Facts About Neptune, the Farthest Planet

What kind of planet is Neptune?

Neptune is the eighth and farthest planet from the Sun, about 4.5 billion kilometers (30 AU) away. It's an ice giant, like its near-twin Uranus: about four times wider than Earth, with no solid surface. NASA's comparison: if Earth were the size of a nickel, Neptune would be about as big as a baseball.

Neptune is dark and cold. NASA says sunlight on Earth is about 900 times brighter than on Neptune, and sunlight takes about four hours to get there. It's named after the Roman god of the sea, a good fit for its deep blue color.

Neptune by the numbers

Source: NASA Planetary Fact Sheet and NASA Science.

A year on Neptune lasts about 165 Earth years, so Neptune has gone around the Sun only once since it was discovered: it completed that first orbit in 2011. Its axis is tilted a lot like Earth's, so it has seasons, but each one lasts more than 40 years.

The planet found by math

Neptune was the first planet found by prediction instead of by looking. After Uranus was discovered, astronomers noticed it wasn't moving exactly as expected, as if something unseen were tugging on it. The French mathematician Urbain Le Verrier worked out where that unknown planet should be. In 1846, the German astronomer Johann Galle pointed his telescope at that spot and found Neptune on his first night of searching.

Fun twist: Galileo had actually sketched Neptune in 1612 and 1613, but he thought it was a star.

The windiest planet

Neptune has the fastest winds in the solar system. NASA says they reach more than 2,000 kilometers per hour (1,200 mph), about three times as fast as Jupiter's and nine times as fast as Earth's. The strongest winds ever measured on Earth are about 400 kilometers per hour.

Those winds drive huge storms. When Voyager 2 flew past in 1989, it saw the Great Dark Spot, an oval storm in the southern hemisphere big enough to hold the whole Earth, along with bright white clouds of methane ice racing above it. A few years later, the Hubble Space Telescope found that the Great Dark Spot had disappeared; new dark storms have formed and faded since.

What's inside Neptune?

Tap the cutaway in the diagram:

1. Atmosphere: mostly hydrogen and helium with a little methane, which absorbs red light and helps make Neptune blue.
2. Icy mantle: NASA says 80 percent or more of Neptune's mass is a hot, dense fluid of water, methane and ammonia. Scientists think there may even be a superheated water ocean under the clouds, kept from boiling away by the huge pressure.
3. Rocky core: a small core with about as much mass as the whole Earth.

Neptune is the densest of the four giant planets. It's slightly smaller than Uranus but has more mass.

Rings and moons

Neptune has at least five main rings, named after astronomers who studied the planet: Galle, Leverrier, Lassell, Arago and Adams. They're faint and dusty. The outermost, Adams, has four bright clumps called arcs, named Liberté, Egalité, Fraternité and Courage. NASA says the rings are probably young and won't last forever.

Neptune has 16 known moons. The biggest, Triton, was discovered by William Lassell just 17 days after Neptune itself. Triton is special:

• It's the only large moon in the solar system that orbits backward, opposite to the way its planet spins. That suggests Neptune captured it, probably from the Kuiper Belt.
• Voyager 2 saw geysers on Triton shooting material more than 8 kilometers high.
• Its surface is about −235 °C, one of the coldest places measured in the solar system.

How cold is Neptune?

Neptune's average temperature at the level where its air pressure equals Earth's is about −200 °C, according to the NASA Planetary Fact Sheet. Yet Neptune is a little warmer than scientists expected for a planet so far from the Sun: it gives off more than twice as much heat as it gets from sunlight, so it has a strong source of heat inside. That inner heat may help power its record-breaking winds.

Neptune and Uranus: twins?

Neptune and Uranus are often called twins. They're almost the same size, both are ice giants of water, methane and ammonia around small rocky cores, and both look blue. But there are differences. Neptune is a bit smaller yet more massive and denser. It's tilted a normal 28 degrees, while Uranus lies on its side. And Neptune's weather is far more active, with dark storms and racing white clouds, while Uranus usually looks calm.

Missions to Neptune

Only Voyager 2 has visited Neptune, flying past in August 1989. It found the Great Dark Spot, confirmed the rings, discovered six new moons and photographed Triton's geysers. Since then, the Hubble and James Webb space telescopes have kept watching Neptune's changing weather from afar.

Fun facts about Neptune

• Neptune is the only planet you can't see without a telescope.
• If you weigh 80 pounds on Earth, you'd weigh about 90 pounds at Neptune's cloud tops.
• Pluto sometimes comes closer to the Sun than Neptune does, but their orbits are timed so they never meet.
• Neptune's magnetic field is tipped about 47 degrees from its spin axis and is about 27 times as powerful as Earth's.

Learning about Neptune at home

Act out Neptune's discovery: hide a magnet under a table and roll a steel ball across the top; when the ball's path bends, kids can predict where the hidden magnet is, just as Le Verrier did. Take the facts quiz above and print the labeled Neptune diagram.

Then compare Neptune with Uranus and Saturn, or see how far out it orbits in the 3D solar system with distances to scale.