8th planet from the Sun
Neptune
At the edge of the Solar System, where the Sun is just a bright star, Neptune defies every expectation: its winds approach 2,100 km/h, the fastest ever measured on a planet. It was discovered in 1846 not with a telescope, but with pen, paper and Newton's laws.
- Diameter
- 49,528 km
- Distance from Sun
- 4,515 M km
- Orbital period
- 164.8 years
- Day length
- 16h 06m
- Moons
- 16
- Average temp.
- -214 °C
The planet discovered on paper
Neptune is the only planet discovered through mathematics before observation. Anomalies in Uranus's orbit hinted at a perturbing body: Urbain Le Verrier calculated its position, and on 23 September 1846 astronomer Johann Galle found it less than one degree from the predicted spot. A triumph of Newton's celestial mechanics.
Since then Neptune has completed barely one orbit: its year lasts 164.8 Earth years. Only one spacecraft has visited it, Voyager 2, which in 1989 revealed its deep blue colour, its faint rings and a giant dark storm the size of Earth, since vanished.
The fastest winds in the Solar System
Winds on Neptune blow at up to 2,100 km/h, nearly twice the speed of sound on Earth: a paradox for a planet that receives 900 times less sunlight than we do. Much of the energy comes from within: Neptune radiates 2.6 times more heat than it receives from the Sun.
Like Uranus, it is an ice giant: beneath the hydrogen, helium and methane atmosphere hides a mantle of water and ammonia at thousands of degrees, squeezed to enormous pressures. Some models predict that in its depths carbon may crystallise into a "diamond rain".
Triton, the rebel moon
Neptune's largest moon, Triton, is unique: it orbits in a retrograde direction, against the planet's rotation — a sign it was not born there but captured from the Kuiper Belt, Pluto's home region. At -235 °C it is one of the coldest places in the Solar System, yet geologically active.
Voyager 2 photographed nitrogen geysers there rising 8 km from a surface of nitrogen and methane ice. Its retrograde orbit comes at a price, though: tides are slowly dragging it towards Neptune, and in billions of years Triton will be torn apart, gifting the planet a great ring.
The 16 moons of Neptune
Neptune has 16 confirmed natural satellites (IAU / Minor Planet Center figures as of June 2026). Of those, 14 have received an official name; the other 2 so far carry only a provisional designation, such as S/2002 N 5 and S/2021 N 1.
Inner regular moons 7
- Naiad
- Thalassa
- Despina
- Galatea
- Larissa
- Hippocamp
- Proteus
Outer moons 7
- Triton
- Nereid
- Halimede
- Sao
- Laomedeia
- Psamathe
- Neso
Frequently asked questions
How was Neptune discovered?
Through mathematics: Urbain Le Verrier deduced the position and mass of an unknown planet from anomalies in Uranus's orbit. Johann Galle observed it from the Berlin Observatory on 23 September 1846, less than a degree from the calculated position.
Why are Neptune's winds so strong?
The main engine is internal heat: Neptune radiates 2.6 times the energy it receives from the Sun. Moreover, its cold, low-turbulence atmosphere offers very little friction: once triggered, winds can accelerate to 2,100 km/h with almost nothing to slow them down.
What is Triton?
It is Neptune's largest moon (2,707 km across) and the only large moon in the Solar System with a retrograde orbit: it circles against the planet's rotation. It is probably a Kuiper Belt world captured by Neptune's gravity, a close relative of Pluto.
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