Lecture Starts at 11:13
PSW Science #2,409
A Planet Definition Debate
Alan Sterns & Ron Ekers
April 29, 2019
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Lecture by Alan Stern
This lecture will describe scientific issues involved in deciding on a definition for the term “planet,” stressing the value of data developed since the 1990s that planetary types are more diverse than had been imagined. The lecture will describe a proposed new formal definition, the Geophysical Planet Definition (GPD), according to which “A planet is a sub-stellar mass body that has never undergone nuclear fusion and that has sufficient self-gravitation to assume a spheroidal shape adequately described by a triaxial ellipsoid regardless of its orbital parameters.” The lecture will explain why planetary scientists created the GPD, its advantages in classifying planetary bodies, and why it is superior to the definition adopted not long ago by the IAU that classifies some planetary bodies, such as Pluto, not as “Planets” but as Dwarf Planets. The lecture will also touch on flaws in the way the IAU’s planet definition was established and why voting is not an appropriate method for making scientific decisions, such as categorization.
Lecture by Ron Ekers
The International Astronomical Union (IAU) has been the arbiter of planetary and satellite nomenclature since its inception 100 years ago in 1919, and this was one of the reasons for forming such an international body. Agreeing on names and conventions is a necessary part of international science, although it is not itself a science. Normally, IAU technical working groups make recommendations and decisions that primarily affect professional astronomers. However, from time to time, the IAU takes decisions and makes recommendations on issues concerning astronomical matters affecting other sciences or the public. Such decisions and recommendations are not enforceable by any national or international law, rather they are international agreements on conventions. As a result IAU decisions have to have broad consensus to be effective.
When Pluto was discovered, in 1930, it was thought, incorrectly as it turned out, to be a few times larger than the Earth. It is now known instead to be smaller than the Earth, indeed, smaller than the Moon. Many new Trans Neptunian Objects (TNOs) similar to Pluto have been discovered since then. One of them, TNO, 2003 UB313, found by Mike Brown in 2006 is estimated to be larger than Pluto. The IAU faced a conundrum naming this object because the naming conventions for minor solar system bodies were different from those for planets and because there had never been a formal definition of a planet. The IAU had to make a decision on the definition of a planet in order to name the new class of TNOs. Were all the TNOs to be called planets or was Pluto, which all experts agreed was one of the TNOs, to have a special status? To establish a clear taxonomy, the IAU established a formal definition for “planet” in the solar system as “(a) a celestial body that is in orbit around the sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighborhood around its orbit.” As is now well known, Pluto did not meet all three criteria and, according to a companion definition, was classified with its fellow TNOs, as a “Dwarf Planet.”
This lecture will discuss some of the considerations of the IAU executive and its working groups during the Prague General Assembly of the IAU in August 2006 that led the IAU to introduce the category of dwarf planets for these TNOs, and to include Pluto in this classification.
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