Here is a quick summary of the components of the solar system -- besides the fact that all of them orbit the Sun, including Earth and two major planets too dim for ancient astronomers to have seen. Four distinct classes of objects are usually recognized:
- Major planets, in order of distance from the Sun -- Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. All but the inner two have satellites, and all four outer ones have rings as well, composed of small orbiting chunks of matter.
- Asteroids or minor planets, most but not all between Mars and Jupiter. Ranging in diameter up to about 500 km.
- The “Kuiper Belt" of icy objects outside the orbit of Neptune, of which the best known (though as of now only the second largest) is Pluto, discovered in 1930 and about the size of our Moon. The belt is named after the Belgian astronomer Gerard Kuiper and may extend to twice the distance of Neptune and is estimated to consist of as many as 100,000 objects (about 1000 of them identified so far), many only 100 km across or smaller.
- Comets, traditionally divided into “non-returning" (official name, “long period comets") and “periodic” ones. Non-returning comets are believed to come from the “Oort cloud," a huge near-spherical collection of frozen chunks on the distant fringes of the solar system. They are loosely bound to the Sun, and now and then the gravity of a distant star is believed to slightly change the motion of some and send them sunwards. They become visible as comets when sunlight evaporates some of their surface to create the comet's glow and tail. Periodic comets were once believed to have started as non-returning ones but to have been diverted by the pull of one of the larger planets. They are now widely held to arrive from the Kuiper belt as a class of objects known as Centaurs.