Pluto, which was until recently regarded as the outermost and smallest planet in the solar system, has never been visited by an exploring spacecraft. So little is known about it that it is difficult to classify. Its distance from Earth is so great that the Hubble Space Telescope cannot reveal its surface features. Appropriately named for the Roman god of the underworld, it must be frozen, dark, and dead. Its mean distance from the Sun is 5,900 million kilometres. In fact, it has the most eccentric orbit in the solar system, bringing it at times closer to the Sun than Neptune. Furthermore, there is evidence that Pluto has an atmosphere, containing methane, and a polar ice cap that increases and decreases in size with Pluto’s seasons. It is not known to have water. The Hubble Space Telescope’s faint-object camera revealed light and dark regions on Pluto, indicating an ice cap at the north pole. It is not known if there is an ice cap at Pluto’s south pole.