Neptune Observatory
Neptune Data Center
Explore detailed data about Neptune's moons, frequently asked questions, mission highlights, and scientific measurements.
Neptune's Moons
| Discoverer | ||||
|---|---|---|---|---|
| Triton | 2,707 | -5.877 (retrograde) | 1846 | William Lassell |
| Proteus | 420 | 1.122 | 1989 | Voyager 2 |
| Nereid | 340 | 360.13 | 1949 | Gerard Kuiper |
| Larissa | 194 | 0.555 | 1981 | Harold Reitsema et al. |
| Galatea | 176 | 0.429 | 1989 | Voyager 2 |
| Despina | 150 | 0.335 | 1989 | Voyager 2 |
| Thalassa | 80 | 0.311 | 1989 | Voyager 2 |
| Naiad | 60 | 0.294 | 1989 | Voyager 2 |
| Hippocamp | 34 | 0.950 | 2013 | Mark Showalter |
Planet Data
Neptune Overview
Neptune is the eighth and farthest known planet from the Sun in the Solar System. It is the fourth-largest planet by diameter, the third-most-massive planet, and the densest giant planet. Neptune is 17 times the mass of Earth and is slightly more massive than its near-twin Uranus.
- Equatorial radius: 24,764 km
- Mass: 1.024 × 10²⁶ kg (17.15 Earth masses)
- Surface gravity: 11.15 m/s²
- Number of known moons: 16
- Ring system: 5 main rings
Atmospheric Composition
Neptune's atmosphere is composed primarily of hydrogen and helium, along with traces of hydrocarbons and possibly nitrogen. It contains a higher proportion of ices such as water, ammonia, and methane compared to Jupiter and Saturn.
- Hydrogen (H₂): 80 ± 3.2%
- Helium (He): 19 ± 3.2%
- Methane (CH₄): 1.5 ± 0.5%
- Hydrogen deuteride (HD): ~0.019%
- Ethane (C₂H₆): ~0.00015%
Orbital Mechanics
Neptune orbits the Sun at an average distance of 30.07 AU (4.5 billion km). Its orbital period is approximately 164.8 Earth years, meaning it has completed just one full orbit since its discovery in 1846.
- Semi-major axis: 30.07 AU
- Orbital period: 164.8 years
- Orbital eccentricity: 0.0086
- Orbital inclination: 1.77°
- Average orbital speed: 5.43 km/s
Interior Structure
Neptune's internal structure resembles that of Uranus. The planet likely has a rocky core composed of silicates and iron-nickel, surrounded by a mantle of water, methane, and ammonia ices. The mantle may contain a vast ocean of super-hot, electrically conductive water.
- Core mass: ~1.2 Earth masses
- Core temperature: ~5,400 K
- Core pressure: ~7 Mbar
- Mantle composition: Water, methane, ammonia ices
- Magnetic field tilt: 47° from rotational axis
Frequently Asked Questions
Neptune's blue color is the result of methane in the atmosphere absorbing red wavelengths of sunlight and reflecting blue wavelengths back into space. However, methane alone doesn't fully explain the vivid blue — an unknown atmospheric component likely contributes to the deeper coloration compared to Uranus.
The distance between Earth and Neptune varies significantly due to their different orbital positions. At its closest approach (opposition), Neptune is about 4.3 billion km (28.8 AU) from Earth. At its farthest, the distance can reach about 4.7 billion km (31.3 AU). Light takes approximately 4 hours to travel from Neptune to Earth.
Only one spacecraft has visited Neptune: Voyager 2, which made its closest approach on August 25, 1989. During its flyby, Voyager 2 discovered six new moons, observed Neptune's ring system, and studied the planet's atmosphere and magnetic field. No dedicated Neptune mission has been launched since, though several have been proposed.
Neptune has the fastest winds in the solar system, reaching speeds of up to 2,100 km/h (580 m/s). These winds blow primarily in a direction opposite to the planet's rotation (westward). The source of energy driving these extreme winds remains a puzzle, as Neptune receives very little solar energy due to its distance from the Sun.
Neptune itself is extremely unlikely to harbor life as we know it, due to extreme temperatures, pressures, and lack of a solid surface. However, its moon Triton is of some interest — with active geysers, a thin atmosphere, and a subsurface ocean possibly maintained by tidal heating, Triton could theoretically have environments where simple life might exist.