Neptune, the most distant major planet, is a dark, cold world whipped by supersonic winds. Notably, it was the first planet predicted by mathematics before being observed by telescope.
Dark, cold, and swept by supersonic winds, Neptune reigns as the most distant planet in our solar system.
Over 30 times farther from the Sun than Earth, it is invisible to the naked eye. In 2011, it completed
its first 165-year orbit since its 1846 discovery.
At this distance, high noon on Neptune resembles a dim twilight on Earth; sunlight is roughly 900 times
fainter there.
Neptune holds the distinction of being the first planet found via mathematical prediction. Urbain Le
Verrier calculated its position, leading Johann Galle to the discovery in 1846. It is named after the
Roman god of the sea.
Voyager 2: Launched in 1977, Voyager 2 is the only spacecraft to visit Neptune. While its twin,
Voyager 1, is also in interstellar space, Voyager 2's tour included all four gas giants.
Its 1989 flyby of Neptune altered its trajectory, sending it below the elliptic plane and eventually
into interstellar space in November 2018. Although operating with reduced instruments to conserve power,
Voyager 2 continues to transmit data and is expected to function until at least 2025.
With a radius of 15,299.4 miles (24,622 kilometers), Neptune is roughly four times wider than Earth. If
Earth were a nickel, Neptune would be a baseball.
Averaging 2.8 billion miles (4.5 billion kilometers) from the Sun, Neptune sits 30 astronomical units
(AU) away. Sunlight takes 4 hours to reach the planet.
A day on Neptune lasts about 16 hours, while a single orbit around the Sun takes 165 Earth years (60,190
days).
Due to Pluto's eccentric orbit, Neptune is sometimes farther from the Sun than the dwarf planet. Every
248 years, Pluto slips inside Neptune's orbit for 20 years (most recently 1979-1999). However, orbital
resonance ensures they never collide; for every three Neptune orbits, Pluto completes two.
Neptune's axis is tilted 28 degrees, similar to Earth/Mars, creating seasons. However, each season lasts
over 40 years.
Neptune formed about 4.5 billion years ago from swirling gas and dust. Like Uranus, it likely formed closer to the Sun before migrating to the outer solar system around 4 billion years ago.
Neptune, like Uranus, is an ice giant. Over 80% of its mass is a hot, dense fluid of "icy" materials
(water, methane, ammonia) surrounding a small rocky core. It is the densest of the giant planets.
A subsurface ocean of superheated water may exist beneath the cold clouds, kept liquid by immense
pressure.
Neptune lacks a solid surface. Its atmosphere merges gradually into water and other melted ices, eventually reaching a solid core with a mass comparable to Earth.
The atmosphere is dominated by hydrogen and helium, with trace methane. While Uranus appears blue-green,
Neptune is a vivid blue; an unknown component likely intensifies the color.
Neptune is the solar system's windiest world, with winds reaching 1,200 mph (2,000 kph)—three times
Jupiter's and nine times Earth's strongest winds.
In 1989, Voyager 2 observed the "Great Dark Spot," a storm large enough to engulf Earth. Though it has
vanished, new storms continue to appear.
Neptune's extreme temperatures, pressures, and volatility make it hostile to life as we know it.
Neptune has 14 known moons. Triton, the largest, was discovered just 17 days after Neptune itself. In
keeping with the Roman sea god theme, moons are named after lesser sea gods and nymphs.
Triton is unique as the only large moon with a retrograde orbit, suggesting it was captured. It is
extremely cold (-391°F / -235°C), yet Voyager 2 observed geysers erupting 5 miles high. Its thin
atmosphere is warming, though the cause remains unknown.
Neptune has at least five main rings (Galle, Leverrier, Lassell, Arago, Adams) and four prominent ring
arcs. The rings are likely young and short-lived.
Unique to Neptune are its ring arcs: Liberté, Egalité, Fraternité, and Courage. These clumps of dust in
the Adams ring should theoretically spread out but are likely stabilized by the moon Galatea.
| Name | Distance from Planet's Center | Radial Width |
|---|---|---|
| Galle | ~26,000 miles (41,900 kilometers) | 9.3 miles (15 kilometers) |
| Leverrier | ~33,100 miles (53,200 kilometers) | 9.3 miles (15 kilometers) |
| Lassell | ~34,400 miles (55,400 kilometers) | - |
| Arago | ~35,800 miles (57,600 kilometers) | - |
| Adams | ~39,100 miles (62,930 kilometers) | < 31 miles (50 kilometers) |
| Liberté (Arc) | ~39,100 miles (62,900 kilometers) | - |
| Egalité (Arc) | ~39,100 miles (62,900 kilometers) | - |
| Fraternité (Arc) | ~39,100 miles (62,900 kilometers) | - |
| Courage (Arc) | ~39,100 miles (62,900 kilometers) | - |
Neptune's magnetic field is tipped 47 degrees from its axis and offset from its center. Like Uranus, this causes wild variations in the magnetosphere as the planet rotates. The field is about 27 times stronger than Earth's.