the monthly newsletter of the Alachua Astronomy Club, Inc. This letter was written in response
to a short news article that had appeared in the January 2001 issue of FirstLight.)
Where does Pluto fit in? Opinions vary
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The January 2001 issue of FirstLight contained an article titled, "Planet Pluto demoted by Hayden Planetarium." This story was apparently prompted by recent reports about Pluto in many newspapers such as the New York Times, Gainesville Sun and Florida Times-Union. (The latter ran this news story on the front page of the January 27, 2001 issue with the headline, "Pluto's status as planet in doubt.") In fact, this is not "new news" since this subject has been around for several years. However, I take issue with these reports (as does the International Astronomical Union) and must come to the defense of the little "plutinos" of our universe. The "demotion of Pluto" is primarily the result of statements made by a few astronomers such as Brian Marsden (Smithsonian Astrophysical Observatory) and Neil De Grasse Tyson (Director, Rose Center at the Hayden Planetarium, New York American Museum of Natural History). Since the opening of the new Rose Center about a year ago, it has become well known that Tyson omitted Pluto in the count of solar system planets. Tyson (St. Petersburg Times) suggests "Pluto is not necessarily a planet at all but a lump of ice." Why? Because Pluto is a small, icy world (smaller than Earth's Moon) and resides in the outer solar system (the Kuiper Belt) where hundreds of other small "ice chunks" roam. In a sense Tyson did not go far enough. Following his thinking, our solar system really contains only "four planets." Which four? Only the four Jovian planets—Jupiter, Saturn, Uranus and Neptune. Why these four? Because the four Terrestrial planets—Mercury, Venus, Earth and Mars—are also small worlds very different from their larger brethren. If Pluto seems "small" compared with other "planets," what about Earth? Yes, our planet is five times the diameter of Pluto and four hundred times more massive. But Earth is similarly "small" when compared with its larger siblings. The Jovian planets are about four to eleven times the diameter of Earth and fifteen to three hundred times more massive. For Mercury, smallest of the Terrestrial planets, the numbers are more extreme. Even the smallest Jovian planet, Neptune, is more than ten times the diameter of Mercury. |
Furthermore, "Jovian planets are gas and liquid giants" with extensive envelopes rich in hydrogen and helium. Their average densities are low (similar to water). Nevertheless, the Earth is nearly all "rock" with virtually no gaseous envelope and an average density more than five times that of water. Clearly the Earth is very unlike a Jovian planet. The Earth, along with the other terrestrial objects, are all small, inconspicuous "rocks" compared with the large, less dense gas giants. So, why not demote the Earth? If we do not wish to demote the terrestrial objects, then perhaps we need to "elevate" the Jovian planets. Indeed, many astronomers compare Jupiter to brown dwarfs or "failed stars." In fact, Jovian planets are more like the Sun than Earth—the Sun is a large, hydrogen rich, gaseous ball with an average density like Jovian planets. So, should we consider the Earth, Venus, Mars and Mercury the only "true" planets? And the Moon? Mercury, a "planet," is only slightly larger than our "satellite." Their diameters are, respectively, about one-quarter and one-third Earth's. In addition, the Moon is also a "rock" with a relatively high, terrestrial-like density. If the Moon orbited the Sun by itself, would we not call the Moon a planet? I concur with Tyson when he says astronomers have yet to agree on a standard definition of a planet. Yes, Pluto is different from the other eight planets. Yet, the Earth is also very different from planets like Jupiter. In the early Twentieth Century, astronomers classified planets into two major groups—the Terrestrial ("Earth-like") and Jovian ("Jupiter-like"). After the discovery of Pluto in 1930, later studies of this planet showed the "ninth planet" was a small, icy world trekking through the dark, outer regions of the solar system. We now recognize that two planet groups are not enough. Pluto's status is not really in doubt. Pluto simply belongs to a third group of "planets," the Plutinos, named for its largest member—Pluto. Howard L. Cohen Associate Professor of Astronomy University of Florida |
