International Astronomical Union/Martin Kornmesser
Sept. 6, 2006
The Name's the Thing
What's a planet, anyway? By helping to find increasingly distant worlds around our star, new telescopes are making this a difficult question to answer.
Most science fiction folks, writers and readers, are fascinated by space. Space travel, space trivia, space discoveries—you name it, they want to know what's new. The most recent "new" part of space science happens to be local—in our own solar system—about what orbits our star, the sun. Quick, how many planets are there? A few weeks ago, the answer was nine. That's probably what's in your textbooks right now. The new number, however, is eight (see "Pluto and the Plutons" or "Doggone! Pluto gets a planetary demotion").
But don't worry. We haven't really lost a planet.
It's all about words and what they mean. In science, words have a precise meaning so they can be used and understood by anyone. For example, water is a molecule made up of one oxygen atom and two hydrogen atoms. This statement means the same to a chemist, a biologist, and a nonscientist.
As new discoveries are made, however, new names are needed. Sometimes, old names need to be changed. The discovery of a new, sun-orbiting world larger than Pluto, called 2003 UB313 (unofficial name Xena), has made astronomers debate just what the word "planet" should mean.
One choice would have given us 12 planets instead of nine, adding the large asteroid Ceres as well as Pluto's moon Charon and the new world. But there was concern about what would happen in the future, given that there may be as many as 50 more objects in orbit as large as Pluto yet to be discovered. Do we really have 62 planets around our star?
Another choice, the one that the astronomers picked, is to add a new term for small orbiting worlds, namely "dwarf planet." At this moment, therefore, our star has eight planets and three dwarf planets. With more dwarf planets to come.
What do words matter? As we look at other stars, we're finding objects in orbit. To properly share information and understand these objects, everyone involved needs to use the same name to mean the same thing. As for science fiction writers? We love this stuff. I can't wait to put a dwarf planet in a story!
Challenge: In Orbit
To improve your orbital vocabulary, try the following word find puzzle. All the words describe or name something that orbits our star. Find them in the grid. Look up any words that you don't know by using a dictionary or going online.
Printable page
| asteroid |
Earth |
Neptune |
Saturn |
| Ceres |
Jupiter |
Oort Cloud |
Sedna |
| Charon |
Kuiper Belt |
Planet |
Uranus |
| comet |
Mars |
Pluto |
Venus |
| dust |
Mercury |
Pluton |
Xena |
Answer
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