Note: After I posted this, another announcement came across my desk: yet another big object past Pluto, maybe bigger than the one discussed below. Designated 2033 UB 313, it may be 3000 km across, comfortably larger than Pluto's 2300 km. Read about both here.
I had something else I was gonna post today, but then this came up.
Checking an old series of images to look for dim, distant objects beyond Pluto, astronomer Jose-Luis Ortiz at the Instituto de AstrofÃsica de AndalucÃa, Spain, found something interesting. It was slow-moving, indicating it was well beyond Pluto, but much brighter than you'd expect for something that far out. That implies (but does not prove) it might be large, even larger than Pluto.
The object was picked up again by American astronomers using the giant Keck 10-meter telescopes in Hawaii, as well as with the also-giant-but-not-quite-as-much 8-meter Gemini telescope. It's been given the designation 2003 EL61. And the plot thickens-- it appears to be the same object found by another group (the same ones who found Quaoar and Sedna, two of the largest objects orbiting the Sun beyond Pluto).
Is this an object bigger than Pluto? That depends. By measuring the orbit, astronomers are confident about its distance. If it is very reflective (think icy) then it doesn't have to be as big to be so bright. If it's non-reflective and dark (as many objects out there are) then it has to be a lot bigger to be as bright as it is as seen by us.