2004 XR190

On December 13, 2005, a team of astronomers (led by Lynne Allen) announced the discovery of a new planetary body between one-fifth to one half the size of Pluto, found during routine operation of the Canada-France Ecliptic Plane Survey (CFEPS) as part of the Legacy Survey on the Canada France Hawaii Telescope (CFEPS press release). The object's brightness indicates that it is likely to be between 500 and 1,000 kilometers (300 to 600 miles) in diameter. Currently designated 2004 XR190 (or XR 190) but nicknamed "Buffy," the object takes about 440 years to move around the Sun at an average distance (semi-major axis) of 57.4 AUs in a relatively circular orbit (e=0.11) that is inclined about 46.7° to the ecliptic. It is the first known object with a relatively circular but highly tilted orbit beyond Neptune and Pluto. This orbit is hard to explain using existing models of Edgeworth-Kuiper Belt formation (more technical details).

CFEPS



Larger image



2004 XR 190 ("Buffy")
has a highly tilted but
relatively circular
orbit (more).

In late 2005, 2004 XR190's highly circular orbit was considered to be unusual for the icy objects found in the outer Solar System. At that time, only one other detected object, Sedna, remains further than 50 AUs from the Sun throughout its entire orbit. Moreover, Sedna has a highly eccentric orbit that takes inward as close as 76 AUs before moving back beyond 900 AUs. In contrast, Buffy stays in a comparative narrow orbital region between 52 and 62 AUs from the Sun. Although astronomers have detected other EKOs whose orbits lie mostly 50 AUs, these objects have very eccentric orbits, and almost all eventually move inward to within 38 AUs of the Sun, which place them within Neptune's gravitational reach and so these EKOs are generally thought to have been scattered out to their present orbits by a gravitational slingshot with Neptune to become part of the "Scattered Disk."

As of late 2005, only a few other EKOs were known to have orbits that stay mostly beyond 50 AUs. Unlike Scattered Disk objects, these EKOs do not move inward within Neptune's gravitational reach of Neptune but have named members of the "Extended Scattered Disk." Two of these objects (1995 TL8 and 2000 YW134) move inward to within to 40 AUs of the Sun but have fairly elliptical orbits that take them back out beyond 60 AUs.

Southwest Research Institute



Larger illustration.



2004 XR190 appears to orbit
in the outer reaches of the
Edgeworth-Kuiper Belt, which
eventually merges into the
inner Oort Cloud.

Extended Scattered Disk objects, however, also include 2000 CR105, which moves inward to within 44 AUs of the Sun but then outward beyond 500 AUs, and Sedna, which never comes closer to the Sun than 76 AUs before moving to around 900 AUs away (into the realm of the inner Oort Cloud). Given the large orbital eccentricities of these two objects (which move beyond 500 AUs of the Sun), some astronomers have argued that they were likely to have been strongly perturbed by a massive celestial object (which is unlikely to have been Neptune as they do not come close enough to its gravitational influence) such as a rogue planet or passing star, which could have dragged the two objects farther out after initial orbital perturbation by Neptune. (More discussion on such a scenario with illustrations from computer models are available from a 2005 Powerpoint presentation by Brett Gladman and Collin Chan.)