Hundreds of millions of comet nuclei forming a spherical region that surrounds the solar system is called Oort cloud.
The Oort cloud revolves at tens of thousands of the distance of the earth from the sun.
Comets are the condensed water and dust particles that orbit around the sun with inward helix leaving behind a tail of vapours and vaporize completely as they get closer to the sun during their revolution around it.
They have a nebulous appearance and extremely elongated orbits.
