Distant object found orbiting Sun backwards

An object in the icy Kuiper belt has been found orbiting the Sun backwards, compared to most other objects in the solar system. It may help explain the origin of an enigmatic family of comets typified by Comet Halley.

The new object, called 2008 KV42, lies in the Kuiper belt, a ring of icy bodies beyond Neptune. Its orbit is inclined 103.5° to the plane of the Earth's orbit, or ecliptic. That means that as it orbits the Sun, it actually travels in the opposite direction to the planets.

Researchers led by Brett Gladman of the University of British Columbia first spotted the maverick object in May. Observations suggest it is about 50 kilometres across and travels on a path that takes it from the distance of Uranus to more than twice that of Neptune (or between 20 and 70 astronomical units from the Sun, with 1 AU being the Earth-Sun distance).

Its orbit appears to have been stable for hundreds of millions of years, but astronomers say it may have been born elsewhere. "It's certainly intriguing to ask where it comes from," says Brian Marsden of the Minor Planet Center in Cambridge, Massachusetts.

Gladman says it was probably born in the same place as Halley-type comets. These comets also travel on retrograde or highly tilted orbits – lasting between 20 and 200 years, but they come closer to the Sun.

It has been unclear where such comets come from. Computer models suggest they do not arise in either of the two birthplaces of other types of comets – the Kuiper belt or the much more distant Oort cloud, a shell of of icy bodies lying between 20,000 and 200,000 AU from the Sun.

Gladman's team calculates that 2008 KV42 arises beyond the Kuiper belt but closer than the Oort cloud, in a region thought to lie between 2000 to 5000 AU from the Sun. Some astronomers call the zone the inner Oort cloud.

A gravitational disturbance likely kicked 2008 KV42 out of the inner Oort cloud and to its present orbit. And Gladman says it might one day be pushed out of that orbit and into one that brings it closer to the Sun, making it a possible "transition object" on its way to becoming a Halley-type comet.

Gladman's team has found more than 20 other Kuiper belt objects with steeply inclined orbits while surveying the sky well away from the ecliptic – but no others with a retrograde orbit.