Why isn’t the asteroid belt a planet?

Planetary scientists have a very specific definition for a planet, and they have a tendency to get rather ‘snippy’ if you try to go outside that definition. As an example, ever since ‘Pluto’ was first discovered, the definition for a planet was a lot more accepting and so they included Pluto as planet in our solar system. Pluto was a favorite for many people, because it was the smallest and the furthest planet, and was kind of the ‘underdog’ of the solar system. When they made the decision to change the definition of what a planet is in 2005, little Pluto was dropped off the list. So you see, it’s important to know what the definition of a planet is, before you can ask why the asteroid belt isn’t included.

An asteroid belt around the bright star Vega.

The official definition of a planet by the International Astronomical Union is: “a body that orbits the Sun, is massive enough for its own gravity to make it round, and has “cleared its neighborhood” of smaller objects around its orbit”. In the case of Pluto, they realized that there were other objects in the asteroid belt that were even larger than Pluto and so they decided that Pluto would be classified as a dwarf planet.

The asteroid belt, also known as the Kuiper Belt (pronounced Ki-per) is a collection of asteroids and icy objects of many sizes in between the planets of Jupiter and Neptune. The belt is closer in orbit to Neptune and is in the shape of a disc. These objects are thought to be leftovers from the creation of the solar system that got caught up in the gravitational pull and just circle around aimlessly. Some of the sizes of the objects are huge, almost the size of planets, while others are incredibly small. It is our planetary neighbor Jupiter, with gravity that is so strong, that it keeps most of these objects from hitting the earth.

Jovian Trojans - asteroids that lap the sun in the same orbit as Jupiter

Some of the objects in the belt might be planet-sized, but now they have to be examined to see if they fit the definition of a planet. When planetary scientists began using stronger and more powerful telescopes as well as the images from the Hubble space telescope, they realized that there were thousands upon thousands of objects in the asteroid belt, but none that should be classified as true ‘planets’.

Many of the objects in the belt do have a real orbit. The problem is that it is very crowded and the objects are always banging and crashing into each other. This changes the size, shape and even the orbit, depending upon who crashed into who. If an asteroid was big enough, it could crash into a larger dwarf planet, and it would probably just create a bigger dwarf planet. If it was big enough it could move the dwarf planet out of orbit.

Since there is so much chaos and the asteroids within the asteroid belt are always changing and being rearranged, it simply doesn’t make sense to classify any of them or the asteroid belt itself, as a planet.

Definition of planet

Kuiper Belt & Oort Cloud

What is a Planet?