The innermost planet, Mercury, is also the smallest of the principal planets – apart from Pluto, which may be in a completely different category. It was once thought that a still closer-in planet existed and it was even given a name: Vulcan. It would be wellnigh impossible to observe except during a total solar eclipse or a transit across the Sun’s disk, but various claims were made, notably by a French amateur, Lescarbault, who stated that he had observed a transit on 26 March 1859. One believer was U. J. J. Le Verrier, whose mathematical work had led to the identification of Neptune in 1846. Vulcan’s distance from the Sun was said to be about 21 000000 km, with a period of 19d 17h. However, it is now certain that Vulcan does not exist. The only bodies which may move within the orbit of Mercury are comets, plus some asteroids such as Phæthon.