Image: Comet
This chart shows the orbital paths (looking along the plane of the solar system) for the comet 2008 KV42 as well as other objects in the outer solar system.
updated 9/4/2008 10:00:27 PM ET 2008-09-05T02:00:27

Halley's Comet, which lights up Earth's sky every 75 years with its glowing tail, is a bit of a scientific mystery.

So far, theories have been at a loss to explain how it acquired its extremely unusual backwards orbit — but the recent discovery of another odd comet orbiting farther out in the solar system may shed light on Halley's origins.

The newly discovered comet, 2008 KV42, circles the sun at a tilt of 104 degrees compared to the main plane in which most of the planets and asteroids travel. The newfound oddball also orbits in reverse compared to almost everything else. Scientists think it might represent an intermediate point between comets like Halley's and their progenitors in the far and totally uncharted reaches of the solar system.

"The big mystery's been, how do you get comets like Halley's comet?" said astronomer JJ Kavelaars of the National Research Council of Canada, who worked on the team that discovered KV42. "It's one of the most famous comets known, and we have no dynamic explanation for how it got into its orbit and how it got to be there. Now we've found an object that could provide a source for a Halley-type comet."

KV42 was discovered at about 32 times the distance from Earth to the sun, and the closest it swings to the sun is about where Uranus is. Researchers suggest the comet may have originated out in the distant Oort cloud of objects thought to swarm in a sphere around the solar system almost 1 light-year out from the sun, or roughly a quarter of the way to the next nearest star.

At some point, they think, gravitational interactions (such as with Uranus or Neptune, or even a nudge from another star in the galaxy passing nearby the sun long ago) could have kicked the comet from its perch in the lower level of the Oort cloud, about a tenth of a light-year away, down to where it is now, orbiting beyond Neptune in a region known as the Kuiper Belt, where several other more normally orbiting rocky objects have been found.

This would explain how it got such a bizarre backward, or retrograde, and tilted orbit. If it originated in the spherical Oort cloud, the comet could develop an orbit with any tilt at all, as opposed to most comets, which originate in the plane of the solar system and thus should have an orbit in line with the planets.

Eventually, researchers predict KV42 could get kicked down even closer to the sun by interactions with the outer planets and the solar system's heavyweight, Jupiter. Ultimately, both Halley's and KV42 are likely to end up slingshot out of the solar system altogether from gravitational interactions with the planets.

Until now, physical models haven't been able to come up with an explanation for a comet like Halley's, but the researchers hope the discovery of KV42 will allow physicists to plug a new halfway point into their models to derive a reasonable evolutionary path for these objects.

"An orbit like this provides the road sign that says there must be a source of objects which could very plausibly be something related to the Oort cloud, which is feeding in slowly over the age of the solar system," research team member Brett Gladman of the University of British Columbia told "It's a pointer hopefully to a coherent model."

The researchers presented their discovery at the 10th "Asteroids, Comets and Meteors" meeting in Baltimore in July 2008. They first observed KV42 with the Canada-France-Hawaii Telescope in Hawaii, and made follow-up observations on the MMT telescope in Arizona, the Cerro Tololo Inter-American Observatory's 4-meter telescope in Chile, and the Gemini South telescope of Canada's Gemini Observatory, also in Chile.

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