Are there eight or nine planets?
When Pluto was discovered in 1930 it was considered to be a planet also, bringing the total to nine; however, as astronomers learned more about the outer solar system Pluto seemed to fit less and less well with the other planets due to its size, weird orbit, and other factors. In 2006 a definition for “planet” was adopted that excluded Pluto, which (along with several other objects roughly the same size as Pluto) is now called a “dwarf planet.
