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Mercury, November/December 2003 Table of Contents

Image courtesy of John Whatmough.

by Gibor B. Basri

Recent discoveries inside and outside the solar system have complicated our perception of what constitutes a "planet."

Even before civilization, people looked at the sky and recognized different celestial objects. The Sun defined daytime and the stars provided a fixed background of twinkling lights at night. Among the stars moved the Moon and a few special steadier lights. The Greeks called the moving lights "planets," which was derived from their word for "wanderer." The Greeks included the Sun and Moon as planets, along with Mercury, Venus, Mars, Jupiter, and Saturn, because motion against the stars was their defining characteristic of "planet." Both the stars and planets were thought to revolve around Earth.

After the Copernican Revolution, we recognize the Moon as the only natural body that orbits Earth. We have also discovered that the Sun is a very nearby example of a star, and the visible planets are other large bodies that orbit the Sun. We see them by reflected sunlight, while stars produce their own visible light. This understanding yields a Webster’s dictionary definition of the word "planet": any heavenly body that shines by reflected sunlight and revolves about the Sun.

In the past century astronomers gained much understanding of our solar system, and robotic spacecraft have visited most of the planets. Yet today, astronomers find themselves unable to agree upon a succinct definition of the word "planet." After the recent discoveries of objects in the outer solar system, extrasolar planets, brown dwarfs, and free-floating objects of planetary mass, we realize the longstanding definition of "planet" is overly simplistic and narrow. There is much, much more than meets the eye.