Hypothetical planet within the orbit of Mercury. It was predicted in 1859 by Urbain-Jean-Joseph Le Verrier (1811–77) to account for an unexplained component of the precession of Mercury's orbit. Sightings were reported between 1859 and 1878, but these were not confirmed subsequently by observations made either during solar eclipses or when the planet was predicted to cross the Sun. The anomalies of Mercury's orbit were later explained by Albert Einstein's general theory of relativity.
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