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Answer: Triton is the largest moon of Neptune and the only large moon in the solar system that orbits in the opposite direction of its planet's rotation, a retrograde orbit. Scientists think it is a Kuiper Belt Object captured by Neptune's gravity millions of years ago.

Key facts

• Discovery: William Lassell discovered Triton on Oct. 10, 1846, just 17 days after Neptune itself was discovered.
• Like Pluto: Triton shares many similarities with Pluto, the best known world of the Kuiper Belt.
• Rotation: like our own moon, Triton is locked in synchronous rotation with Neptune, so one side faces the planet at all times.
• Atmosphere: a thin atmosphere composed mainly of nitrogen with small amounts of methane.
• Very cold: most of Triton's nitrogen is condensed as frost, giving its surface an icy sheen that reflects 70 percent of the sunlight that hits it.

Active geology

Voyager 2 is the only spacecraft to fly past Neptune and Triton. During its 1989 flyby it found surface temperatures of -391 degrees Fahrenheit (-235 degrees Celsius) and active geysers, making Triton one of the few geologically active moons in the solar system. Triton, Io and Venus are the only bodies in the solar system besides Earth that are known to be volcanically active at the present time.

A note on the moon count

The two NASA pages disagree: the Triton page calls it the largest of Neptune's 13 moons, while the Neptune moons page says Neptune has 16 officially recognized moons.

Claims

• id: C1 claim: Triton's thin atmosphere is mainly nitrogen with some methane. source: S1 evidence: "Triton's thin atmosphere is composed mainly of nitrogen with small amounts of methane."
• id: C2 claim: Triton shares many similarities with Pluto, a Kuiper Belt world. source: S1 evidence: "It shares many similarities with Pluto, the best known world of the Kuiper Belt."
• id: C3 claim: Lassell discovered Triton on Oct. 10, 1846. source: S1 evidence: "Triton was discovered on Oct. 10, 1846 by British astronomer William Lassell"
• id: C4 claim: Neptune has 16 officially recognized moons. source: S2 evidence: "Neptune has 16 officially recognized moons."

Sources

• [S1] Triton. NASA. https://science.nasa.gov/neptune/moons/triton/ . License: US government work, public domain. Accessed 2026-11-04.
• [S2] Neptune Moons. NASA. https://science.nasa.gov/neptune/moons/ . License: US government work, public domain. Accessed 2026-11-04.

License: this note is CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Summary by iinventedlight (AI-assisted).

@misc{iil_note_ece7e0yd,
  author = {{iinventedlight}},
  title = {{Triton: Neptune’s backward-orbiting moon}},
  howpublished = {iinventedlight Library},
  year = {2026},
  date = {2026-11-04},
  url = {https://iinventedlight.com/library/notes/triton-moon/},
  note = {Permanent ID iil:note:ece7e0yd. License: CC-BY-4.0.},
}

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