The Definition of Planet
Warning: this is not an official definition, merely a proposal.
- The Definition:A planet (in our Solar System) is
an object gravitationally bound to the Sun which has the same diameter
as Pluto or a larger diameter.
- The Planets: Mercury, Venus, Earth and Luna (the
Moon), Mars, Jupiter, Ganymede, Io, Callisto, Europa, Saturn, Titan,
Uranus, Neptune, Triton, Pluto, Xena [as we know it should be called;
why can't a planet be a warrior princess?]. I have checked this and I
believe I have the correct list of bodies, and moreover that the list
remains the same if "diameter" is replaced by "mass" (unless Xena is
paradoxically lighter than Pluto, which I have not checked).
- Advantages of this Definition: It is readily
checked by observation and it is likely to be stable. The
modifications of textbooks that it entails are virtuous: it is good
for students to know at least the Galilean satellites of Jupiter and
the largest moons of the other outer planets. No object which is
a planet under the old definition ceases to be a planet.
- Disadvantages of other definitions: The newest
definition is open to challenge on the grounds that it is unclear that
the classical planets have really all "cleared their orbits" in any
precise sense. There are plenty of asteroids that cross the orbits of
Earth and Mars, and Pluto crosses the orbit of Neptune.
The other definition proposed in the IAU is perverse in making Charon
a planet without making Luna (our Moon) a planet and likely to be
unstable in that numerous asteroids and large Kuiper belt objects
become debatable cases: the textbooks will constantly be adjusted.
- A question: Should it be "mass" rather than
"diameter"? Mass may be more easily observable; does it give a
different class of objects (there seems not to be an object with
smaller diameter than Pluto which is more massive; it appears
that Xena is heavier than Pluto).
- The Boring Alternative My sons pointed out (as I
knew perfectly well, of course) that adding the additional clause that
a planet should not be gravitationally bound to anything larger than
itself other than the Sun neatly restricts us to the familiar planets
and Xena. But I've always thought (ever since learning that Ganymede
has a larger diameter than Mercury when I was a child) that the major
satellites are worlds in their own right...