The object, dubbed Quaoar, inhabits the Kuiper Belt, an area beyond the orbit of Neptune that is dotted with rubble left over from the building of the Solar System.

Quaoar has a diameter of about 1,300 kilometers (800 miles), making it about half the size of Pluto, the most distant planet from the Sun.

In a study published on Thursday in the British journal Nature, US astronomers David Jewitt and Jane Luu of the Institute for Astronomy in Hawaii and the Massachusetts Institute of Technology (MIT) say Quaoar has crystalline water on its surface, and possibly ammonia too.

That finding, derived by infrared photography, is a surprise, for the Kuiper Belt is so far from the Sun that the temperature there is less than 50 C (90 F) above absolute zero (-273 C, -459 F).

Crystalline ice is thought to form at 100 C (180 C F) above absolute zero.

Jewitt and Luu conclude that the ices may have been in Quaoar's interior and were exposed by an impact with another rock that smashed away part of its surface.

Alternatively, the ices could have been formed by outgassing from a volcano, they theorise.

Quaoar (pronounced kwah-o-wahr), spotted in 2002, was named in honour of the Tongva people who inhabited southern California before the arrival of Europeans.

In their creation mythology, Quaoar is a primeval force who, by singing and dancing, creates the deities.

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