Proper motion is the apparent angular rate of motion of a star or other object across the line of sight on the celestial sphere. It is a combination of its actual motion through space (see space velocity) and its motion relative to the solar system.
Most stars are so distant that the proper motion is almost negligible. If the stars distance is also known, its transverse velocity can be calculated. Only 200 stars have proper motions exceeding one arc-second per year. Barnard's Star has the largest proper motion (10.3 arcseconds per year), followed by Kapteyn's Star.
Periodic perturbations, or wobbles, in a star's proper motion are indicative of an unseen companion, which may be a dim star, a brown dwarf, or a planet. In 1844, Friedrich Bessel reported variations in the proper motions of Sirius and Procyon, now known to be due to white dwarf companions. The use of astrometry to detect extrasolar planets, however, is much more problematic, because of the extremely small deviations involved.