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Moons of Neptune

Triton: Neptune's backwards moon

Neptune has 16 known moons, but one dominates: Triton holds more than 99% of the mass of everything orbiting Neptune. And it goes around the wrong way.

Triton's pinkish and grey icy south polar cap with dark streaks from nitrogen geysers

NASA/JPL/USGS, Voyager 2 (PIA00317), 1989. Public domain.

Meet the moons

Triton

Diameter
2,707 km
Surface temperature
about −235 °C
Orbit
retrograde (backwards)

Triton orbits Neptune in the opposite direction to Neptune's spin, the only large moon in the solar system that does. That almost certainly means it did not form there: Triton was probably a dwarf planet from the Kuiper Belt, like Pluto, captured by Neptune's gravity.

When Voyager 2 flew past in August 1989 it found a young surface with "cantaloupe" terrain and dark streaks left by geysers of nitrogen gas, shooting about 8 km high from a frozen surface.

Tides are slowly dragging Triton inwards. In a few billion years it will get close enough to be torn apart, possibly giving Neptune a spectacular ring system.

Thin crescents of Neptune and its moon Triton seen from behind by a departing spacecraft
The thin crescents of Neptune and Triton, seen by Voyager 2 as it left the system in 1989.NASA/JPL, Voyager 2 (PIA02215), Aug 1989. Public domain.

The other moons

Proteus (about 420 km) is one of the largest irregularly shaped moons known. Nereid has one of the most stretched-out orbits of any moon. Several small moons orbit among Neptune's faint rings, and more distant ones were found by telescopes on Earth.

Could life be here?

Our rough guide, from 0 (no chance) to 5 (among the best places to look). No life has been found beyond Earth.

Could life be here? Triton

  • May have an ocean under its ice shell, warmed by tides from its capture
  • Active geysers mean energy reaches the surface
  • Extremely cold surface
  • Hardly any data since 1989

Verdict: Possible ocean world, worth a future mission.

Quick answers

Why does Triton orbit backwards?

Because it was probably captured from the Kuiper Belt rather than forming around Neptune.

Does Triton have geysers?

Yes. Voyager 2 saw plumes of nitrogen gas and dark dust rising about 8 km above its surface in 1989.

How many moons does Neptune have?

16 known moons as of 2026.

Sources and credits

  1. NASA Science, Triton.
  2. NASA/JPL Voyager 2 Neptune encounter, August 1989.

Photos credited to NASA, NOAA and other public-domain sources are used with thanks; BotBuhdy adds a small botbuhdy.com mark. Pictures labelled AI-generated illustration are artist-style illustrations made for this page, not photographs.