Pluto Demoted

March 02, 2020

Pluto Demoted

Image: Wikimedia Commons

Lewis Jones


‘My Very Educated Mother Just Served Us…’ If you were born before 1998, you will likely complete that sentence with ‘Nine Pizzas’; however, if you were born after 1998 and went to school in America, the answer is ‘Nachos’, while if you were born after 1998 and grew up in Australia, the answer is ‘Noodles’.

Of course, I’m referring to the ordering of planets in our solar system, and, in particular, to the definition of ‘planet’ adopted by the International Astronomical Union (IAU), meeting in Prague, on 24 August 2006, by which our beloved Pluto, ninth from the sun (at least, when it's not eighth as a result of crossing Neptune’s orbit!) was de-planetised. We had to forget our mnemonic devices. Oh, the irony.

Resolution 5A(1) reads as follows:

A ‘planet’ is a celestial body that (a) 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 neighbourhood around its orbit.[1]

The discussion at the IAU revolved around (c), so let me clarify. Point (c) does not rule out the idea that a planet will have moons, but rather states that if a planet shares an orbit, the planet is in charge; other objects will have been captured by the planet.

Before the vote, the excitement ramped up when one delegate pointed out that by the proposed definition, neither Neptune nor Pluto are planets. They both fail the test at (c). Neptune crosses the orbit of Pluto; hence, Pluto has not cleared its orbit and is disqualified. However, if Neptune’s presence serves to disqualify Pluto as a planet, then Pluto, by the same logic, disqualifies Neptune!

The voice of the chair rang out over the PA system, ‘See Footnote 1. Footnote 1.’ There was a footnote. Here it is.

[1] The eight planets are: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.

Well, nothing like a flat contradiction to fix the problem! Community beliefs about which planets are planets meets the attempt to generalise that belief in a definition, in order to apply that learning to other systems. The resolution passed and there was no more pizza.

The moral of this story is that there are no given definitions in science. We make them up and sometimes we even vote on them. Definitions are necessary. They allow us to label and draw boundaries around the patterns and structures we observe in our world. They consolidate in a sentence or an equation what we think we already know and give us a common language as we seek to add new ideas into the system. Definitions are also problematic. Every definition is a form of pretending that the world is black and white; not grey. They train us to miss things about our world that don’t fit. They train us to scorn attempts to admit grey.

Another aspect of definition-making is the ‘we’. Science is several things—a method, a body of knowledge—but science is also a human endeavour. The global nature of the community is a great strength of science. The belief in objective truth draws people out of their own presuppositions as they have to test and interact with the observations of people, similarly committed, but with a different set of biases. However, every community, even within science, develops its own unjustifiable internal presuppositions. Some are foundational beliefs (such as that creation is orderly, without which the edifice would crumble) while others are less central (that Neptune is a planet and Pluto isn’t). Whatever the topic, the inescapable sociological reality of describing what we believe to be true, whether about the world, ourselves, or about God, should make us humble in our interactions with those who disagree with our descriptions and forever vigilant to regularly re-examine our own beliefs.


Dr Lewis Jones received his PhD in Astrophysics from the University of North Carolina, and moved to Australia to do postdoctoral research at the University of NSW. He subsequently completed the Bachelor of Divinity at Moore Theological College, and is now the Director of The Simeon Network, the postgraduate and academic arm of AFES. Lewis also serves on the Human Research Ethics Committee at UNSW, and attends Randwick Presbyterian Church.


[1] (accessed January 2020).

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