Hypothetical solar system. Artwork of the inner solar system with the orbits of known (blue) and hypothetical (red) planets and moons. The Sun is at the centre of the solar system. The four known planets are (from inner to outer): Mercury, Venus, Earth and Mars. The two hypothetical planets are Vulcan (innermost orbit) and Neith (orbiting near Venus). Some theorists have suggested that Mercury has a moon and that Earth has two (some people call the supposed second moon Lilith). Alleged sightings of hypothetical planets and moons can usually be ascribed to inaccurate observations and the occurrence of previously unknown phenomena.