Why is Pluto no longer classified as a major planet?
The word "planet" has been divided into two newer concepts:
major planets and dwarf planets.
The definition of major planet excludes Pluto because a
major planet must have cleared its orbit of asteroids, comets, and
other debris. Pluto is a part of the Kuiper belt, and has not
absorbed most of the cometary bodies into itself, or captured them
as satellites. Pluto is therefore a dwarf planet.
The rules of a planet are:
- It orbits a star or the remnants of a star
- It is large enough for gravity to squash it into a sphere-like
- It must have cleared its orbit
Pluto's orbit has many icy bodies in its path, most of them in
the Kuiper Belt where Pluto spends most of its time.
The definition of planet was changed in 2006 by the
International Astronomical Union (IAU). As a dwarf planet Pluto is
in the same category as Eris (which is bigger than Pluto), Ceres
(king of the asteroids), and possibly other bodies being discovered
in the Kuiper Belt.
Pluto is much smaller than was believed originally, when it was
discovered. The modern definition of a major planet, however, is
that a planet has somehow "cleared the neighborhod" of its orbit,
whatever that means.