Overview Proteus is one of the largest of Neptune's known moons, although it is not as big as Triton. The moon has an odd box-like shape and if it had just a little more mass it would be able to transform into a sphere. Proteus orbits Neptune about every 27 hours.
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Scientists think Triton is a Kuiper Belt Object captured by Neptune's gravity millions of years ago. It shares many similarities with Pluto, the best known world of the Kuiper Belt. Like our own moon, Triton is locked in synchronous rotation with Neptune―one side faces the planet at all times.
As Uranus and Neptune drifted farther outward, they passed through the dense disk of small, icy bodies left over after the giant planets formed. Neptune's orbit was the farthest out, and its gravity bent the paths of countless icy bodies inward toward the other giants.
In the outer solar system, the gas giants Jupiter and Saturn and the ice giants Uranus and Neptune have dozens of moons. As these planets grew in the early solar system, they were able to capture smaller objects with their large gravitational fields.
Our solar system consists of our star, the Sun, and everything bound to it by gravity – the planets Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune; dwarf planets such as Pluto; dozens of moons; and millions of asteroids, comets, and meteoroids.
In 1943, astronomer Kenneth Edgeworth hypothesized that many small, icy bodies exist in a disc in the region beyond Neptune, having condensed from widely spaced ancient material, and that from time to time one of them visits the inner solar system.
