The Universe in the Classroom

No. 59 - Fall 2002

© 2002, Astronomical Society of the Pacific, 390 Ashton Avenue, San Francisco, CA 94112.

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A Good Definition of the Word "Planet": Mission Impossible?

by Gibor Basri, UC Berkeley

Looking (it) Up
The Problem with Pluto
Planets and Brown Dwarfs
Born Into the Right Class?
Runts and Runaways

Mission Definition

Ask anyone on the street to give an example of a planet, and they'll consider you a bit dim. Perhaps they'll answer with "Earth", or maybe one of the other obvious choices like Venus or Mars. If you ask "How about Pluto?" they may pause, having heard that there is a bit of a debate about that. If you ask, "What is the difference between planets and stars?", the number of people able to answer well drops dramatically,  being a subset of the scientifically literate population (which we wish was much larger). Finally, if you ask "What is the exact scientific definition of 'planet'?", it turns out that nobody can answer it, because there really isn't one.

1) Looking (it) Up

Of course, you can certainly look "planet" up in the dictionary (have your students do this). Mine says that a planet is "any heavenly body that shines by reflected sunlight and revolves about the Sun". It also notes that "planet" originally meant any heavenly body that moves with respect to the fixed stars, which included the Sun and Moon. The word itself means "wanderer" in Greek, and other cultures generally have words for "planet" with a similar meaning. So why isn't that good enough for us today? I suppose we can blame Copernicus and Galileo. The former taught us that basing everything on what we at Earth can see is a mistake, and the latter showed that using telescopes gives us far more information about the cosmos than our naked eyes. The discoveries since Galileo (and especially from the last decade) leave us with knowledge that renders the old definition of "planet" completely inadequate.

Nicholas Copernicus (1473-1543) published "On the Revolutions of the Heavenly Spheres" in the year of his death. In this work he proposed that the Earth is not the center of the Universe, but revolves around the Sun as do the other planets.
Galileo Galilei (1564-1642) used his telescope to make observations of the planets that supported the revolutionary ideas of Copernicus.

What's the problem? First of all, we now know that all heavenly bodies shine. It is just a matter of how brightly and with what kind of light. Anything with a temperature above absolute zero will emit light (more technically: electromagnetic radiation). Even you do! Objects at the temperature of people or the planets in our solar system do so primarily in the infrared (what we call "heat radiation": the stuff that night-vision goggles use). It is true that in visible light (the stuff your eye works with), the planets are much brighter from reflected sunlight than from their own luminosity. Actually, Jupiter emits more total radiation from its internal source than the amount of sunlight it reflects, but its internal radiation is mostly infrared. In any case, that is nothing fundamental, since it also depends on the distance of Jupiter from the Sun (ask your students why?). And it is certainly true that the Sun is vastly more luminous than any of the planets.

Infrared head
Infrared Jupiter
Red and Pink colors indicate regions where more heat is being emitted and yellow and green are cooler areas. Note that this person was wearing eye glasses when this picture was taken.
Image courtesy Teletherm Infrared, Florida
The bright areas in this infrared image of Jupiter show regions where heat is escaping through gaps in the clouds. Jupiter has an internal heat source, and it emits twice as much heat as it receives from the Sun.

This argument may seem like a quibble, as does the second problem, which is caused by saying that planets must revolve about (orbit) the Sun. There was no problem with that until we began finding planets around other stars (in 1995). Now the count of extrasolar planets is roughly 100, and it will continue to increase rapidly in the foreseeable future. (See Of course, one can generalize to "revolves about a star" rather than explicitly mentioning the Sun. We can thus fix the dictionary definition to something like "A planet is an object whose own luminosity is much fainter than the star which it orbits." This seems to improve the dictionary definition to be more in accord with current science. But alas, this is also completely inadequate!

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