As the planetesimals grew larger, they began to act as vacuum cleaners, ridding the solar system of debris and eventually becoming the current planets. Asteroids are small rocky worlds. Most asteroids orbit the sun in a region between Mars and Jupiter called the Asteroid Belt. This belt is thought to be the remains of a planet that failed to form at a distance of 2.8 AU from the sun. There are about 200 asteroids larger than 60 miles in diameter, about 200 larger than 6 miles, and 500,000 larger than 0.6 miles.
Comets are large dirty snowballs.
They consist of three parts. The nucleus is a ball of frozen water and carbon dioxide, and is usually a few dozen Km in diameter. The coma is the bright area surrounding the nucleus, and may be as large as Neptune. The tail is a luminous trail of debris left behind as the comet melts. As a comet falls into the sun from the Oort Cloud or the Kuiper Belt , it steadily becomes warmer.