From 1838 he studied the long-term stability of the Solar System, in particular the causes of perturbations, leading to a revision of the masses of the planets, the dimensions of the Solar System, and the velocity of light. In 1845 D. J. F. Arago alerted him to irregularities in observed positions of Uranus. The next year he ascribed these to the presence of an undiscovered planet, and supplied a predicted position to J. G. Galle who located Neptune in 1846 November. In 1859 Le Verrier suggested that a small planet or belt of asteroids within Mercury’s orbit would account for irregularities in Mercury’s observed positions (see vulcan).