Neptune

Neptune was found after astronomers noticed that Uranus did not move exactly as expected. Calculations by Urbain Le Verrier and John Couch Adams pointed to the position of an unseen planet, which was first observed through a telescope on 23 September 1846.

In amateur instruments Neptune appears as a faint bluish disk. At about magnitude 7.7 it can be reached with binoculars under suitable conditions, but its surface cannot be resolved from Earth. Voyager 2 images from 1989 revealed a large oval cloud feature, the Great Dark Spot, and showed that atmospheric features move around the planet in roughly 16 hours, with strong latitude-dependent differences.

Like Uranus, Neptune is an ice giant dominated by hydrogen, with substantial methane and helium in its atmosphere. The methane contributes to its blue color, and the planet is encircled by a very faint ring system.

Before Voyager 2 arrived, only Triton and Nereid were known. The spacecraft added six more faint moons, all dimmer than 20th magnitude. Triton is much brighter at about magnitude 13.6, slightly smaller than Earth's Moon, and among the coldest worlds measured in the solar system at roughly -236 °C.