Neptune
Neptune is the eighth and most distant planet — a deep-blue ice giant with the fastest winds in the Solar System (up to 2,100 km/h). It was the first planet found by mathematical prediction.
Key facts
Science
Physical
- Mass
- 1.02 × 10²⁶ kg · 17.1 M⊕
- Radius
- 24,622 km · 3.86× wider than Earth
- Density
- 1,638 kg/m³ · 1.6× water
- Surface gravity
- 11.3 m/s² · 1.15× Earth's
- Escape velocity
- 23.6 km/s · 2.11× Earth's
- Axial tilt
- 28.32 °
Orbital
- Distance from the Sun
- 29.9 AU
- Semi-major axis
- 30.1 AU
- Eccentricity
- 0.01022
- Inclination
- 22.30 °
- Orbital period
- 164.97 years
- Perihelion
- 29.8 AU
- Aphelion
- 30.4 AU
- Mean orbital velocity
- 5.43 km/s
Rotation
- Rotation period
- 16.11 hours
Atmosphere
- Composition
- 80% H₂, 19% He, 1.5% CH₄
- Scale height
- 19.7 km
Rings
- Composition & structure
- Dark, reddish particles — likely water ice coated with a radiation-darkened, carbon-rich (organic) material, mixed with a high fraction of microscopic dust, so the rings are faint. The outermost Adams ring holds the bright, clumpy 'arcs' (Fraternité, Égalité, Liberté, Courage), confined by the shepherd moon Galatea. Established from stellar-occultation studies and Voyager 2 (1989). The precise composition of the dark component is not established.
Temperature
- Mean
- 72 K (-201 °C)
Composition
- Bulk composition
- Water/ammonia/methane ices over a rocky core
Radiation & temperature
- Insolation
- 0.00111 S⊕ · 1/905 of the sunlight Earth receives
- Bond albedo
- 0.29 · reflects 29% of incoming sunlight
- Equilibrium temperature
- 47 K (-227 °C)
- Measured mean temperature
- 72 K (-201 °C)
- Greenhouse warming
- +25 K
- Surface magnetic field
- 14 µT · 28% of Earth's
Discovery & history
- Discovered
- September 23, 1846
- By
- Johann Galle, from predictions by Urbain Le Verrier and John Couch Adams
- Method
- Mathematical prediction (gravity)
- Name
- The Roman god of the sea, for its deep blue colour.
Neptune is the only planet found by mathematics before it was seen. Uranus was drifting slightly off its predicted path, and Urbain Le Verrier (in France) and John Couch Adams (in England) independently calculated that the gravity of an unseen planet could explain it. On 23 September 1846 Johann Galle at the Berlin Observatory pointed his telescope where Le Verrier predicted and found Neptune within a degree of the spot — a stunning vindication of Newtonian gravity. Voyager 2 flew past in 1989, revealing supersonic winds and the moon Triton.
Images
Images from open archives · credit shown per image