Throughout history, stargazers focused on the Sun, Moon, planets, and comets. Those were the objects in Earth's "neighborhood" and easy to spot in the sky. However, it turns out there are other interesting objects in the solar system that aren't comets, planets or moons. They're small worlds orbiting out in the darkness. They got the general name "minor planet".
Sorting the Solar System
Prior to 2006, every object in orbit around our Sun was sorted into specific categories: planet, minor planet, asteroid, or a comet. However, when the issue of Pluto's planetary status was raised that year, a new term, dwarf planet, was introduced and immediately some astronomers began to apply it to Pluto.
Since then, the most well-known minor planets were reclassified as dwarf planets, leaving behind only a few minor planets that populate the gulfs between planets. As a category they are numerous, with more than 540,000 officially known to date. Their sheer numbers make them still rather important objects to study in 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. It's almost like playing "process of elimination". Still, knowing something is a minor planet vs. a comet or dwarf planet is rather useful. Each object has a unique formation and evolutionary history.
The first object to be classified a minor planet was the object Ceres, which orbits 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). It has been visited by a spacecraft called Dawn, which has solved some of the mystery surrounding Cerean formation and evolution.
How Many Minor Planets are there?
The minor planets catalogued by the IAU Minor Planet Center, located at the Smithsonian Astrophysical Observatory. The vast majority of these little worlds are in the Asteroid Belt and are also considered asteroids. There are also populations elsewhere in the solar system, including the Apollo and Aten asteroids, which orbit inside or near Earth's orbit, the Centaurs - which exist between Jupiter and Neptune, and many of the objects known to exist in the Kuiper Belt and Oört Cloud regions.
Are Minor Planets Just Asteroids?
Just because asteroid belt objects are considered minor planets it does not mean that all of them are simply asteroids. Ultimately there are lots of objects, including asteroids, that fall into the minor planet category. Some, such as the so-called "Trojan Asteroids", orbit in the plane of another world, and are studied closely by planetary scientists. Each object in each category has a specific history, composition, and orbital characteristics. While they may seem similar, their classification is a matter of great importance.
What about Comets?
The one non-planet hold out are comets. These are objects made almost entirely of ice, mixed with dust and small rocky particles. Like asteroids, they date back to the earliest epochs of solar system history. Most comet chunks (called nuclei) exist in the Kuiper Belt or Oört Cloud, orbiting happily until they are nudged into a sunward orbit by gravitational influences. Until relatively recently, no one had explored a comet up close, but beginning in 1986 that changed. Comet Halley was explored by a small flotilla of spacecraft. Most recently, Comet 67P/Churyumov-Gerasimenko was visited and studied by the Rosetta spacecraft.
Classifications of objects in the solar system are always subject to change. Nothing is set in stone (so to speak). Pluto, for example, has been a planet and a dwarf planet, and may well regain its planetary classification in light of the New Horizons missions discoveries in 2015.
Exploration has a way of giving astronomers new information about objects. That data, covering such topics as surface characteristics, size, mass, orbital parameters, atmospheric composition (and activity), and other subjects, immediately changes our perspective on such places as Pluto and Ceres. It tells us more about how they formed and what shaped their surfaces. With new information, astronomers can tweak their definitions of these worlds, which helps us understand the hierarchy and evolution of objects in the solar system.
Edited and expanded by Carolyn Collins Petersen