Pluto was ruled not a planet anymore because it is a "dwarf planet"
Apparently, the International Astronomical Union's (IAU) definition of a planet includes that a planet should be part of a body that would orbit the sun.
According to the IAU, there are three criteria's that contribute to a planet being called a planet.
1. It is within orbit around the Sun.
2. It has a sufficient mass to assume a nearly round shape
and 3. It has "cleared the neighborhood" around it's orbit.
Pluto only so happens to meet 2 of these requirements, not including the third option.
