An astronomical unit, or AU, is defined as the average distance between Earth and the Sun. In 2012, the International Astronomical Union defined the length of one AU as 149,597,870,700 meters, which is just about equal to 93 million miles. Most astronomers approximate 1 AU = 150 million kilometers. For comparison, Mercury is about 0.4 AU from the Sun; Neptune is about 30 AU from the Sun; and the Alpha Centauri system, which contains the stars closest to the Sun, is about 270,000 AU away.