Canberra, April 23 : Astronomers have found a mysterious clan of icy objects in the outer solar system, which look much younger than their years.
According to a report in New Scientist, the youthful looking family of objects were found in the uiper Belt, a ring of icy objects beyond Neptune.
They appear puzzlingly fresh-faced, despite the fact that they probably formed in a collision more than a billion years ago.
The largest member of the family, a rapidly tumbling blimp-shaped object called 2003 EL61, was discovered in 2005.
In 2007, astronomers found five smaller objects travelling in similar orbits. Their paths suggested they all formed a single object that was broken apart in a collision more than a billion years ago.
Now, a team led by David Rabinowitz of Yale University in New Haven, Connecticut, US, reported that the brightness of the large object and four of the smaller ones changes little when observed from various points along Earth's orbit.
That suggests their surfaces are covered with fresh powdery ice no more than 100 million years old. The researchers also suspect it means the surfaces are bright, though they haven't directly measured how much light the objects reflect.
"A fresh surface is understandable for the biggest object, 2003 EL61, because it is large enough to hold an atmosphere that vaporises and refreezes regularly. But the four smaller family members are too small to hold atmospheres," said Rabinowitz.
In fact, no other small objects in the outer solar system have been found with bright young surfaces.
They are thought to darken over time as solar ultraviolet radiation and charged particles called cosmic rays break down carbon-rich ices such as methane. This 'space weathering' leaves behind dark, reddish carbon compounds.
"Something fishy is going on with this family," said Mike Brown of Caltech in Pasadena, US. "We sure have a lot to learn about what happens to ice in the outer solar system," he added.
For now, the objects' youthful appearance may have to remain a mystery.
"You'd have to go there with a spacecraft to learn what the surface really is like," Rabinowitz told New Scientist.