Many people reacted to the boisterous debate in Prague Aug. 14-25 concerning the definition of what constitutes a planet. Most folks figured that issue had been settled a long time ago and were surprised, occasionally outraged, to find out that astronomers were discussing it at all.
The word “planet” comes from a Greek term meaning “wanderer.” Before electric lighting, planets were a part of every evening’s activities. In the dark sky, they stand out clearly and change position against the fixed background stars. Those we see with our eyes: Mercury, Venus, Mars, Jupiter and Saturn, were the only planets known through most of human history. Once Galileo pointed his telescope skyward in 1609, the hunt began for other, fainter compatriots to the familiar wanderers.
Two hundred years of searching were required before another planet was found. Uranus was discovered in 1781 by Englishman William Herschel and Neptune in 1846 by Johann Gottfried Galle (a German) and Heinrich Louis d’Arrest, (a Dane) at a location predicted by Urbain Le Verrier from observed discrepancies in the orbit of Uranus.
Pluto was not discovered until 1930. Clyde Tombaugh, working in Flagstaff, Ariz. , made photographic images of the night sky using a telescope at the Lowell observatory. Based on calculations by others (later shown to be incorrect), Tombaugh executed a systematic sky survey, turning up Pluto, a comet and a range of other objects in the sky (including a supercluster of galaxies). To find Pluto, he spent thousands of hours meticulously comparing photographs, finally concluding his effort 17 years after he began. This was very hard work.
Technology has given our quest for solar system objects greater reach. A number of objects similar to Pluto, including one likely larger than Pluto, have been found in the outer solar system. These discoveries, along with Pluto’s oddities of size, composition and orbit, are what sparked the International Astronomical Union to consider just what we mean when we say “planet.”
The debate in Prague was itself an interesting undertaking. An executive body, sensitive to likely pressure from the media, kept the proposal under wraps until the triennial meeting of the world’s astronomers was under way. Wandering the halls in Prague or screening e-mail from colleagues around the world revealed that consensus was going to be difficult to attain. Meetings by relevant committees were held, the definition was revised and a vote finally taken. The result, that we now have eight planets in our solar system, not nine, may upset some and please others. Even though we have lost something familiar, we have gained something of value in the process.
What we have gained is a better understanding of our solar system as uncovered by new observations and discoveries. This understanding can now be conveyed to all in the context of the new definition. The citizens of our solar system are the eight planets that dominate their region of space, the as-yet-incompletely revealed population of dwarf planets headed up by Pluto, and the retinue of small solar system bodies such as comets and asteroids.
At the core of science is discovery followed by understanding and revision of our old taxonomies. Tradition, as comforting as it may be, will continually be overturned by new discovery. The new definition of planets orbiting the sun is carefully aimed at bodies in our solar system, but it can easily be generalized to the many recently discovered extra-solar planets by replacing the word “sun” with “star.” There are speculations about earth-like bodies drifting far from any star that are capable of harboring life energized by inner radioactivity. Are these planets by this or any other definition? Does it matter compared to the wonder of scientific discovery? In a small way Pluto has lost some prestige, but it is now the first of a new class rather than the last of an old one. What we have gained in knowledge far outweighs any loss. The exciting race for the discovery of all the dwarf planets can now begin. Deeper understanding of our solar system and our place in the universe will follow.
Kevin B. Marvel is executive officer and J. Craig Wheeler is president of the American Astronomical Society.