Astronomers are puzzling over an unusually shiny space rock orbiting billions of kilometres out from the Sun.

A team led by Professor James Elliot of Massachusetts Institute of Technology report their study of an object in the Kuiper Belt today in the journal Nature.

The Kuiper Belt, which orbits just beyond Neptune, is home to thousands of small remnants from the Solar System's formation called Kuiper Belt Objects, or KBOs.

Because they are so tiny and far away, it's difficult to examine KBOs directly, but scientists can study them when they block the light of a star they pass in front of.

They can work out its diameter by comparing the transit times seen at two or more telescopes that fall under the path of the shadow.

Stellar occultation

Elliot and colleagues used this method of 'stellar occultation' to study one KBO, labelled 55636.

Using a network of 20 telescopes across the globe, the team had 10 seconds during which to observe KBO 55636 and calculate the mean radius as 143 kilometres.

They then combined this diameter measurement with well-known measurements of KBO 55636's brightness to calculate the albedo, or reflectivity, of the object.

The albedo was a lot greater than previously thought implying the presence of pristine water-ice on its surface.

KBO 55636 is one of a number of KBOs formed from a strange football-shaped dwarf planet, called Haumea, which spins once every four hours.

It is thought that Haumea was involved in a collision about 1 billion years ago. There is some evidence that the resulting 'chips' are shinier than other KBOs.

But this is the first time a Haumea chip's exact diameter and albedo has been measured accurately, and it's confirming the group of KBOs are a strange lot indeed.

"The surprising thing is that the ice has somehow remained fresh for a billion years," says Michele Bannister, an Australian researcher in Kuiper Belt Objects, also known as Trans-Neptunian Objects, at Mount Stromlo Observatory in Canberra.

Space-weathering

She says ice-covered KBOs generally get less shiny with what is called "space-weathering" by the solar wind, dust and other space debris.

"If you think of the snowfields in the Snowys [Snowy Mountains], you have everything from dust to gum leaves dropping on it," says Bannister.

"A snow field won't stay beautiful and bright once it's fallen out of the sky."

But for some reason a billion years of space-weathering doesn't seem to have affected KBO 55636.

"We don't understand why this object is as shiny and bright as it appears to be. It should have been weathered into being quite dark," says Bannister.

Bannister says the new findings might help scientists in their search to understand how these Haumea chips were formed and why they are so dark.

She says one explanation is cryovolcanism - volcanoes made of water ice - as seen on Saturn's moon Enceladus. But, she says, KBO 55636 is way too small to support such volcanoes.

"Basically we don't understand how this space weathering process works," says Bannister.

"This is exciting. Hopefully we'll now have information to tell us a new theory about space weathering."

This is the first time occultation has been used for an object of this size.

"The logistics involved in organising 20 telescopes spread half way around the world to observe something like this are horrendous," says Bannister.