Population I stars are those young stars whose metallicity is highest. The Earth's Sun is an example of a metal-rich star. These are common in the spiral arms of the Milky Way galaxy. Usually, the youngest stars, the extreme Population I, are found farther in and intermediate Population I stars are farther out, etc. The Sun is considered an intermediate Population I star. Population I stars have regular elliptical orbits of the galactic centre, with a low relative velocity. The high metallicity of Population I stars makes them more likely to possess planetary systems than the other two populations, since planets, particularly terrestrial planets, are thought to be formed by the accretion of metals.