Comets are the remainders of material formed in the coldest part of our solar system.
Impacts from comets played a major role in the evolution of the Earth, primarily during its early history billions of years ago. Some believe that they brought water and a variety of organic molecules to Earth. Comets are visible for two reasons. Dust driven from a comet's nucleus reflects sunlight as it travels through space. Secondly, certain gases, stimulated by the sun, give off light like a fluorescent light bulbs.