The Problem with Pluto

Abstract

In 2006, for the first time in its history, the International Astronomical Union defined the characteristics of a ‘planet’ in our Solar System and consequently demoted Pluto from ‘planet’ to ‘dwarf planet’. Scientific and popular discussion leading up to and following the formal definition of planet demonstrated that astronomers, amateurs, educators, and school children had been employing Pluto in different ways to construct multiple scientific and cultural cosmologies. Formalizing the definition brought these cosmologies into direct collision, necessarily privileging some cosmologies over others. The story of Pluto, from discovery to demotion, illustrates the discursive disruption that stemmed from forced consensus. Before ‘planet’ received a formal definition, it was a tacitly understood term that encompassed several meanings and acted as part of the contact language in a trading zone, as described by Peter Galison. Social groups were able to clearly communicate using the word ‘planet’ despite differing cosmologies. This paper takes a closer look at Pluto’s history, highlighting three moments when Pluto’s planetary status was challenged. Each moment — a controversial exhibit opening that excluded Pluto from the planetary display, the discovery of objects in the Solar System larger than Pluto, and the International Astronomical Union conference at which astronomers crafted the definition of planet — spurred discourse that accentuated conflicting cosmologies. Actors involved in these incidents directed the discussion in order to maintain clear communication or to make sure their cosmologies remained relevant. Only with the formal definition of planet, which excluded several popular and scientific cosmologies and shifted discussion about classification in our Solar System from a prototypical to an Aristotelian language, was Pluto demoted.