May 29, 2020

Is Pluto a Planet? by David Weintraub

The issue of planet-hood has become a hot button topic in the astronomical community as of late.

In is “Pluto a Planet?” Author David Weintraub tackles the thorny issue that has plagued astronomers and school children alike. 

The book is aptly subtitled A Historical Journey through the Solar System, and Mr. Weintraub delivers just that. The tale weaves a fascinating path through history, our concept of the Solar System in general, and the definition of a planet in particular. For instance, at one time, the Sun & Moon were considered planets; the Earth, however, were not! Other temporary visitors to the planetary role call include Ceres, Juno, and Vesta (asteroids). The illusive Vulcan (a hypothetical inter-mercurial planet; so much for Mr. Spock!) and most recently, Eris, which sparked the current debate. Through-out the reading, one comes to realize that the concept of “planet” is purely cultural one, without a truly definitive scientific basis. This has long been known, but astronomers were reluctant to do anything about it. Perhaps, in hindsight, the discovery of Pluto and its initial assessment as a planet in 1930 may have been premature.

As astronomers scrutinized the tiny world, it began to seem less planet-like. Initially estimated to be ten times the earth’s mass, later studies and the identification of its moon, Charon, sunk it in the 1970′s to less than 0.01 of an earth mass. Astronomers joked that at the current rate of shrinkage, Pluto’s’ mass would become negative by 1984!

The author is almost right up to date on the cusp of the current debate, as the book was published in 2007, after the discovery of UB313 and the International Astronomical Unions’ (IAU) string of definitions, the most recent of which was last month . The book also recognizes that further refinement of the definition is probably necessary, as more remote Pluto-like bodies, or “Plutoids” are uncovered. Also considered are exo-planets; first discovered in 1992, they are proving to be bizarre beasts as well, from “hot Jupiters” to pulsar planets. The universe is becoming a very strange place! Will we ever be able to say “Captain, we’re entering orbit around a class M planet,” ala Star Trek? (OK, I know, that was two trek references in one post!)

Amid all this, the New Horizons spacecraft is due to arrive at Pluto in 2015. Will this shed more light on the subject? Pluto remains the last of the “Big 9″ to have its picture taken with any kind of clarity.

I know of some school children that were seriously hurt by the IAU’s decision. We all like to root for poor little underdog Pluto. Certainly, a nine planet solar system is tidy and simple. But, as pointed out in the book, Pluto is tinier than several moons, including our own! Incidentally, two more miniscule moons have been discovered recently orbiting Pluto; Nix & Hydra; that brings the compliment of satellites up to three. Will more be discovered during the New Horizons flyby?

The author also illustrates the dilemma of using different criteria for defining planetary status. Does it directly orbit its parent star? Many objects do that, from dust grains to pulsars. Is it basically round? A little better, but density has a lot to do with minimum diameter to achieve “hydrostatic equilibrium”. Does it clear its orbit? As it stands, this is still a confusing definition that begs a clear explanation.

And so the question remains; is Pluto a planet? Like a good open ended mystery novel, the author ultimately leaves it up to the reader to decide. We haven’t heard the end of this issue; discoveries are coming at such a swift pace in the field of astronomy, doubtless more edicts will be issued. All the while, objects like Pluto will remain what they are, without regard for human definitions. Perhaps, rather than fretting about clean categories, we should instead take the tack of descriptive field geologists, and simply observe and digest what we see, without pre-judgment. At very least, sub categories of objects are needed; “Plutoids” are a step in the right direction. But Pluto will still take its place in the cultural pantheon of the “Big 9″. I notice that the magazines Sky & Telescope and Astronomy haven’t been in a hurry to take Pluto off of the monthly planetary role call!

As an aspiring science educator and one who has been “in the trenches” educating the public one telescopic view at a time, I would heartily recommend is Pluto a Planet? As a method to arm yourself for this quintessential 1st grade question. One thing is certain; they will grow up in a universe that much more wondrous and bizarre than the old Del Rey: the Solar System view that we had! Believe me, it seems to be what they’re all taking about; and it comes up with the public more than once!


  1. [...] How Old is the Universe? out by Princeton Press. Fans of this site will remember our review of Is Pluto a Planet? also by Mr. Weintraub a few years [...]

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