Prior to 2006 every object in orbit around our Sun was either a Planet, a Minor Planet or a Comet. However, when the issue of Pluto's planet-hood was raised that year, a new term, Dwarf Planet, was introduced.
Since then, the most well known Minor Planets were reclassified as Dwarf Planets, leaving behind only a few Minor Planets that anyone ever hears about. But as a category they are numerous, with more than 540,000 officially known to date. And this makes them still rather important to our Solar System.
What is a Minor Planet?
Simply, a Minor Planet is any object in orbit around our Sun that is not a Planet, Dwarf Planet, or a Comet.
The first object to be classified a Minor Planet was the object Ceres, now known to be orbiting in the asteroid belt between Mars and Jupiter.
However, in 2006 Ceres was officially re-classified as a Dwarf Planet by the International Astronomical Union (IAU).
How Many Minor Planets are there?
There are now more than 540,000 Minor Planets Catalogued by the Minor Planet Center operated by the Smithsonian Astrophysical Observatory.
The vast majority of these objects are in the asteroid belt, a region of space between the orbits of Mars and Jupiter where the remnants of a planet orbit the Sun.
It is thought that the gravitational pull of Jupiter was so great that during the formation of, what was to be a planet, it was ripped apart. The pieces that are left behind now constitute the asteroid belt.
Are Minor Planets Just Asteroids?
Just because asteroid belt objects are considered Minor Planets it does not mean that all Minor Planets are simply asteroids. There are objects out beyond the orbit of Neptune that are considered Minor Planets, however many astronomers do not consider them asteroids.
Ultimately there are lots of objects, including asteroids, that fall into the Minor Planet category. The one non-planet hold out are comets.
These objects are unique enough in their composition and placement in the Solar system, that they justify their own category.