The major planets in the solar system are greatly outnumbered by the swarms of smaller bodies called minor planets, or asteroids, and by the even more numerous and smaller bodies known as meteoroids. Most of the asteroids exist within the relatively large gap lying between the orbits of Mars and Jupiter, whereas meteoroids are randomly distributed. A few large asteroids have radiuses of a few hundred kilometers, but most are much smaller. The smaller meteoroids produce meteor trails when they enter the Earth's atmosphere, and the larger ones that survive atmospheric entry become known as meteorites. Some of them strike the earth and form meteorite craters.
A large number of the asteroids appear similar to the carbonaceous chondritic meteorites, and they are probably of relatively lower density than ordinary rocks. Nearly 2,000 have accurately determined orbits and have been given names. It is generally believed that most smaller asteroidal bodies have been created in collisions involving larger asteroids. Probably very many still smaller bodies exist that have not been detected by photographic surveys because of their size.
Many asteroids have orbits that cross the orbit of Mars. Some cross the orbit of the Earth or go even deeper into the inner solar system. These are called the Apollo asteroids. It has been suggested that many of the meteorites that strike the Earth are chips of the Apollo asteroids caused by collisions. These asteroids can also collide with the Earth or one of the other terrestrial planets. Some of the major craters that exist on these planets have more than likely been caused by such collisions, and chips from such collisions with Mars and the Moon have reached the Earth's surface as meteorites as well. Astronomers in fact regard most meteorites as basically asteroid-derived chips whose orbits eventually intersect with that of the Earth through a combination of processes, including the effects of the gravitational pull of Jupiter and Saturn and an effect, called the Yarkovsky effect, involving the differential absorption and reradiation of solar energy.
Other asteroidal bodies, called Trojans, have been observed both 60 degrees ahead of Jupiter in its orbit and 60 degrees behind. These positions of special orbital stability are called Lagrangian points. It is possible that similar swarms of dust particles are concentrated in the Moon's orbit, both 60 degrees ahead of the motion of the Moon and 60 degrees behind it (sometimes called the L4 and L5 Lagrangian points), but there has been no clear confirmation of this.
Until recently it was believed that minor planets were confined to the inner solar system. In 1977, however, an object was discovered and called Chiron, a body some hundreds of kilometers in radius that orbits between Saturn and Uranus. This object has since been classified as a huge comet. Other objects of minor-planet size have since been sighted in the outer solar system; they are called Trans-Neptunian Objects (TNOs) or plutinos, because they have orbits beyond that of Pluto (and Neptune) and their orbits are comparable to that of Pluto. (Pluto itself is considered by many astronomers as the largest of these plutinos.) One of the most remarkable of these discoveries, temporarily labeled EB173, is a reddish sphere, about 600 km (370 mi) in diameter, that lies between the orbits of Neptune and Pluto. It is comparable in size to the asteroids Ceres and Pallas. Why some outlying objects such as EB173 are reddish and others are gray, indicating different compositions, is another solar-system puzzle that remains to be solved. One TNO is 20000 Varuna - named by the International Astronomical Union for the Vedic Hindu deity Varuna and for the fact that it was the 20,000th minor body discovered in the solar system.
These objects of the outer solar system are also known as Kuiper-belt objects, because the Dutch American astronomer Gerard Kuiper further suggested that a ring of cometary material exists at this range of distances, with its inner edge about 37 astronomical units from the Sun (the distance from the Earth to the Sun is defined as one astronomical unit).The object called Chiron is now conjectured to have had such an origin. Beginning in the 1990s, several objects thought to be members of the Kuiper belt have been sighted beyond Pluto, such as 2001 KX76, determined in 2001 to exceed the size of Ceres, and 2002 Quaoar, about half the size of Pluto but as large as the rest of the asteroids combined. Traveling around the Sun in a nearly circular orbit, Quaoar is both the largest solar-system object to be discovered since Pluto and the farthest solar-system object yet to be resolved by a telescope.