Le Verrier's correct prediction of the position of Neptune was a personal triumph for him, there have been suggestions that there was a large element of luck about it, but it cannot be denied that the actual discovery, by Galle and D'Arrest, was due entirely to Le Verrier.

The discovery of Pluto

Despite the fact that the new planet did not move at the distance required by Bode's law, then generally believed to be significant rather than coincidental, the Solar System could well be regarded as complete. Yet there was no proof of this, and as early as 1846 Le Verrier himself suggested that there might well be a planet moving beyond the orbit of the recently discovered Neptune. The first systematic search was made in 1877 by David Peck Todd, from the US Naval Observatory. From perturbations of Uranus, he predicted a planet at a distance of 52 a.u. from the Sun, with a diameter of 80 000 km. He conducted a visual search, using the 66-cm USNO reflector with powers of ×400 and ×600, hoping to detect an object showing a definite disc. He continued the hunt for 30 clear, moonless nights between 3 November 1877 and 5 March 1878, but with negative results. A second investigation was conducted in 1879 by the French astronomer Camille Flammarion, who based his suggestion upon the fact that several comets appeared to have their aphelia at approximately the same distance, well beyond the orbit of Neptune.