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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