Triton orbits Neptune in a retrograde orbit—revolving in the opposite direction to the parent planet's rotation—the only large moon in the Solar System to do so.

Triton is differentiated, with a crust of primarily ice atop a probable subsurface ocean of liquid water and a solid rocky-metallic core at its center.

When John Herschel received news of Neptune's discovery, he wrote to Lassell suggesting he search for possible moons.

A brewer by trade, Lassell spotted Triton with his self-built 61 cm (24 in) aperture metal mirror reflecting telescope (also known as the "two-foot" reflector).

Viscoelastic damping from tides alone is not thought to be capable of circularizing Triton's orbit in the time since the origin of the system, and gas drag from a prograde debris disc is likely to have played a substantial role.

[26] This will result in either a collision with Neptune's atmosphere or the breakup of Triton, forming a new ring system similar to that found around Saturn.

Astrophysicists believe it might have originated in the Kuiper belt,[14] a ring of small icy objects extending from just inside the orbit of Neptune to about 50 AU from the Sun.

[27] This has been further supported in a 2024 study of the chemical composition of Pluto and Triton which suggests they originated in the same region of the outer Solar System before the latter was pulled into Neptune’s orbit.

Kathleen Mandt at NASA's Goddard Space Flight Center in Maryland and her colleagues hypothesize that Triton and Pluto formed close to each other before the Solar System settled down.

"They had to have formed beyond the water-ice line," says Mandt, referring to the distance from the sun where water would freeze into ice or snow, which is why Triton and Pluto have similar amounts of certain key elements.

"One possibility is that the giant planets moved closer to the sun early in the first 100 million years or so of the Solar System, which may have disrupted the orbits of some bodies like Triton.

To be gravitationally captured by a planet, a passing body must lose sufficient energy to be slowed down to a speed less than that required to escape.

[32][33] Triton is the seventh-largest moon and sixteenth-largest object in the Solar System and is modestly larger than the dwarf planets Pluto and Eris.

[39][40] Triton's reddish color is thought to be the result of methane ice, which is converted to tholins under exposure to ultraviolet radiation.

Water, the most abundant volatile in the Solar System, comprises Triton's mantle, enveloping a core of rock and metal.

There is enough rock in Triton's interior for radioactive decay to maintain a liquid subsurface ocean to this day, similar to what is thought to exist beneath the surface of Europa and several other icy outer Solar System worlds.

However, the strong obliquity tides are believed to generate enough additional heat to accomplish this and produce the observed signs of recent surface geological activity.

[44] The black material ejected is suspected to contain organic compounds,[43] and if liquid water is present on Triton, it has been speculated that this could make it habitable for some form of life.

Streaks on Triton's surface left by geyser plumes suggest that the troposphere is driven by seasonal winds capable of moving material over a micrometer in size.

[7] The temperature of Triton's upper atmosphere, at 95±5 K, is higher than that at its surface, due to heat absorbed from solar radiation and Neptune's magnetosphere.

[48][53] A haze permeates most of Triton's troposphere, thought to be composed largely of hydrocarbons and nitriles created by the action of sunlight on methane.

[55] These observations indicated Triton was approaching an unusually warm southern hemisphere summer season that happens only once every few hundred years.

[58] The 40% of Triton's surface imaged by Voyager 2 revealed blocky outcrops, ridges, troughs, furrows, hollows, plateaus, icy plains, and a few craters.

[66] The Voyager 2 probe in 1989 observed a handful of geyser-like eruptions of nitrogen gas or water and entrained dust from beneath the surface of Triton in plumes up to 8 km high.

[71] However, the significant geological activity on Triton has led to alternative proposals that the plumes may be cryovolcanic in nature, rather than driven by solar radiation.

Triton's south polar region is covered by a highly reflective cap of frozen nitrogen and methane sprinkled by impact craters and openings of geysers.

[50] The high plains found on Triton's eastern hemisphere, such as Cipango Planum, cover over and blot out older features, and are therefore almost certainly the result of icy lava washing over the previous landscape.

[73] There are long double ridges of ice with central troughs bearing a strong resemblance to Europan lineae (although they have a larger scale[74]), and which may have a similar origin,[7] possibly shear heating from strike-slip motion along faults caused by diurnal tidal stresses experienced before Triton's orbit was fully circularized.

[76] Due to constant erasure and modification by ongoing geological activity, impact craters on Triton's surface are relatively rare.

[82] In the 1990s, various observations from Earth were made of the limb of Triton using the occultation of nearby stars, which indicated the presence of an atmosphere and an exotic surface.