Thursday, July 23, 2015

Earth Was Once, Like Pluto, a Non-Planet

Surveys say Pluto is still a planet, in spite of a formal action by a small number of astronomers to classify it as a non-planet.

This partly reflects people not keeping up with science, but mostly reflects the belief that the International Astronomical Union (IAU) made a mistake in adopting a definition of planet so narrow that it excludes most bodies that people would think of as planets.

Most people, I believe, think of a planet as a kind of material object. That is, if you see a photo of an object and it looks like a planet, and it is the right size to be a planet and made of the right materials, then it must be a planet.

By contrast, the definition adopted by the IAU defines a planet based not on what it is but on what it does. The core of the IAU definition is that a planet must be in a highly regular orbit around the sun. Pluto would be a planet under the IAU definition if its orbit were almost perfectly circular. At least, I think it would. There are other complexities in the definition, and even with the benefit of my degree in mathematics, my skill at calculus is not strong enough to work out the myriad details of applying the IAU definition to this hypothetical case.

There are other obvious problems with the IAU definition of a planet. Following conventional theories of planet formation, every planet must at one time have been a non-planet. At one time Earth would have been considered a dwarf planet, and it went along that way for tens of millions of years, until one day gravitational effects from Jupiter stabilized its orbit enough that it qualified as a planet. It is not that Earth got larger or changed in any material sense to go from being a dwarf planet to being a planet, but that its orbit lined up better. Unfortunately, figuring out exactly what day that would be, even if we could directly observe it, would involve computations that went to 25 decimal places. A planet like Earth would be a transitional edge case, a candidate planet if you will, subject to argument for thousands of years before new measurements and computations showed with a degree of confidence that it had become a planet. To imagine an even more problematic edge case, a disturbance in the orbit of a planet could cause it to become a non-planet for several million years, after which it would be a planet again.

The worries over the IAU definition of a planet might have been allayed if the new photos of Pluto showed that it looked like an oversized comet. The fact that Pluto turned out to look exactly like a planet has to be an embarrassment to the IAU, as every story about Pluto has to incorporate the strained explanation of why Pluto is considered a non-planet.

A definition of a simple object that requires calculus to apply in the most ordinary cases and allows for this degree of fuzziness in actual, well-known edge cases is a poorly chosen definition. The fact is, the IAU was overreaching in making such a big change in the definition of planet without checking with the public or the scientific community or even forming a consensus among its own members. This is a problem that could most easily be solved by the IAU itself, reopening its definition of planet and choosing a broader definition that is a better fit with the intuitive understanding of a planet as a physical object. My guess is that they have about 15 years to make that correction. It could be almost as simple as inventing a term such as “major planet” for something close to the current definition of planet, then including every kind of “planet” in the word planet. If they fail to act, what is likely to happen is that the news media and public will adopt something closer to the popular understanding of a what a planet is, and devise an awkward term such as “IAU planet” for the IAU’s broken definition.