Neptune

Neptune is the eighth and most distant planet from the Sun. Its position was predicted by math before it was ever seen, and this blue ice giant has the fastest winds in the Solar System.

What kind of planet is it?

Neptune is the eighth and outermost planet in the Solar System, orbiting at an average distance of about 30.1 AU, or about 4.5 billion km, from the Sun. It's about 49,528 km across, roughly 3.9 times Earth's diameter: a little smaller than Uranus, but more massive. It takes about 165 years to circle the Sun, so after its discovery in 1846, it didn't complete its first full orbit until 2011.

Like Uranus, Neptune is an ice giant. Beneath an atmosphere of hydrogen, helium and a little methane lies a thick layer of water, ammonia and methane mixed under high pressure, and there's thought to be a rocky core at the center. Thanks to its large mass, gravity at the cloud tops is about 1.14 times Earth's, the highest of any planet after Jupiter.

A day (rotation) lasts about 16 hours 7 minutes, and its axis is tilted about 28.32°, so it has seasons like Earth. But since a year lasts about 165 years, each season goes on for more than 40 years.

Atmosphere and weather

The average temperature at Neptune's cloud tops is about -200°C (-328°F). Methane in the atmosphere absorbs red light, making the planet look blue. The deep-blue images that became widely known were actually color-enhanced to make features stand out, and recent research has found that its true color is closer to a pale blue-green, much like Uranus.

Neptune is the farthest planet from the Sun and receives only about 1/900 of the sunlight Earth does, yet it has the fastest winds in the Solar System, reaching around 2,000 km/h. Neptune radiates more than twice as much heat from its interior as it receives from the Sun, and this inner heat is thought to help power its violent weather.

In 1989, Voyager 2 discovered the Great Dark Spot, a dark swirling storm about the size of Earth, in the southern hemisphere. But when the Hubble Space Telescope looked a few years later, the storm was gone, and new dark spots have since been seen appearing and disappearing elsewhere. White clouds of methane ice sometimes drift around the dark spots.

Moons and rings

As of 2025, Neptune has more than 15 known moons. The standout among them is Triton.

• Triton: Discovered in 1846 by British astronomer William Lassell, just 17 days after Neptune itself was found. About 2,707 km across, it makes up most of the total mass of Neptune's moons. It's the only large moon in the Solar System with a retrograde orbit, traveling opposite to its planet's rotation, so it's thought to be a captured object that once wandered the Kuiper Belt beyond Neptune. With a surface temperature of around -235°C, it's one of the coldest places in the Solar System, and Voyager 2 observed eruptions there shooting nitrogen gas and dust upward.
• Nereid: Discovered in 1949, it follows a very elongated, oval orbit.
• Proteus: Neptune's second-largest moon, but because its surface is dark and it orbits close to Neptune, it wasn't discovered until Voyager 2 arrived in 1989.

Neptune also has five faint rings, named after astronomers who contributed to Neptune's discovery and study: Galle, Le Verrier, Lassell, Arago and Adams. The outermost Adams ring is notable for clumps of material that appear as bright arcs. In 2022, the James Webb Space Telescope captured a sharp image of Neptune's rings.

Exploration

Neptune wasn't stumbled upon with a telescope; it was found by calculation first. Astronomers noticed that Uranus was drifting slightly from its predicted path and suspected that an unseen planet beyond it was tugging on it. France's Urbain Le Verrier calculated its position, and in September 1846 Johann Galle at the Berlin Observatory in Germany found Neptune close to the predicted spot. England's John Couch Adams is also known to have made similar calculations. In fact, Galileo saw Neptune in 1612–1613 but took it for a star and simply noted it down.

To date, only one spacecraft has visited Neptune: Voyager 2.

• Launched in 1977, Voyager 2 passed Jupiter, Saturn and Uranus in turn before flying by Neptune in August 1989. It's the only spacecraft to have visited all four giant planets.
• It discovered the Great Dark Spot, the fast winds, the structure of the rings and Triton's eruptions, and found several new moons.
• After skimming very close over Neptune's north pole and turning toward Triton, it headed out of the Solar System.

Since then, the Hubble and James Webb space telescopes and large ground-based telescopes have kept watch on Neptune's weather and rings.

Fun facts

• Neptune was the first planet discovered through mathematical prediction.
• A year on Neptune lasts about 165 years, so since its discovery it has completed only a little more than one orbit around the Sun.
• Part of Pluto's orbit dips inside Neptune's, but thanks to a resonance in which Pluto orbits twice for every three Neptune orbits, the two never collide.
• Triton is very slowly drawing closer to Neptune, and some predictions say that in the very distant future it could break apart and become a new ring around Neptune.
• Neptune is named after Neptunus, the Roman god of the sea: a fitting name for a blue planet.

See it in Planetica

In Solar System view, Neptune appears on the eighth and outermost orbit. Like the other planets, it's placed at its real position for today's date, calculated from J2000 orbital elements, but since one orbit takes about 165 years, it looks almost frozen in place. Compare its orbit with those of the inner planets and you'll get a real sense of how vast the Solar System is.

Pick Neptune and fly in close to see moons approximating the real ones circling with it. The moons are drawn larger than their true proportions so they're easy to see. Size, gravity, day length, axial tilt, average temperature and orbital period are based on real values.

The surface patterns are created by Planetica's planet generator, so they aren't a map of Neptune's actual clouds or dark spots. Instead, you get to compare your very own system, made from a name and birthday, side by side with the real outer planets of the Solar System.

Other planets

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• Mercury
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• Earth
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• Jupiter
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• Uranus
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