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Is Pluto a Planet?
A Historical Journey through the Solar System
By David A. Weintraub PRINCETON UNIVERSITY PRESS
Copyright © 2007 Princeton University Press
All rights reserved.
ISBN: 978-1-4008-5297-0
CHAPTER 1
What Is a Planet?
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Is Pluto a planet? This question appears so simple—clearly the answer is either yes or no—yet the simplicity is misleading. Logically, we must know what a planet is and determine whether Pluto fits those criteria if we are to construct a well-reasoned answer.
The question Is Pluto a planet? has stirred the passions of professional astronomers since this enigmatic object was discovered in 1930. In order to understand why this question vexes the professionals, we will first follow the path of early intellectual discovery along which scientists came to recognize that the Earth is a planet. Then we will walk the historical path that led to the discovery of much of the solar system, including the planets Uranus and Neptune, the asteroid belt, and Pluto. Finally, we will focus our attention on important astrophysical discoveries since the discovery of Pluto that have culminated in widespread, popular confusion and impassioned professional debate over the status of Pluto.
At the end of the twentieth century, the debate over the answer to the question Is Pluto a planet? spilled into the public domain because several new discoveries—large objects in the Kuiper Belt in our solar system, giant planets orbiting other stars, possible planet-sized objects floating freely through space—all provide new and important ways to think about how to answer this question. We will look at these new discoveries, especially at their impact on how we think about planets and planetary systems, including, of course, how we think about our own solar system.
The question Is Pluto a planet? illustrates a difficult challenge common to all areas of research and thought: how do we draw the lines we use to categorize objects and ideas? Categorization is one of the first steps in learning: we organize information by similarities and differences. We know that mammals give birth to live young; yet, a platypus lays eggs and is a mammal. Despite its name, a koala bear is a marsupial, not a bear. So which similarities are most important and fundamentally determine membership in a group or class? Which differences are incidental? As is often said, the devil is in the details.
Astronomers like to joke that knowledge of one object—a bright point of light in the nighttime sky—defines a class of objects: stars. The discovery of a second bright point of light that is not identical to the first forces us to create two distinct classes of objects—for example, red stars and blue stars. In this example, the two stars share one important characteristic—they are both bright points of light in the sky—but differ in the apparently important characteristic of color. If we then discover a third object in the sky sharing the original characteristic—a bright point of light in the nighttime sky—but differing in the second characteristic—this third object is yellow—do we conclude that these three objects belong to three distinct categories of objects, with each group being represented by only one example? Or, might we realize that we have discovered three similar objects that differ only in the incidental quality of color? Which is more important, recognizing the similarities among the objects (they are all stars) or emphasizing the differences (colors) between them? At what point should we discard the categories we are using and start anew?
By defining finer and finer categories, we learn about the physical universe in which we live. We use our knowledge both to expand our understanding (Wow, three kinds of stars exist!) and to delimit what we do not understand (Why do stars have different colors?). The problem for us lies in how we define a group (for example, planets) when we know very little about the individual objects in the group and almost nothing about the processes that made or make similar objects and have very few examples of objects that presumably are members of the group.
We cannot answer our question Is Pluto a planet? unless we are able to determine the qualities that define the boundaries of the category planet. Once we agree on how to define planet, we can ask whether a particular object, in this case Pluto, satisfies our criteria. Since we need to define planet, we could turn to a dictionary.
The Oxford English Dictionary (OED) defines the modern word planet as deriving from the Old French planete out of the Latin planeta; in turn, the Latin is derived from the Greek word for wandering star, planetos, which in turn evolved from planasthai, the verb "to wander." Clearly, we need to understand what the ancient Greeks meant by their word, and the OED tells us, giving the Old Astronomy (i.e., archaic and no longer used) usage:
A heavenly body distinguished from the fixed stars by having an apparent motion of its own among them; each planet, according to the Ptolemaic system, being carried round the Earth by the rotation of the particular sphere or orb in which it was placed. The seven planets, in the order of their accepted distance from the Earth, were the Moon, Mercury, Venus, the Sun, Mars, Jupiter, and Saturn.
If you are reading carefully, you will have noticed that according to the ancient Greeks, the Sun was a planet but the Earth was not! Certainly, no person in the twenty-first century thinks of the Sun as a planet. We all were taught that the Sun is a star, not a planet.
