Planets, pluralism, and conceptual lineage

• The conceptual history of the term ‘planet’ is discussed.
• The importance of branching kind-concepts to the pluralism debate is argued for.
• Sociological reasons are given to explain pluralism’s lack of popularity.

Conceptual change can occur for a variety of reasons; some more scientifically significant than others. The 2006 definition of ‘planet’, which saw Pluto reclassified as a dwarf planet, is an example toward the more mundane end of the scale. I argue however that this case serves as a useful example of a related phenomenon, whereby what appears to be a single kind term conceals two or more distinct concepts with independent scientific utility. I examine the historical background to this case, as a template for developing additional evidence for pluralist approaches to conceptual disputes within science and elsewhere.