In 2002 Caltech astronomer Mike Brown and colleagues found a Kuiper Belt object half the size of Pluto. In 2004 Sedna another world three-quarters the size of Pluto. Brown kept on looking using computers to do the work of Tombaugh's blink comparator.

Brown said, " We do exactly same thing Clyde Tombaugh is the one who basically figured out the system for finding these things. But we do all by the computer instead there are three images in a row that are blinking on the screen just like Clyde Tombaugh would blink. On the morning January 5th, 2005, I was looking at these very images and I click the button saying this one's no good and I suddenly see an object there very clearly moving across the frame in  dead center and at another point, something is one where I almost literally fell out of my chair because it is close to being the brightest object we have ever seen out there and it is also moving extremely slowly. Slow means it is far away slow and bright means it is really big. If we combined more distant with anything we have ever found and almost the brightest thing we had ever found. We knew immediately that thing had to be the biggest thing we have ever found in our solar system. The very first thing I did is grab my telephone and called up my son and I said I found the tenth planet."

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2003 UB313 the random name assigned by Brown's computer was twenty-five percent larger than Pluto. Until an official name was chosen Brown came up with a temporary nickname commemorating Percival Lowell search for Planet-X. Further Brown explained that " We had actually been reserving the name Xena for the first object we found larger than Pluto. We wanted something started with an X for Planet-X. We wanted something that was mythological, now Xena is only TV mythology, not real mythology but we figured Pluto is named after cartoon dog that is ok and we wanted something that we decided there were not enough female planets. We were going to name another. So if you take those three together you get Xena."

Xena has now officially been named Eris, goddess of discord which seems appropriate. Some astronomers argue that Pluto and Eris as Kuiper Belt objects should not be classified as true planets others disagree.