Explained in 60 Seconds: Why is Pluto not a Planet? 60 Seconds Explained in Explained in Chris Marshall Keywords Science writer Dwarf planet, Pluto, planet, Solar System, Eris [email protected] On 24 August 2006 an International Pluto became the smallest planet, and, a state of hydrostatic equilibrium prevails, Astronomical Union (IAU) resolution for most of its orbit, the furthest out from indicating that inward gravitational forces changed the shape of the Solar System the Sun, following its discovery in 1930 by are balanced by an outward pressure. and redefined what it meant to be a Clyde Tombaugh. However, the early 90s planet. As a result, Pluto, the smallest and saw the discovery of a spate of objects The final hurdle, however, proved fatal. To most recently discovered planet in the that resided beyond Neptune and Pluto. be a planet, the object must have cleared Solar System, became a dwarf planet. Their existence cast doubt upon the valid- the neighbourhood around its orbit. Pluto ity of this initial decision. Pluto’s pedigree is surrounded by shards of rock and ice, In the months that followed, the initial was called into question once and for all and so failed miserably to achieve this; its surprise over this decision gave way to with the discovery of Eris in January 2005. cluttered surroundings and poor house- acceptance, although many remembered Although slightly smaller than Pluto, Eris keeping were its undoing. Pluto the planet fondly. was found to be more massive, and ques- tions about what it really means to be a This third criterion is, in effect, a determina- These memories surfaced again just planet again came to the fore. tion of size. Pluto is too small to gravitation- recently, when NASA’s New Horizons ally dominate its environment by absorb- spacecraft flew by Pluto and captured Pluto was reclassified in Prague at the ing and ejecting material in its path. It is for the distant world with vivid clarity. People IAU XXVI General Assembly. A resolution this reason that when we look upon the around the globe asked anew: Why is Pluto passed on the closing day of the assem- New Horizon mission’s high-definition not a planet? bly provided a threefold definition of a images of Pluto, we see more that a dwarf planet. Pluto met the first two criteria with planet among many; we see a planet admirable ease: it is a celestial body that that once was, and an important piece of orbits the Sun, it is round, indicating that astronomy’s history. Figure 1. In both of these views of the dwarf planet Pluto, four images from New Horizons’ Long Range Reconnaissance Imager (LORRI) were combined with colour data from the Ralph instrument. In the left image this was done to create a sharper view of Pluto and on the right to create an enhanced colour view. 4 CAPjournal, No. 18, September 2015.