Wednesday, February 22, 2017

"All These Worlds..."

I'm fascinated that the argument over the definition of the archaic word "planet" simply refuses to die. On the one side of this ongoing non-controversy we have those who respect the current IAU definition and argue that we have eight planets in the Solar System plus an odd number of dwarf-planets (including Pluto, Ceres, Eris, etc.). On the other side of acrimony we have those who pine of the good 'ole days of "My Very Educated Mother Just Served Us Nine Pizzas" and can't abide the IAU decision.

But wait. Now we have a THIRD side, demanding that EVERY object that orbits the Sun is a planet--which will give us an impressive catalogue indeed!

Personally, I think that we need to dispense with this entire debate. We are making the mistake here of trying to squeeze the incredibly numerous and varied bodies that orbit the Sun into very small taxonomical boxes by forcing them all into some new, all-encompassing, impossible definition of a single word.


The term itself is unfit for our purposes. Meaning "wanderer" It's based on an ancient word applied in its era to the five naked-eye bodies that wandered in the skies of Babylon and Athens. Sometimes also applied to our Moon and passing comets, it simply isn't enough for today's Solar System.

Try on a different word: "Worlds".

When I speak to students who visit the Abbitt Observatory I use this word rather than "planets" because it better represents the bodies of the Solar System as we know them today; not as unknown and un-knowable points of light that inexplicably brightened and dimmed as they traced mysterious paths across the ancient heavens but as what they are to us in this modern era; actual places that can be visited and explored, studied and understood in all their sizes and varieties.

Earth is a world; there can be no debate on this. So is Mars. And the Moon, Vesta and Ganymede; all worlds. Tiny Pluto, giant Jupiter, icy Comet Halley and all the myriad objects in the Kuiper Belt...the word applies to them all broadly, without need of division or amplification.

Within the massive catalogue of worlds there are giants and pygmies, from super-Jovian exoplanets down through the scales to the smallest aggregations of rubble to be found in the Main Belt. Of course there is need of classification within the broad context of Worlds, but no requirement for time-and-energy wasting argument regard the very meaning of the term!

"Planets" are passe; open your mind to other Worlds.

1 comment:

  1. You've either forgotten or ignored the genuine position of planetary scientists who oppose the IAU definition in favor of a geophysical definition. This group does not seek to define everything that orbits the Sun as a planet. According to the geophysical planet definition they support, a planet is any non-self-luminous spheroidal body orbiting a star, free floating in space, or even orbiting another planet. This definition give primacy to objects' intrinsic properties rather than to their location in classifying them. If an object is large enough and massive enough to be squeezed into a round or nearly round shape by its own gravity--a condition known as hydrostatic equilibrium--according to this definition, it is a planet. Objects in hydrostatic equlibrium are very different from those that do not attain this status. The former are geologically complex, often differentiated into core, mantle, and crust. In contrast, asteroids and comets are poorly held together rubble piles. Just classifying everything in the Kuiper Belt by its location while ignoring the real differences between these two very different types of objects is simply bad science.

    Within the broad umbrella category of planet, adherents of the geophysical definition divide objects into subcategories based on their intrinsic properties. These subcategories include terrestrials, gas giants, ice giants, dwarf planets, hot Jupiters, Super Earths, etc.

    What is most concerning about this entry is that it completely misrepresents the position of professional planetary scientists who oppose the IAU definition. I urge you to contact Alan Stern or Kirby Runyon to learn more about their viewpoint and to read Alan Boyle's book "The Case for Pluto."