Solar System

Neptune

Ice giant · The windswept deep · Johann Galle, 1846 — found within 1° of Le Verrier's mathematical prediction

The most distant planet, Neptune is a deep-blue ice giant with the fastest winds in the Solar System, found by mathematics before it was ever seen.

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Texture: Solar System Scope (CC BY 4.0)

Key facts

Distance from Sun 30.07AU 4.50 billion km
Radius 24,622km 3.9× Earth
Mean temperature −200°C cloud-top average

Time & motion

A day (one spin) 16.1 hours A short day for such a distant world.
A year (one orbit) 164.8 Earth years Neptune completed its first orbit since discovery only in 2011.
Surface gravity vs Earth
11.0 m/s² 1.12 g
0Earth = 1 g2.5×

Mass & physical characteristics

Mass 1.02 × 10²⁶kg 17.1× Earth
Density 1.64g/cm³ Water = 1.0 g/cm³
Escape velocity 23.5km/s Speed to break free
Orbital velocity 5.4km/s Speed around the Sun
Axial tilt 28.3° An Earth-like tilt, so Neptune has seasons — each lasting ~40 years.

Atmosphere & magnetosphere

Atmosphere

About 80% hydrogen, 19% helium and 1.5% methane. Methane deepens its blue; winds top 2,000 km/h — the fastest of any planet.

Magnetic field

A tilted, off-centre field, inclined about 47° from the rotation axis.

Moons & rings

Known moons 16 16 confirmed moons, the largest being Triton.
Notable moons
TritonProteusNereid

Rings

Five faint rings of dark dust, named after astronomers such as Adams and Galle.

Make-up

Composition

Water, methane and ammonia ices; methane gives its vivid blue colour.

Notable features

  • The Great Dark Spot — an Earth-sized storm seen by Voyager 2 in 1989.

Fun facts

  • Neptune was predicted by mathematics from Uranus's orbit, then found within a degree.
  • It has the fastest winds in the Solar System — over 2,000 km/h.
  • Neptune radiates more heat than it receives from the distant Sun.
  • Its moon Triton orbits backwards and has active nitrogen geysers.
  • Neptune completed its first orbit since discovery only in 2011.

Exploration

Voyager 2 1989 The only visit — revealed the Great Dark Spot and Triton's geysers.
Hubble Telescope 1994– Tracks the birth and death of its giant storms.