Why was Pluto reclassified as a dwarf?

Answer 1

Because it was way too small to be a real planet.

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Some may be annoyed by my nit-picking. It is not that Pluto was singled out and redefined as a dwarf planet (and by the way it is not Pluto's size that is at issue). A definition of planet was decided upon, and according to the definition Pluto does not meet criteria to be considered a major planet. Pluto has not cleared its neighborhood (the Kuiper Belt) of debris. It may well be that Pluto's then-disputed place among the major planets fueled the need for a definition, but the definition was needed and the current one seems reasonable to most.

Answer 2

Contrary to popular belief, the size of Pluto has nothing to do with it. In fact, Pluto passes the only test for being a planet that is at all related to size: under the pull of its own gravity it has formed into a body that is very close to spherical in shape. The reason it is not a major planet is because it has not cleared its neighborhood (the Kuiper Belt) of debris.

Answer 3

It turns out that there are several dwarf planets in our solar system, not just Pluto. If Pluto is to be considered a planet, than Eris and a few others are just as deserving. To some extent it is an arbitrary choice, whether to recognize a larger number of planets, or to create new qualifications by which Pluto is reclassified as a dwarf planet, and the latter is the what astronomers chose to do.

Answer 4

It's been decided to stick with eight major planets and then to call the new ones that are discovered dwarf planets. They tend to have rather strange orbits, with high inclination and eccentricity, and they are quite small, although Pluto has five moons.

The eight major planets have stable almost-circular orbits, and they are big enough have hoovered up most of the debris in their paths.

Answer 5

Pluto was excluded from the list of planets in 2006 after the International Astronomical Union (IAU) released it's decision on what defines a planet. In sum, the IAU now defines a planet as being a celestial body within a solar system that:

1. Orbits the sun

2. Is large enough to form into a round-ish shape

3. Has "cleared the neighbourhood" of it's orbit

Since Pluto did not meet the third criteria, it can no longer be considered a planet; instead, it has been reclassified as a dwarf planet.