Space · Planets
Neptune
The outermost planet — predicted by mathematics before it was seen, with the fastest winds in the solar system (up to 2,100 km/h).
Quick facts
Type
Ice giant
Distance from Sun
30.070 AU
4495.1 million km
Diameter
49,528 km
Mass
17.1 Earth masses
1.024 × 10²⁶ kg
Surface gravity
11.15 m/s²
Day length
16.11 Earth hours
Year length
59800 Earth days
163.7 Earth years
Mean surface temperature
−201 (cloud-top) °C
Atmosphere
80% H₂, 19% He, 1.5% CH₄
Confirmed moons
16
Naming origin
Roman god of the sea
What's there
Neptune is a deep azure ice giant similar in composition and size to Uranus, but more dynamic. Its atmosphere drives the fastest winds measured on any planet — sustained at 580 m/s (about 2,100 km/h), nearly five times the strongest tropical cyclone winds on Earth — despite receiving only 0.001 of Earth's solar flux. Voyager 2's flyby in 1989 imaged a Great Dark Spot, a Jupiter-Great-Red-Spot-style anticyclonic storm, that had dissipated by the time Hubble re-observed in the mid-1990s; Neptune routinely grows and loses such storms on the scale of years.
Neptune's largest moon, Triton, orbits backward (retrograde) relative to Neptune's rotation, the only large moon in the solar system to do so. The strong consensus is that Triton is a captured Kuiper Belt object similar to Pluto, not a native Neptunian moon — when Neptune's gravity caught Triton, it disrupted the planet's original moon system and may have ejected or destroyed most of the original satellites. Voyager 2 in 1989 remains Neptune's only visitor; a Trident mission concept (Neptune flyby with Triton focus) was an early-2020s NASA finalist but did not advance to flight.
Who's been there
| Mission | Encounter | Year | Status | Primary objective |
|---|---|---|---|---|
|
Voyager 2 NASA |
Flyby | 1989 | Completed | Only spacecraft ever to visit Neptune; imaged Triton's geysers; discovered 6 of the 16 known moons. |
Moons
| Name | Diameter | Orbital radius | Discovered | Discoverer |
|---|---|---|---|---|
| Naiad | 66 km | 48,227 km | 1989 | Voyager 2 imaging team |
| Thalassa | 82 km | 50,074 km | 1989 | Voyager 2 imaging team |
| Despina | 150 km | 52,526 km | 1989 | Voyager 2 imaging team |
| Galatea | 175 km | 61,953 km | 1989 | Voyager 2 imaging team |
| Larissa | 194 km | 73,548 km | 1989 | Voyager 2 imaging team (Harold Reitsema observed 1981) |
| Hippocamp | 17 km | 105,283 km | 2013 | Mark R. Showalter (Hubble Space Telescope) |
| Proteus | 420 km | 117,647 km | 1989 | Voyager 2 imaging team |
| Triton | 2707 km | 354,759 km | 1846 | William Lassell |
| Nereid | 340 km | 5,513,400 km | 1949 | Gerard P. Kuiper |
| Halimede | 62 km | 16,611,000 km | 2002 | Matthew J. Holman et al. |
| Sao | 44 km | 22,228,000 km | 2002 | Matthew J. Holman et al. |
| Laomedeia | 42 km | 23,567,000 km | 2002 | Matthew J. Holman et al. |
| S/2002 N 5 | 23 km | 23,571,000 km | 2021 | Scott S. Sheppard et al. (announced 2024) |
| Psamathe | 40 km | 46,695,000 km | 2003 | Scott S. Sheppard et al. |
| Neso | 60 km | 50,258,000 km | 2002 | Matthew J. Holman et al. |
| S/2021 N 1 | 14 km | 50,300,000 km | 2021 | Scott S. Sheppard et al. (announced 2024) |
Naming etymology
Neptune was the first planet found by mathematical prediction rather than by accidental observation. In the 1840s, irregularities in Uranus's orbit suggested an unseen outer body perturbing it; Urbain Le Verrier in France and John Couch Adams in England independently calculated where the perturber would have to be. Johann Galle at the Berlin Observatory pointed his telescope at Le Verrier's coordinates on the night of September 23, 1846, and found the new planet within 1° of the prediction. Le Verrier proposed 'Neptune' — the Roman god of the sea — for the deep-blue color the planet showed in early telescopes, and the name stuck within weeks across all the major European observatories. Neptune's moons are named after Greek sea deities and nymphs (Triton, Nereid, Naiad, etc.) to match the planet's namesake.
Methodology & sources
Numerical data drawn from the NASA Planetary Fact Sheets; satellite parameters from JPL Solar System Dynamics — Physical Parameters and the JPL Satellite Discovery Circumstances table. Mission history cross-referenced against NASA's mission catalog and individual mission pages. Naming etymology from the IAU Working Group for Planetary System Nomenclature. Hero rendering is a stylized SVG composed from primary-source visual reference (NASA / JPL imagery) — no photographs are reproduced.