Proteus is one of the darkest objects in the solar system—“as dark as soot” is a good description. It reflects only 6% of the sunlight that strikes it. Proteus is an ellipsoid about 258 mi (416 km) in diameter, larger than Nereid. It circles Neptune at a distance of about 57,700 mi (92,800 km) above the cloud tops, and completes one orbit in 26 hours and 54 minutes. Scientists say that it is about as large as a satellite can be without being pulled into a spherical shape by its own gravity.
Proteus and its tiny companions are cratered and irregularly shaped—they are not round—and show no signs of any geologic modifications. All circle the planet in the same direction as Neptune rotates and remain close to Neptune's equatorial plane.