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Is Pluto a Planet?: A Historical Journey through the Solar System

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"Is Pluto a Planet?" provides valuable historical context for the recent IAU decision to demote Pluto from planet status. The book's main point is that the progress of astronomy has forced several previous mutations in our definition of 'planet'. Several times newly-found objects have been accepted as planets, until it was realized they were just too different and represented a new class of objects. When we first discovered objects circling Jupiter they were first called planets, until it was decided that a planet had to have its own orbit around the sun, and thus these objects were redefined as moons. When we first found asteroids they were first called planets, since they did indeed have their own orbits around the sun, but then it was decided that asteroids were in a class of their own. And when we first found Pluto it was thought to be much larger than it turned out to be, but now it too seems to be much less typical of the other planets than of a new class of smaller bodies on the outskirts of the solar system. The author clearly holds the opinion that Pluto sneaked into planethood through historical accident and didn't deserve it, but he does try to offer the 'pro-Pluto' viewpoint. Readers should be aware that this book was written before the IAU decision and doesn't contain any account of it; for readers seeking a Pluto-specific book the long discussion of our evolving concepts of the solar system may be more than they wanted. But this book is also a unique study in astronomical and intellectual history, showing how people from many times and places have conscientiously tried to deal with the tension between old habits and new information.

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QUESTION: What Is the ultimate goal of this book?
ANSWER: Our solar system historically has had nine major plants. They are, as you proceed outwards from the Sun, as follows:

(1) Mercury (2) Venus (3) Earth (4) Mars (5) Jupiter (6) Saturn (7) Uranus (8) Neptune (9) Pluto.

({1 to 4} are the inner or terrestrial planets and {5 to 8} are the outer or giant planets.)

The controversy this book deals concerns the last planet on this list, Pluto. Is it indeed a planet or is it...something else?

This is the question that is answered in this informative, well-written book authored by David Weintraub, a professor of astronomy at Vanderbilt University.

To answer the question, "Is Pluto a planet?," it only makes sense that you have to answer a more basic question: "What is a planet?" This is actually the question at the heart of this book. Despite its simplicity, this question is not easy to answer.

This book tells how the meaning of the word "planet" has changed from ancient times to the present day, as new solar system objects (moons, asteroids, comets, centaurs, Kuiper Belt objects, etc.) have been discovered. Weintraub provides the historical, philosophical, and astronomical background that allows the reader to decide whether Pluto deserves to be called a planet.

A hallmark of this book is how it weaves the historical with the scientific into a single, intriguing story.

Think of the planets as tiny specks circling the sun. If you backed off to see the solar system as a whole, the four outer giant planets would hardly be noticeable and the four inner planets including Pluto would be lost in the sun's glare.

This conception of the solar system was accepted only after one of the major intellectual upheavals in human history took place about four centuries ago: the Copernican revolution (after Nicolaus Copernicus). The key to this revolution was the work of five scientists who collectively lived between 1470 and 1730. These people paved the way for the solar system discoveries that were to follow.

It should be noted that this book was written just before the International Astronomical Union (IAU) decision regarding Pluto. Weintraub anticipated this decision and says:

"Making such a decision should not be a arbitrary activity. `Is Pluto a planet?' is a scientific question, not a matter of public opinion or a decision to be made by NASA or a panel of distinguished astronomers [which the IAU is]. Science moves forward at a pace dictated by progress in understanding, not by fiat or a majority vote of a committee."

The above statement by Weintraub should alert the potential reader to the high standard of excellence this book aspires too, a book grounded in science and not opinion or majority vote.

Finally, there are pictures, graphs, portraits, etc. found throughout. These help in both understanding and add another dimension to the book.

In conclusion, this is a book for those people seeking a fuller understanding of the history of our solar system, the science surrounding Pluto, and the tantalizing, provocative, recent discoveries in our outer solar system.

(first published late 2006; preface; 14 chapters; postscript; main narrative 230 pages; appendix: "What we know about Pluto;" notes; index)

<<Stephen Pletko, London, Ontario, Canada>>

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When is an object in the heavens a planet? At its heart, that's what David Weintraub's book, "Is Pluto a Planet?" is about. This topic, while of limited significance astronomically, has been much in the news and the public eye since the International Astronomical Union (IAU) decided to reclassify Pluto, along with a few other objects, as a minor planet.

Weintraub does an excellent job of working through the historical answers to this question beginning with the ancient Greeks and working into the twentieth century ideas and definitions. In doing so, he encourages the reader to understand that today's popular definition (i.e.-the one held by most casual thinkers) is clearly tied to ancient definitions that have long lost their relevance in today's discussion. Covered in the first 150 pages of the book, this narrative clearly outlines the progression from where humankind began in its journey of understanding the heavens to a modern view of the solar system and the forces and processes that govern and shape it.

