Six Words: Appearance, Limit, Donor, Acute, Planet, Filter
Prior to the 20th century people observed the appearance of a planet closer to the sun than Mercury. Acute observations, limited by the technologies of the 17th through the early 20th century, seemed to reinforce those observations. Unexplained variations in the orbit of Mercury donated to the theory.
Eventually, filtered by better equipment and Einstein’s Theory of Relativity, the existence of the planet was disproved. It did gain renewed life with the development of the Star Trek series. The theoretical planet was named Vulcan and we all know that that was the home of Mr. Spock!