Dr Tyson excluded Pluto from the American Museum of National History exhibit because it is considered an icy body and not a planet.
Originally, he identified Pluto as a comet from the Kuiper Belt of Comets, since its composition its mostly ice, and stated that if it came near the sun it would evaporate and grow a tail, an uncommon behavior for a planet.
Eventually, in the 1990s, icy bodies were found in the outer solar system, and the similarity between Pluto and them was evident. The conclusion was that both are made of ice, have similar strange orbits, and cross the orbit of other planets.
