Vulcan, in astronomy, hypothetical planet whose existence was proposed by Leverrier 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 explains 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.

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