Why Pluto is considered a dwarf planet?

Answer 1

Because it is not a true planet, it's designation has been changed after much debate and a vote by the leading experts in the field of astronomy. There is some disagreement still over the requirements of planet-hood. For example, there is still a lot of debris floating around out there, much of it in the vicinity of planetary orbits. But the remaining debris is relatively tiny compared to the planets, and they cannot now gather enough material to ever be in contention for possession of a given orbit.

All major planets have cleared their paths of debris. Pluto spends the great majority of its time in what is called the Kuiper Belt, a huge field of rocky and icy bodies beyond Neptune, at least one of which is larger than Pluto. As a result, Pluto is considered not to have 'cleared its path'. It is one of many interesting Kuiper Objects.

A popular misconception is that Neptune is involved in Pluto's reclassification because Pluto's orbit is, for roughly 15 years per cycle, within Neptune's orbit, suggesting that Neptune is 'debris' in Pluto's path. This makes no sense, because one could conclude that Neptune has not cleared its path of debris (Pluto) and is therefore not a planet. And in comparison, Pluto would more easily be defined as debris of Neptune, rather than calling the huge Neptune debris of Pluto. The fact is that neither Pluto nor Neptune is 'debris' in the other's path. The orbits are not coplanar, and they have a special and exact harmonic relationship that prevents them from ever colliding. During exactly two orbits of Pluto, Neptune orbits exactly three times. There are other similar harmonic relationships in the solar system. This arrangement between Pluto and Neptune is stable, and only some spectacular disturbance by some force outside of the solar system could disrupt it.

Answer 2

In 2006 the International Astronomical Union, in response to the discovery of several new objects that might be considered planets, developed a new definition for a planet with three criteria:

1. The object must orbit the sun
2. The object must have enough mass to be made round by its own gravity
3. The object must be able to "clear the neighborhood" of debris.

Objects that meet the first two criteria but fail the third are called dwarf planets. Pluto orbits in the Kuiper Belt, a disk-shaped region of smaller, icy objects, so it has not cleared the neighborhood.