Ex-planets and exo-planets: Why Pluto is not a planet anymore

Abstract

The word "planet" was used by the ancients to describe wanderers or moving lights in the sky. There have been efforts to refine the definition of a planet to better reflect our contemporary scientific understanding of planetary systems.
The main documents from the International Astronomical Union on this issue are: 1) the "Position Statement on the Definition of a Planet" by the Working Group on Extrasolar Planets (WGESP) of the International Astronomical Union last modified on 28 February 2003, and 2) Resolution 5 ("Definition of a Planet in the Solar System") and Resolution 6 ("Pluto") approved by the XXVIth General Assembly of the International Astronomical Union in Prague on 24 August 2006. The WGESP statement emphasized "a gradualist approach with an evolving definition, guided by the observations that will decide all in the end," and proposed as parts of a working definition that an object is a planet if it orbits stars or stellar remnants, its mass is below the limiting mass for thermonuclear fusion but above a certain minimum mass/size. The working group did not specify a value for minimum mass/size but instead recommended that "The minimum mass/size required for an extrasolar object to be considered a planet should be the same as that used in our Solar System." Resolution 5 did not designate a specific numerical value for the minimum size or mass but instead set the criteria: "a planet in our Solar system is a celestial body that a) is in orbit around the Sun, b) has sufficient mass for its self-gravity to overcome rigid body so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighbourhood around its orbit." In addition to setting the criteria for a planet Resolution 5 also created two other distinct categories of bodies in our Solar System: "dwarf planets" and "small solar system bodies." Resolution 6 acknowledged that by the criteria set in Resolution 5, there are only eight planets in our Solar System and that Pluto is a "dwarf planet." Additionally Resolution 6 recognized Pluto as the prototype of a new, and yet unnamed, category of trans-Neptunian Objects.
The talk will highlight the discoveries within and beyond the Solar System that eventually lead to the creation of the above IAU documents and the developments that came after the adoption of the August 2006 IAU Resolutions. The discoveries include dynamical resonance between Pluto and Neptune, the discovery of Charon, 2003 UB313, and other Kuiper Belt Objects, and the discovery of hundreds of planets outside our Solar System.