"Buffy" is one of the most tilted, or inclined, Kuiper Belt Objects

A large object has been found beyond Pluto travelling in an orbit tilted by 47 degrees to most other bodies in the solar system. Astronomers are at a loss to explain why the object’s orbit is so off-kilter while being almost circular.

Researchers led by Lynne Allen at the University of British Columbia in Vancouver, Canada, first spotted the object in observations made with the Canada-France-Hawaii Telescope in December 2004. Since October 2005, they have made follow-up observations that have revealed the object’s perplexing path.

Tentatively named 2004 XR190, the object appears to have a diameter of between 500 and 1000 kilometres, making it somewhere between a fifth and nearly half as wide as Pluto. It lies in a vast ring of icy bodies beyond Neptune called the Kuiper Belt, most of which orbit in nearly the same plane as Earth.

But at 47 degrees, 2004 XR190’s orbit is one of the most tilted, or inclined, Kuiper Belt Objects known. That suggests it was flung out of the solar system’s main disc after a close encounter with another object - such as Neptune or perhaps a star that passed by the Sun billions of years ago.

Scatter effect

Neptune has been blamed for scattering many other KBOs into tilted paths. But these tend to show other signs of a past interaction with the giant planet, such as moving in elliptical paths and having one part of their orbit pass near Neptune’s at 30 astronomical units (AU) from the Sun (1 AU is the distance between the Earth and the Sun).

2004 XR190, however, follows a nearly circular path. And it is too distant to have come into direct contact with Neptune, travelling between 52 and 62 AU from the Sun. Its orbit is also too circular - and too small - to have been tilted by a passing star, says Allen.

These traits make the object, nicknamed "Buffy" after the US television series about a vampire slayer, hard to explain. "Maybe Buffy is going to be a bit of a theory slayer," Allen told New Scientist.

But she suggests one theory that might account for the space rock’s strange orbit. It involves a commonly held notion that early in the solar system, Neptune itself moved outward into its present orbit, from around Uranus’s current location.

Gravitational kick

As it did so, its gravitational reach extended outwards, as well. This reach comes in the form of zones, or resonances, where an object’s orbital period happens to be an integer multiple of Neptune’s. So when one of these outward-expanding resonances swept past 2004 XR190, it could have kicked the object out of a fairly circular, flat orbit into a more elongated, tilted one.

Then, over time, the orbit might have grown more circular as the tilt increased. "These interactions can cause some Kuiper Belt Objects to circularise and tilt," says Allen. But she remains cautious: "We don’t know if Buffy’s orbit really was created in this manner - because it could be too far away from a resonance or the resonance could not be strong enough - but this seems like the best shot."

Renu Malhotra, a planetary scientist at the University of Arizona in Tucson, US, says most resonances simply elongate an object’s orbit. She says a few objects could "trade off" some of their elongation, or eccentricity, for a higher tilt, but the effect would be small. "I find it hard to see how you would get a large inclination out of a modest eccentricity," Malhotra told New Scientist. "There’s a limit on how much inclination you can trade off."

Hal Levison, a planetary scientist at the Southwest Research Institute in Boulder, Colorado, US, says he and others have produced objects like Buffy using models of such special resonances. "However, I do have some problems with the idea," he admits.

Hidden objects

He points out that this object was found when it happened to be passing through the plane of the solar system - where it spends just 2% of its orbit. That suggests many more such objects remain undiscovered, tilted at orbits where most surveys do not search for them. "I just don’t think these mechanisms can deliver that much stuff," Levison told New Scientist.

He ventures another possible explanation - that the Sun had a twin and that both stars followed circular orbits around each other. "That could excite inclinations without exciting the eccentricities," he says. "However, this idea creates more problems than it solves, by far."

Allen and her colleagues will continue to observe the new object to pin down its orbit more accurately. And following the convention for naming bodies in the Kuiper Belt, they have suggested several Inuit names for the object to the International Astronomical Union.