Hypothetical solar system. Artwork of the outer solar system with the orbits of known (blue) and hypothetical (red) planets. The Sun is at the cen- tre of the solar system. The 5 known planets are (from inner to outer): Jupiter, Saturn, Uranus, Neptune and Pluto. The four hypothetical planets are: Oceanus, Hades, Planet 'O' and Planet 'P'. It has also been suggested that Nemesis, a dim companion star of our Sun, lies far beyond the obit of Planet 'P'. Hypothetical planets in the outermost regions of the solar system would be very difficult to detect because their great distance from the Sun means that there is only very weak sunlight for them to reflect.