We hear about asteroids all the time, especially the ones that are streaking by Earth on their way to (or back from) the inner part of the solar system.
Asteroids, or minor planets as they are sometimes called, are irregularly shaped (that is, not round) objects made primarily of rock, but can sometimes contain chunks of metal.
Most asteroids orbit the Sun in a region between Mars and Jupiter known as the Asteroid Belt. Sometimes this is also called the Main Belt or the Main Asteroid Belt, but there are other points at which asteroids will orbit the Sun; most notably the Trojan asteroids that orbit the Sun along Jupiter's path.
(The Trojan asteroids orbit at 60 degrees in front of and behind the gas giant. Some literature refers to the group of asteroids in front of Jupiter as the Greek Asteroids, differentiating them from the Trojan asteroids that follow behind, though this is less common.)
How Did the Asteroid Belt Form?
The asteroid belt began its formation in the same way that the rest of the planets in our solar system formed. Soon after the Sun had formed, energy was injected into the surrounding gas and dust leftover from original solar nebula. It began to clump together during collisions to form planetesmals - larger and larger chunks of rock and metal.
Eventually, these planetesmals would become large enough that their own gravity would begin to attract surrounding matter and they would grow in size, and sometimes combine with other planetesmals to form planets.
However, Jupiter, the largest planet in our solar system formed very rapidly. And, because of its large size had a very strong gravitational field. The energy from this field caused the planetesmals forming between Mars and Jupiter to collide at energies that were too violent for them to stick together. Rather, they began to break apart, sending some pieces hurling throughout the solar system.
With much of the mass ejected from the asteroid belt during this process, only a small fraction remained totaling about 0.05% of the Earth's mass.
There are millions of asteroids in the belt, with more than 1 million of them having a diameter of at least 1 kilometer. There are at least 200 asteroids that have a diameter larger than 100 kilometers, with the largest object in the belt, Ceres, measuring 950 kilometers.
The Largest Objects in the Asteroid Belt
At least one of the objects in the asteroid belt is large enough to have pulled itself into a spherical shape due to its own gravitational field. This object, Ceres, is therefore classified as a dwarf planet, the same classification as Pluto.
There are three other objects in the asteroid belt (Vesta, Pallas and Hygiea) that are also large enough to have formed into a near-spherical shape and are considered candidates to be included as dwarf planets, though as the time of this writing, had not been officially accepted as such.
Together, these four asteroid belt objects contain about half of the total mass of the asteroid belt, while Ceres alone accounts for one third the mass.