Asteroids and Comets
In addition to the nine planets orbiting the Sun together with their moons, there are a large number of lesser bodies in the Solar System. Between the orbits of Mars and Jupiter is a gap where there is no planet where one should be expected. In 1801 a small object less than 1000 Km in diameter was found, and was called Ceres. This turned out to be the largest member of a large population of small objects orbiting the Sun between Mars and Jupiter. These objects are called asteroids or minor planets. Even Ceres, which is considerably larger than the next largest asteroid, is considerably smaller than the smallest planet Pluto. Information on the asteroids can be found on the Nine Planets website.
Although most of the asteroids orbit between Mars and Jupiter, there are a number of small bodies that are in other orbits, including those that can even come closer to the Sun than there Earth, some of which may pose a threat of a collision with the Earth in the future. The Earth has suffered impacts from asteroids and comets in the past, including big impacts, and there is strong evidence that an impact either wiped out the dinosaurs directly about 65 million years ago, or contributed to their demise.
In addition to asteroids, small icy objects in usually highly eccentric orbits around the Sun are present, which are known as comets. When one of these objects approaches the Sun, the frozen gases in the comet evaporate and sometime produce a spectacular tail. Details of comets can be found on the Nine Planets website.
Finally, in addition to asteroids and comets, there are what are known as the Kuiper Belt and Oort Cloud. The first is a type of ice asteroid belt beyond the orbit of Neptune, and as stated in the page on the planets, Pluto may be considered to be the largest member of this class of objects. The second is an outer cloud containing a large resevoir of comets. Unlike the Kuiper Belt, the Oort Cloud has not been seen directly, but its presence has been deduced by the orbits of long period comets.