Definition of a Planet
After having to deal with this question countless thousands of times,
I thought I'd write down what I think a good definition of the
word planet would be.
A planet is an object that satisfies both of the following two rules:
- Rule 1: A planet must not be so large that it can support or sustain
fusion reactions (can't be a star). A secondary requirement is that a
planet must not contain
degenerate matter (such as a core of solid neutronium that might be left over
from an exploded star).
- Rule 2: A planet must be large enough to have a shape determined by
gravity and not the strength of its material (usually this means it is
The word planet has come to be one of those fundamental words we use
to label some of the objects in the universe. To be useful, I think this
definition needs to be kept as simple as possible. The historical development
of this word basically boils down to a label for large things that could be
places we could visit. By this definition, we can imagine technology that
would let humans interact directly with anything called a planet. Of course
we can also visit smaller things but they seem to fall in a different category
becuase of their extremely low surface gravity. In the end, that's really
what I'm using as the underpinning of my definition and that's the mass
and density and thus gravity of an object.
There are some interesting consequences of my definition which a sharp
reader will have already considered.
- The Sun or any other star does not appear in the definition.
I don't care where a planet is or what its orbit is --- if it fits this
definition, it's a planet.
That means our Moon is a planet. All of the Galilean satellites are
planets. Many of the larger Saturnian satellites are planets, especially
Titan. Now, to make sense of the objects we see, we'll need some extra
categories for our planets.
- Primary planet
- This is a planet that is in orbit around a star. (Like the Earth.)
- Secondary planet
- This is a planet that is in orbit around another planet. (Like our Moon).
- Wandering planet
- This is a planet that is not in orbit around any particular object.
This could be a planet that was once in orbit around a star
but was stripped away and left to wander in the cold and dark between the stars. It
may still be in an orbit around the center of a galaxy but doesn't orbit
anything in particular.
- How many planets are in our solar system? Here's the list I came up with:
Mercury, Venus, Earth, Moon, Mars, Ceres, Jupiter, Io, Europa, Ganymede,
Callisto, Saturn, Enceladus, Tethys, Dione, Rhea, Titan, Iapetus, Uranus,
Ariel, Umbriel, Titania, Oberon, Neptune, Triton, Pluto, and Charon. Mimas,
Miranda, and perhaps Sedna should be included as well but they're getting
pretty close to the lower bound. That's a total of 27 for sure, maybe more.
- Ok, so how many primary planets are there? Well, there are at least 10 that
we know of and a few more where we don't have enough information yet. Aside
from the nine most of us are already familiar with, Ceres is almost certainly
big enough to be counted a primary planet and Sedna might be big enough too.
This definition is not universally accepted in the astronomical
community but I think it is a good definition that can be defended and could
actually be useful. I have tried to come up with something that captures
the spirit (not the law) of past usage of the word while being mindful of
what know about the universe around us. If this definition were to be
accepted and still useful (and used) 10,000 years from now by our distant
descendents (or any other beings for that matter), then I would consider
this definition to be a success.
Marc W. Buie, Southwest Research Insitute, March 2005
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