Vulcan, in astronomy, hypothetical planet whose existence was proposed by Le Verrier to explain part of the advance of the perihelion of Mercury , not all of which could be accounted for by gravitational effects of the other planets under the Newtonian theory of gravitation . The general theory of relativity , which explain the observed advance of the perihelion of Mercury as being caused by the curvature of space in the vicinity of the sun as a result of the sun's large mass, ended the search for Vulcan.

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