Finding new objects in the Kuiper Belt is getting to be almost routine. But what makes the latest find intriguing is the shape of its orbit. Designated 2004 XR 190 by the International Astronomical Union and nicknamed ‘Buffy,’ the new object is currently 58 AU from the Sun, about twice the distance to Neptune. But an analysis of its orbit shows that it does not approach closer than 50 AU, complicating theories on how Kuiper Belt objects wind up in the positions they occupy.

Here’s the problem: the few objects discovered beyond 50 AU (where the main Kuiper Belt seems to end), have all been in extremely eccentric, or non-circular orbits. Bear in mind that these high eccentricity orbits have been assumed to be the result of gravitational interactions with Neptune or some other outer Solar System body. The encounter was assumed to have acted as a gravitational slingshot to fling the KBO objects into the deep.

But Buffy confounds these theories by coming nowhere near Neptune; another issue is that its orbit is tilted 47 degrees compared to the rest of the Solar System. And in a final gesture of irreverence toward our theories, its orbit is remarkably circular, although it will take several years of observation to measure the orbital parameters precisely. A circular orbit makes the ‘slingshot’ hypothesis far-fetched even if we could figure out around what object Buffy had been slung.

The enigmatic Buffy was discovered during routine operation of the Canada-France Ecliptic Plane Survey. From a news release from the Canada-France-Hawaii Telescope (CFHT):

Astronomer Lynne Allen of the University of British Columbia was the first to lay eyes on the new object, as she completed the initial identification in the course of processing CFEPS data from December 2004. “It was quite bright compared to the usual Kuiper belt objects we find”, said Dr. Allen, “but what was more interesting was how far away it was.”

Brightness is useful in allowing some inferences as to the object’s size; this one is thought to be between 500 and 1000 kilometers in diameter (300 to 600 miles). A useful comparison is Sedna, the only other detected object that stays further than 50 AU out throughout its entire orbit. In sharp contrast to Buffy, Sedna’s orbit is quite elliptical, moving between 76 AU and a whopping 900 AU from the Sun. Buffy’s range of 52 to 62 AU is a puzzle.

Centauri Dreams‘ take: Nothing should surprise us about the Kuiper Belt these days. Each new discovery seems to bring its own share of uncertainties, and what it will take to resolve them is a systematic exploration of the Kuiper Belt that will amass sufficient data to fit objects like Buffy into our current theories of Solar System formation. We know precious little about the so-called ‘Extended Scattered Disk,’ made up of those objects — like Sedna — whose orbits seem to have been perturbed by an object other than Neptune. What that object was — a passing star? — is only one subject for future work.

A more extended discussion of Buffy can be found here.