So did the Sun change from a planet into a star? Of course not. Apparently, though, our understanding of what is meant by planet changed; hence, at the moment when our understanding changed, we reclassified the Sun as a star and the Earth as a planet and discarded the Old Astronomy usage.
Next, we find the Modern Astronomy definition:
The name given to each of the heavenly bodies that revolve in approximately circular orbits round the Sun (primary planets), and to those that revolve round these (secondary planets or satellites). The primary planets comprise the major planets, of which nine are known, viz., in order of distance from the Sun, Mercury, Venus, the Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto, and the minor planets or asteroids, the orbits of which lie between those of Mars and Jupiter.
Another OED definition, given in the New Shorter OED, is similar:
Any of various rocky or gaseous bodies that revolve in elliptical orbits about the Sun and are visible by its reflective light, esp. each of the nine major planets (see below); any of various smaller bodies that revolve around these; a similar body revolving around another star.
The "see below" points to a list of primary planets, a definition of minor planet as an asteroid, and a definition of secondary planet as "a planet that orbits another planet, a satellite, a moon."
Whoa! The Moon revolves around the Earth; therefore, the Moon must be considered a secondary planet? Certainly, a secondary planet is some kind of planet just as a sweet potato is some kind of potato. Do we normally think of our Moon as a planet? No.
Mars has two potato-shaped moons, Phobos and Deimos, each no larger than a small city, both of which also match this definition. Are objects like Phobos and Deimos, with diameters of only a few miles, planets? No.
The Hubble Space Telescope most definitely is a satellite revolving around a primary planet, the Earth. Is the manufactured Hubble Space Telescope, which astronomers would consider to be a "rocky body," a heavenly body? If not, what if NASA hauled a ten-ton boulder into space and launched that boulder into a terrestrial orbit? The boulder is certainly a naturally made rocky body that would be in orbit around a primary planet. What meaningful difference would permit us to distinguish between these two orbiting objects, or would both qualify as secondary planets?
According to the latter two definitions, all of these objects might be planets, as are the asteroids in the asteroid belt, even the ones that are smaller than a house or car, since most of these objects revolve in approximately circular orbits around the Sun. However, according to the Modern Astronomy definition, a Sun-orbiting asteroid whose orbit keeps it in between the orbits of Earth and Mars, rather than in between the orbits of Mars and Jupiter, would not be a planet. Similarly, an asteroid in a moderately elliptical orbit between Mars and Jupiter, traveling from an outermost distance just outside of Jupiter's orbit to an innermost distance just inside of Mars's orbit, also would be disqualified as a planet. The New Shorter OED definition would allow asteroids with more elliptical orbits and those outside the Mars and Jupiter boundaries to be secondary planets, but why are such objects considered planets at all?
When I read these definitions, I want to know: Who are these Old and Modern astronomers? Am I one? Surely, the Old astronomers were not to be trusted, as they included the Moon and Sun as planets. And what of these Modern astronomers whose definition appears to include objects big and small, natural and manufactured, but only if those objects are in nearly circular orbits and in preferred locations? If either of the modern definitions is correct, far more than nine planets orbit the Sun. In fact, there must be hundreds of thousands, perhaps even millions, of planets in our solar system. Clearly, the commonly accepted notion that nine planets orbit the Sun does not match the OED version of the universe.
I, for one, am very dissatisfied when I read the OED definitions. I hope you are similarly bothered. Because the dictionary definition of planet is essentially worthless, scientifically, we have to probe much deeper into astronomy in order to answer what initially appeared to be a simple question. That is what we will do in the rest of this book.
CHAPTER 2
Seven Perfect Planets Made of Aether
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What would you learn if you could look up and observe the sky, night after night, year after year, unencumbered by the accumulated intellectual burdens and prejudices of the twenty-first-century world? The first thing you would notice is that, like clockwork, the Sun rises every morning and sets every evening. The exact moments and directions of sunrise and sunset change in cyclical patterns that mark the seasons; yet, no matter the season, the Sun appears to sweep out one great circle, moving from east to west in the sky, from exact noon one day to exact noon the next day, in exactly twenty-four hours.
Ancient peoples everywhere must have watched the Sun. By doing so, they learned how to predict the onset of seasonal changes based on the height of the Sun above the southern horizon at noon, the position of the Sun along the eastern horizon at sunrise or the western horizon at sunset, or the lengths and directions of shadows cast by the Sun at different times of day. Those peoples with more advanced skills in measuring time—times of day and times of year—were more likely to prosper and survive, as their ability to stay warm and dry and to find, store, and secure adequate food supplies depended on their expertise in predicting seasonal changes.
