius
- about 30 AU
- Orbital period
- about 165 years
- Discovered
- 1846