Solar mass also has a special use when estimating orbital periods and distances of 2 bodies using Kepler's laws : a = M total T, where a is length of semi-major axis in AU, T is orbital period in years and M total is the combined mass of objects in M ☉. In case of planet orbiting a star, M total can be approximated to mean the mass of the central object. More specifically in the case of Sun and Earth the numbers reduce to M total ~ 1, a ~ 1 and T ~ 1.