The windswept frontier
Neptune
Neptune is the eighth and most distant planet, orbiting about 4.5 billion km from the Sun. It is an ice giant with a deep blue atmosphere and the fastest winds measured anywhere in the Solar System.
It was also the first planet found through mathematics rather than by chance: astronomers predicted its position from tiny irregularities in Uranus's orbit before anyone saw it through a telescope.
Key facts
- Diameter
- 49,528 km3.88× Earth
- Mass
- 1.02 × 10²⁶ kg17× Earth
- Gravity
- 11 m/s²1.12 g
- Mean distance from Sun
- 30.05 AU≈ 4,495 million km
- Length of year
- 163.72 yrEarth days or years
- Length of day
- 16.1 hSunrise to sunrise
- Mean temperature
- -200°CAt the 1-bar level
- Known moons
- 16+As of 2025
- Axial tilt
- 28.3°
1 astronomical unit (AU) is Earth's average distance from the Sun, about 149.6 million km. Values follow the NASA Planetary Fact Sheet; many are averages and vary with how and where they are measured.
Physical characteristics
Neptune is 49,528 km across, slightly smaller than Uranus, but 17 times as massive as Earth — making it the densest of the giant planets at 1,638 kg/m³.
Gravity at the 1-bar level is about 11 m/s², a little more than Earth's.
Atmosphere and climate
Neptune's atmosphere is mainly hydrogen and helium with about 1.5% methane, which helps give it its blue colour. Recent reprocessing of Voyager 2 and telescope images suggests Neptune and Uranus are closer in colour than the strikingly deep blue of older images implied.
Winds reach about 2,000 km/h, faster than anywhere else in the Solar System. Voyager 2 photographed a storm system the size of Earth, the Great Dark Spot, in 1989; by the time Hubble looked in 1994, it had vanished. Neptune also gives off about 2.6 times as much energy as it receives from the Sun, revealing a strong internal heat source.
Composition: H₂ 80% · He 19% · CH₄ 1.5%
Internal structure
Fluid ices and a rocky core
Like Uranus, Neptune is thought to have a deep mantle of hot, dense water, ammonia and methane surrounding a rocky core about the size of Earth.
A tilted magnetic field
Its magnetic field is tilted about 47° from its rotation axis and offset from the centre, another trait it shares with Uranus.
Moons and rings
Neptune has at least 16 known moons. The largest, Triton, orbits backwards — opposite to Neptune's rotation — which strongly suggests it was captured from the Kuiper Belt. Voyager 2 saw nitrogen geysers erupting from its frozen surface, one of the coldest ever measured at about −235°C. Tidal forces are slowly drawing Triton inward, and in the far future it is expected to be torn apart.
Neptune has five main rings, named after astronomers involved in its discovery and study. The outermost, the Adams ring, contains unusual bright clumps called arcs.
Orbit and rotation
Neptune orbits at an average of 30.05 AU and takes about 165 Earth years to circle the Sun. Since its discovery in 1846, it completed its first full orbit only in 2011.
A day lasts about 16 hours. Its axial tilt of 28° is similar to Earth's, so Neptune has seasons — but each one lasts around 40 years.
Exploration and missions
Neptune has been visited once, by Voyager 2, and is otherwise studied with large telescopes.
- 1612–1613
Galileo's observations · —
Galileo recorded Neptune in his notes but mistook it for a star.
- 1846
Discovery · —
Johann Galle found Neptune close to the position predicted by Urbain Le Verrier; John Couch Adams had made similar calculations.
- 1989
Voyager 2 · NASA
The only spacecraft to visit; it found the Great Dark Spot, Neptune's rings and Triton's geysers.
- 2022–
James Webb Space Telescope · NASA / ESA / CSA
Captured the clearest view of Neptune's rings in decades.
Surprising facts about Neptune
- Neptune was discovered with mathematics before it was seen.
- It has completed only one orbit since its discovery.
- Its winds are the fastest in the Solar System.
- Its largest moon, Triton, orbits in the opposite direction to the planet's spin.
- Neptune radiates more than twice as much energy as it gets from the Sun.
Neptune compared with Earth
| Property | Neptune | Earth |
|---|---|---|
| Diameter | 49,528 km | 12,756 km |
| Mass (Earth = 1) | 17.1 | 1.000 |
| Gravity | 1.12 g | 1.00 g |
| Day | 16.1 h | 24.0 h |
| Year | 163.72 yr | 365.2 d |
| Mean temperature | -200°C | 15°C |
| Moons | 16 | 1 |