Orbiting the Solar System Backwards

The solar system we're familiar with from elementary school posters and dioramas has nine (now eight) planets and an Asteroid Belt all orbiting the sun rather neatly, chugging along on nearly the same plane. Sure, Pluto was a bit more eccentric, occasionally coming closer to the sun than Neptune, but it got kicked out of the club. But the further you go out into the distant, cold rocks beyond the planets, the better your chances to find something that ventures off the beaten path.

Last year, Brett Gladman, a University of British Columbia astronomer, discovered a 30-mile-wide asteroid, KV42, that actually orbits the sun backwards, but only just barely. Anything with an orbit that is tilted over 90 degrees relative to the orbit of the solar system's planets is considered to have a retrograde, or backwards, orbit, and KV42's orbit is tilted at 104 degrees. The rock was the first of its kind ever to be seen orbiting the sun backwards, and joined a select group of distant objects, including Halley's Comet, that astronomers have ever spotted accomplishing the feat.

When Gladman and his team first spotted the asteroid back in July, he knew it was retrograde, but it wasn't easy to find.

But where did this weirdo come from? Way, way out there. Between 30 and 50 AU from Earth one enters the Kuiper Belt, which contains a slew of cold, small objects like Pluto. The Oort Cloud, which begins about 50,000 AU away, is the source of most of the comets in our solar system. The region is actually hypothetical; it's far too distant for us to visually confirm its inhabitants. But neither place looks like the probable source of Drac, Gladman says. He thinks it came from the vast swath of space between here and the Oort Cloud that astronomers just don't know that much about. It's probable that a gravitational disturbance knocked Drac into its peculiar course, and that such things occur all the time.

However, while other backwards orbiters are flying around waiting to be seen, there's bad news for fans of of astronomical anomalies, or charting your own course: irregularities like these are hard to come by in our solar system. The gravity of the giant planets keeps things on a tight course, and makes life hard for a free roamer like Drac. Over the course of millions of years, Gladman says, an aberrant orbit like Drac's is unstable because it will make passes close enough to gas giants like Uranus and Neptune that their gravity will pull energy out of its orbit, and someday it will no longer orbit the solar system.

Gladman's team is going to keep look for oddball objects. But the inner solar system, it seems, is a hard place to go rogue—the powers that be are always pushing you around.