We report on a 5-year study of the historical literature from Galileo until today regarding the concept of a planet. Bibliometrics show that scientific productivity in astronomy grew exponentially from at least 1700 until the present except for a long period of non-growth from 1894 until 1955. Contained within that was a period of deep reduction of scientific productivity over planets and satellites lasting from 1910 until 1955, which we have labeled the “Great Depression of Planetary Science”. During that period, astronomers’ interests were focused largely on other, more exciting topics enabled by new technologies. Records show it was during this period that astronomers stopped using — and indeed lost memory of — the concept of “planet” that had been taught since Galileo. They began using in its place a folk concept of planets that had evolved during the first half 1800s in astrological and other non-scientific writings, and which had spread through the general public by the end of the 1800s. According to that folk concept, planets were defined as only the large primaries that are orderly and directly orbit the Sun because those are the bodies that suggest orderliness in the cosmos consistent with non-reductionist views that the public favored. Non-scientific publications were arguing that planets in our Solar System were created to serve the Earth, even though it was acknowledged that they do not orbit the Earth. Like scientists, the public was aligning planetary taxonomy with theories they held most important to understand the cosmos, but unlike the scientists their theories were not reductionist. Apparently, pedagogical neglect during the Great Depression of Planetary Science coupled with a lack of theory advancement (and thus no recognized need for a utilitarian taxonomy) produced a generation of astronomers who thought that the folk concept had always been the only concept. In recent years, astronomers have fallen into historical presentism, teaching that the folk concept was actually developed during the Copernican Revolution (despite what the literature clearly shows), that the reductionist concept never existed, and that it has been normative to have a “planet” concept that is not scientifically useful. However, since 1955 there has been movement in the planetary science literature toward a concept that is broader than the folk one and is aligned again with the historic insight and purposes initiated by Galileo. Failure to understand this history has contributed to the recent controversy over the definition of a planet.