Even before the discovery of Eris, an icy body of the outer solar
system larger than Pluto, a heated debate was raging over how we should
define a planet. Something had to be done. Either Eris was a planet, or
So the International Astronomical Union set up a committee to consider the issue, to report back to the IAU conference.
The committee came back with the new definition that a planet had to
be orbiting a star, big enough to have collapsed into a spherical shape
under its own gravity, and not be the satellite of a planet.
I had to think about it for a while, but in the end it was clear
that this was a good definition, not because it saved Pluto, but
because it was a robust, physical definition. It could be applied to
new discoveries without the need for further arguments and conferences.
This definition included the nine existing planets and added Eris,
Ceres and Charon making a total of 12 planets.
Alas, the IAU didn’t ratify the recommendation. Instead of thinking
about what a planet really is, physically, it decided instead to draw
an arbitrary line in order to preserve an arbitrary and by no means
broadly-held perception of what a planet should be.
The change to the committee’s recommendation was to invent a new
category called “Dwarf Planets” to apply to Pluto, Eris and other
Even the name doesn’t make sense. Dwarf planets, the ruling goes,
are not determined by size. In fact, they’re the same as normal
planets, except that they have not “cleared out the neighbourhood
around their orbit” of other bodies.
The orbital neighbourhoods of all planets are cluttered with other
bodies. What exactly is an orbital neighbourhood? Here again we have an
arbitrary decision and description, not a physical one.
Even Jupiter has a whole retinue of asteroids which share its orbit,
the Trojans. We now need to insert a special “secondary” set of
definitions - again arbitrary - to exclude the Trojans, just so that
Jupiter can still be called a planet.
Space in the outer solar system is huge. If we finally find out, in
years to come, that there is nothing else sharing Pluto’s orbit, will
we then call it a planet after all? How close to it does another body
have to be before it is deemed to have not been “cleared out”?
And what about ellipticity? Just because the orbits of the planets
out to Neptune have low ellipticities and obliquities, what logic is
there in deciding that all planets have to have this? How small an
orbital ellipticity and obliquity does a body need to have before it is
a planet? Here is yet another arbitrary, non-physical, line which has
What if we find a body the size of Earth out in the Kuiper Belt? Is
this still a “dwarf planet”? What if we find a body the size of Neptune
out there? Will we suddenly insert some post hoc reason to call it a
planet after all? Maybe its ellipticity will be low, so we could just
“tweak” the definition to include it that way, without including all
that other icy riff-raff out past Neptune.
The author will be among the astronomers and enthusiasts
converging on Bendigo from March 23-25, 2007, for VASTROC the Victorian
Astronomy Conference. To find out more about the conference vist the website.
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