For many, many years Pluto's status as a planet was questionable.

Not only is it small for a planet but its orbit is unusual. The planets go around the sun in an elliptical orbit and do not intersect the other planets orbits.

Pluto's orbit is an exaggerated elliptical shape and its path also goes through the other planet's orbits.
It is also very, very far away from the other planets.

These characteristics are more like a comet, which is what scientist believe Pluto is more like.

The diagram below illustrates the orbits. Pluto's orbit represented by the red orbit.