Loads of school textbooks are going to have to be rewritten after it was decided Pluto is NOT a planet anymore.
Pluto has been called a planet since it was discovered in 1930, but now a meeting of important astronomers has decided it isn't.
Until Thursday it was the smallest of nine planets in the solar system. Now there are only eight left.
The decision was made by about 3,000 space experts in the Czech Republic, who have re-named Pluto a dwarf planet.
At the meeting the experts agreed for the first time a set of rules that will decide what is and what isn't a planet.
What makes a planet?
It must orbit the Sun
it must be nearly round
Its orbit shouldn't cross another planet's
Pluto will now be known as a dwarf planet, as will two other space objects that were being discussed at the meeting, Ceres and Xena.
The decision to take away Pluto's status hasn't been made because it is much smaller than the remaining eight planets.
Instead, the rule that it broke is about its orbit, which crosses the orbit of its neighbouring planet Neptune.