Mean distance from Sun: (2,795,084,80 miles) 4,498,252,900 km
Orbital period (year): 164.79 years
Rotation period (day): 16.11 hours
Diameter at equator: (30,776 miles) 49,528 km
Tilt of axis: 28.3 degrees
Neptune was the first planet to be discovered mathematically.
Neptune is the only planet we can't see without a telescope. It was discovered using a telescope and math. The planet Uranus wasn't moving in its orbit as astronomers expected it to do. French astronomer Urbain Le Verrier thought it was being affected by the gravity of an unknown planet. In 1846 he worked out where this planet should be and told Johann Galle at the Berlin Observatory. Galle found it that evening.
Neptune was seen long before it was discovered.
Galle is considered the discoverer of Neptune because he saw it and recognized it as a planet. Others had seen it, but just thought it was a star, including Galileo all the way back in 1612. If English astronomer John Herschel had looked more closely at in in 1830, he'd have realized it was a planet. That would have been a good story, because his father William Herschel discovered Uranus.
Neptune is about thirty times farther from the Sun than Earth is.
It's much farther away than Uranus, which is nineteen times farther from the Sun than Earth. You'd think Neptune would be a lot colder than Uranus, but they both have cloud-top temperatures of -220 degrees Celsius (-365 degrees Fahrenheit). In common with Jupiter and Saturn, Neptune is producing heat, but we're not sure exactly how. It gives out more heat energy than it receives from the Sun.
Neptune is an ice giant.
The four giant planets in the Solar System are Jupiter, Saturn, Uranus and Neptune. Jupiter and Saturn are made mainly of hydrogen and helium, the two lightest elements. However although Uranus and Neptune have a lot of hydrogen and helium, they also have an icy layer of frozen water, ammonia and methane. This why they are called "ice giants."
Neptune has seasons as Earth does.
Earth is tilted by 23 degrees on its axis and we have four seasons as we orbit the Sun, as shown here. For example, when your hemisphere is tilted towards the Sun, you have summer and people living in the opposite hemisphere have winter.
Neptune's axis is tilted about 28 degrees, which is similar to Earth's 23-degree tilt. But each season lasts more than forty years. If you were born at the start of the winter, it would be winter for most of your life. Also of course, it would be a whole lot colder than Earth.
Neptune is blue.
Neptune's atmosphere is almost all hydrogen and helium, but it has clouds of frozen methane. The methane soaks up red light and reflects blue light.
Neptune has at least six rings.
All four of the gas giants have rings. Neptune's rings are made of dark material, so they're difficult to see. Voyager 2 took pictures of them in 1989.
Neptune has thirteen known moons and one of them is a lot like Pluto.
Astronomers think that Pluto and Neptune's moon Triton are probably the same kind of object - Kuiper Belt Objects. Pluto stayed in the Kuiper Belt and is now a dwarf planet. Triton was captured long ago by Neptune's gravity and became a moon.
Neptune is the windiest planet in the Solar System.
Neptune's atmosphere is very active, with rapidly changing patterns. It also has the highest winds of any Solar System planet - up to 2000 km/hr (1200 mph). This is five times the highest wind speed ever recorded on Earth.
(1) J J O'Connor and E F Robertson "Mathematical Discovery of Planets"
(2) "Neptune's Rings and Moons" http://www.windows2universe.org/neptune/moons_and_rings.html (accessed 11.01.10)