Oort Cloud. Artwork of the Oort cloud of cometary nuclei around the Sun, with the Sun's hypothetical companion star Nemesis. The Oort cloud consists of billions of lumps of ice and dust (white dots) which never condensed into planets. These lie in a halo around the Sun (centre right) and its planets (the outer planets' orbits are shown inset) on the very edge of the solar system. Being so far from the Sun, the orbits of these lumps are sometimes perturbed due to the gravitational effects of passing stars or a body like Nemesis (lower left). These new elliptical orbits (white line in inset) may take them close to the Sun where they vaporise & grow a gas tail; they are then known as comets.