Galactic rim planets. Artwork of alien planets in the outer reaches of the Milky Way Galaxy. The galactic plane (with black dust lanes) runs across centre, where the central galactic bulge is seen. It is thought that the environment required for habitable planets to form exists in a ring-shaped area of the galaxy. This ring-shaped area is neither too close to the lethal radiation from the core, nor too far out. These planets are too far out, and because of the type of stars and material found out here, are too small to retain the atmosphere and liquid water needed for life.