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7 Earth-Size Planets Orbit Dwarf Star

In 1976 the International Astronomical Union (IAU) defined the astronomical unit as the distance from the Sun at which a massless particle in a circular orbit would have a period of one year. This definition relied on a solely Newtonian model of the solar system. However, such a definition proved difficult to implement in general relativity, in which different values of the astronomical unit were obtained depending on an observer’s frame of reference. Through Kepler’s third law of planetary motion, the 1976 definition also depended on the mass of the Sun, which is always decreasing because the Sun shines through converting mass into energy. Increasing precision in measurements of the Sun’s mass meant that the astronomical unit would eventually become a time-varying unit. Because of these problems and because distances in the solar system were known so accurately that the astronomical unit was no longer needed to provide a relative scale, in 2012 the IAU fixed the astronomical unit at 149,597,870.7 km.