What is 2003 UB313? I heard that it was the 10th planet but isn't Sedna the 10th planet?

Answer 1

Sedna is 90377, Eris is 2003 UB313. At the time it was called the 10th planet, some people unofficially called 2003 UB313 Xena, but that was never officially accepted. However Sedna, which was discovered before 2003 UB313, did get more press. Anyway, under the new definitions, there are 8 planets and 3 dwarf planets. The dwarf planets are Ceres, Pluto and Eris. (Ceres is the largest asteroid in the main belt between Mars and Jupiter). Sedna is too small to be a dwarf planet.

Answer 2

The planet which you are talking about is renamed to eris