Certainly, in times long before written records were kept, our ancestors also must have recognized that the points of light that illuminate the nighttime sky rise and set, just like the Sun. They also would have noticed that different stars rise and set at different times of the year and are organized in recognizable patterns. Furthermore, they would have discovered that these stars, as they wheel around the Earth from east to west, never change positions with respect to each other. Their positions seem to be fixed; the stars themselves seem to be attached to an unimaginably large and distant celestial sphere, with the Earth at the center, which makes one complete revolution in twenty-four hours.
At first glance, the Sun seems to be attached to the celestial sphere, as the Sun appears to be carried around the Earth from east to west with the stars, once per day. Yet, the most attentive of the ancient sky watchers would have discovered that the stars appear to go around the Earth just a little more quickly than the Sun. On our modern clocks, we would note that the time for a complete cycle of the stars is almost exactly 23 hours, 56 minutes, and 4 seconds, nearly 4 minutes less than the 24 hours required for the passage of the Sun from the position in the sky we refer to as noon to its next noontime appearance. Thus, we might imagine that the Sun has two motions. One motion takes the Sun completely around the heavens from east to west in 23 hours, 56 minutes, and 4 seconds; the second motion takes it much more slowly in the opposite direction, from west to east, by the equivalent of almost 4 minutes each day.
Since 24 hours is 1,440 minutes, and 1,440 minutes divided by 4 minutes is 360, and since the circumference of a circle has 360 degrees, the Sun moves about 1 degree per day through the fixed stars. At the end of one year, or very nearly 365 days, not only is the Sun back to its starting point among the patterns of the stars, the seasons have also completed one cycle. Thus, the ancient astronomers would have discovered that they could track the seasons by watching the stars, which for some peoples may have been easier than watching the Sun. The warmth of the Sun obviously influences the weather, the growth of plant life, and the regulation of the seasons, whereas the stars do not obviously cause any of these effects; yet, one can easily understand how ancient peoples would have assumed that the stars, like the Sun, somehow must affect life on Earth. This primitive logic is ultimately the origin of the practice of astrology and many ancient religious practices.
Once ancient peoples had associated the ability to predict the onset of the changing seasons with the nighttime sky, watching and observing the patterns of the heavens and knowing how to interpret these observations would have become one of the most important jobs within all premodern societies. The Inca, for example, began their calendar year and agricultural year with the first annual predawn appearance of the Pleiades star cluster (in the modern month of June). Furthermore, if the Sun and stars appear to guide and control the day and the seasons, might other celestial objects that are neither Sun nor star exist that control other terrestrial phenomena, or even our lives?
All ancient observers would have noticed the presence and changing appearance of the Moon. Indeed, the 29.5-day period for the phases of the Moon was one of the easiest astronomical periods to quantify, for observers in virtually all ancient cultures. Peoples living along seacoasts would, early on, have associated the Moon with the tides. Now we have a second celestial object of great importance. The Moon, though it operates on a different schedule than the Sun, rising about 50 minutes later every day while paying no attention to the seasons, also rises and sets every day and is visible at night when it is up at night. Whereas the Sun moves through the fixed pattern of stars in 365 days, the Moon zips along a path through the stars and returns to its starting point in just over 27 days. The ancient nighttime observers would have noticed that the path of the Moon, as it travels through the stars, is very similar, though not identical, to the path of the Sun.
The circular path of the Moon, when drawn on the inside surface of our imaginary celestial sphere, intersects the circular path of the Sun at two points. At one of these points, the Moon can pass in front of, and thereby eclipse, the Sun. When this happens, we experience a solar eclipse. At the other intersection point, the Moon can disappear into the Earth's shadow and thereby be eclipsed by the Earth; at such times, we experience a lunar eclipse. Over a period of about eighteen years, these two points of intersection move synchronously around the full circular path of the Sun; that is, they occur at different times of the year. All the points on the celestial sphere at which solar and lunar eclipses can occur fall along the line followed by the Sun. Thus, the thin line along which the Sun travels was given the name ecliptic.
(Continues...)
Excerpted from Is Pluto a Planet? by David A. Weintraub. Copyright © 2007 Princeton University Press. Excerpted by permission of PRINCETON UNIVERSITY PRESS.
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