First, 2009 is YE7 bright. In absolute terms, it is the 9th brightest object, which is what led to the reasonable assumption that it is likely the 9th largest object (by absolute brightness here, I mean the brightness things would have if they were all the same distance away; some objects are bright just by virtue of being close). Second, the orbit of 2009 YE7 is tilted relative to the planets by 29 degrees. Following the position of an object for only 2 weeks doesn't give you a precise measurement of much about its orbit, but that tilt is one thing that is solidly known even with this limited data. An angle of 29 degrees is an unusually high angle. Not too many objects are tilted by that much. But one that is is Haumea. Haumea with its family of shards all going around the sun on orbits just like it. Tilted by 29 degrees.
2009 YE7, the brightest object discovered in the Kuiper belt in almost 5 years, is almost certainly one of the large shards (perhaps even the largest) blasted off of the surface of Haumea 4 billion years ago. 2009 YE7 and the other shards have been circling the sun on their own ever since. It is bright not because it is particularly large, but because all of the fragments of Haumea have extremely bright, reflective, icy surfaces which make them stand out against the more common darker Kuiper belt objects. 2009 YE7 is not the 9th largest Kuiper belt object; it is probably about 440 km in diameter and so in the top 50. Read more
2009 YE7 is a trans-Neptunian object with an absolute magnitude of 2.8. This qualifies it as a dwarf planet candidate. It was discovered by David Rabinowitz on December 17, 2009 at the La Silla Observatory in Chile. 2009 YE7 is the first large KBO found from the southern hemisphere. Read more