The orbit of 2006 SQ372 is an ellipse four times longer than it is wide, says University of Washington astronomer Andrew Becker, who led the research team.

Sedna, a distant, Pluto-like dwarf planet discovered in 2003, is the only other object with a similar orbit, but not nearly as stretched out.

The new object is about 100 kilometres in diameter.

"It's basically a comet, but it never gets close enough to the Sun to develop a long, bright tail of evaporated gas and dust," says Becker.

Given the boot

University of Washington graduate student Nathan Kaib says it is unclear how the object formed.

"It could have formed, like Pluto, in the belt of icy debris beyond Neptune, then been kicked a large distance by a gravitational encounter with Neptune or Uranus," says Kaib.

More likely, he says, it came from the Oort Cloud, a distant reservoir of icy, asteroid-like bodies that orbit the Sun at distances of several trillion kilometres.

"One of our goals is to understand the origin of comets, which are among the most spectacular celestial events. But the deeper goal is to look back into the early history of our solar system and piece together what was happening when the planets formed," says Kaib.

One of many

"Almost everything people have discovered in these surveys has nice circular orbits around the Sun," he says. "There are only three or four like this one that have elongated orbits that are currently known."

Francis believes there are probably thousands more of these objects orbiting the outer edge of our solar system, yet to be found.

"We've seen this one at its closest encounter. For every one we see, there must be hundreds 240 billion kilometres away," Francis says.

Despite being several billion kilometres from Earth, we may see one of these distant objects close up courtesy of a spacecraft on its way to Pluto.

"NASA has a probe called New Horizons currently on the way to Pluto," says Francis. "Its plan after it goes past Pluto is to maybe visit one of these icy worlds, but probably not this one."