A bizarre mini-planet that orbits the Sun in deep space is covered with crystallised water, European astronomers reported on Thursday.

Around 2,000 kilometres (1,200 miles) long, the "dwarf planet" of Haumea lurks in the Kuiper Belt, which comprises rocks that move in a lonely orbit beyond Neptune.

Haumea, named after a Hawaiian goddess of fertility and childbirth, was discovered in 2004 but detailed views of it are very rare and even now its precise orbit and size are unknown.

New sightings by a team from the European Southern Observatory (ESO) using a giant telescope in the Chilean desert describe Haumea as having the shape of a "flattened rugby ball."

Despite its immense distance from Earth, Haumea shines brightly in space thanks to a reflective surface of water ice that covers a rocky body.

Three-quarters of Haumea and all of Hi'aka, one of two satellites along with Namaka, have this icy coating, ESO said.

The ice is unusual because it is crystalline, meaning that it has an ordered structure, it said in a press release.

Crystalline structures are destroyed by sunlight, so other energy sources are needed to keep the ice organised on Haumea and Hi'aka, rather than meld into shapeless, amorphous frost.

The ESO scientists believe that two sources are available for doing this -- a trio of radiogenic elements (potassium-40, thorium-232 and uranium-238) that lie beneath the icy crust and "tidal forces" provided by the gravitational give-and-take between Haumea and her daughters.

Haumea spins completely in less than four hours, giving it one of the fastest rotation speeds in the Solar System.

The mini-planet's strange shape may derive from the ultra-fast spin, which could have been conferred by a tangential collision with another space rock.

The whack could also have gouged out two splinters which became Hi'aka and Namaka.

It is the fifth largest "dwarf planet," a category that was created in 2006 by the International Astronomical Union (IAU) and is headed by Pluto, which was formerly designated a full-fledged planet.

The two satellites of Haumea are named after the goddess's daughters. Hi'aka measures around 400 kms (250 miles) across, and Namaka is around 200 kms (120 miles).