. "hypothetical planet, Vulcan" . . "Vulcan" . "the hypothetical planet of Vulcan" . "Volcano Island" . "Vulcan (hypothetical planet)" . "intra-Mercurial planet" . . . . . . . . . "Vulcan (hypothetical planet)" . "Vulcan_(hypothetical planet)" . "intermercurial planet" . . . . . "a hypothetical planet" . "hypothetical planet" . . . "another planet even closer to the Sun" . . "hypothetical planet Vulcan" .