In 1878 he believed he had observed two Vulcan-type planets (planets within the orbit of Mercury), but he was mistaken.
== Življenje in delo ==Med letoma 1862 in 1899 je Swift odkril 13 kometov, med njimi periodične komete: 11P/Tempel-Swift-LINEAR, 64P/Swift-Gehrels in 109P/Swift-Tuttle.