Perhaps the best part of the book however lies in the last four chapters. It is in these chapters that Weintraub foes into the science of the last 10-15 years that has led to the questioning of Pluto as a planet and what other things it might be. While I disagree with the author's final conclusions regarding the matter, I do think he does an excellent job in asking the reader to consider what makes a planet a planet and challenging whether such a term has any relevance in today's astronomy.

My only disappointment in the book was that the author did not include the criteria that the IAU used in designating Pluto and other objects as "minor planets". I feel that while the author seems to disagree with the IAU's final determinations, it would have been much more helpful to have both sides of the argument to compare. By leaving the the IAU's position out of the text, the author seems to be somewhat afraid of allowing his readers the chance to understand all aspects of the debate.

I would definitely recommend this book to all readers who are interested in the history of solar system astronomy, are curious about the most recent discoveries at the outer edges of our solar system and who have been energized by the debate over Pluto's status. This book is an excellent contribution to the conversation.

David Weintraub has presented us with a thorough history and scientific analysis of Pluto and the question of whether Pluto is a planet or not. However, I recently received a strange e-mail that may have been misdirected:

Dear Puny Third-Object aka Earth Inhabitants,

Greetings from the Four Worlds

Here on the Four Worlds, we greet your squabblings with derision. We do not even bother to name the pitiful objects including your insignificant third-object aka Earth home that orbit sunward of Jupiter. If we wished for amusement, we would toss your tiny pebble into Jupiter's red spot to see if it will float. But I digress.

Our scientists have determined that by definition there are four planets in our star system. As every school child knows, these are the Four Worlds, the ones you call Jupiter, Saturn, Uranus, and Neptune (Jolly Savants Understand Notation). That you bother to dispute whether Neptune's Dog, the rock you call Pluto, is a planet or not is a laughable matter. Your third-object aka Earth home does not qualify either. It is merely the largest of the Sub-Jovian Clutter.

For an object to be considered for planethood it must have at least thirteen natural moons, a presentable set of rings, and an atmosphere of hydrogen, helium, or methane. If you think that you can meet these minimum requirements, you may contact our Four Worlds Planetary Status Committee for an application form.

Best Regards,
Elzar of Neptune

Since Weintraub's book was written, the IAU has declared that the planetary solar system is complete at eight planets. It is no surprise that Neil deGrasse Tyson, the director of the Hayden Planetarium in New York, is said to get hate mail from third graders. He and the IAU have told the children of the world that they cannot discover a new planet, and by the way, their favorite planet no longer counts. What a terrible message.

Is Pluto a planet? The IAU definition says that a planet is a celestial body that:
- is in orbit around the Sun,
- has sufficient mass so that it assumes a hydrostatic equilibrium (nearly round) shape, and
- has "cleared the neighborhood" around its orbit.

Pluto seems to meet the definition of a planet depending on what you mean by "cleared the neighborhoood," but does the Earth? Even though this definition was crafted to include the Earth as a planet, I do not see that the Earth has "cleared the neighborhood" around its orbit. An estimate of the number of Earth-crossing asteroids includes 1500 larger than one kilometer and 135,000 larger than 100 meters in diameter. Has the Earth cleared its neighborhood? Ask the dinosaurs who had one of these objects fall on their heads 60 million years ago.

In addition, the Earth has a large body which co-inhabits its orbit around the Sun. We call it the Moon.

What about the Moon? It is not a planet because it orbits the Earth, not the Sun. Or does it? Viewed from the Earth, the Moon does orbit the Earth. However, the geocentric solar system was dispelled in 1543 by Copernicus. (See, for example, The Book Nobody Read: Chasing the Revolutions of Nicolaus Copernicus.) Viewed from the Sun, the orbit of the Earth traces a wavy ellipse. The undulations result from the gravitational influence of the Moon. What about the Moon's orbit? It too traces a wavy line. The Moon's orbit never loops back over its own path. This Earth-Moon system behavior is unique in the solar system. The Moon orbits the Sun just as the Earth does. The two form a co-planetary system. If the Earth is a planet by the IAU definition, then so is the Moon.

Weintraub's arguments are good, but I believe that anyone's definition of what is a planet depends as much on cultural and political context as on scientific analysis. I conclude that Pluto is still our ninth planet and will remain so. However, the IAU's decision brings up another question. What will we call it, planet or not, if an Earth-sized or larger planet is discovered at the edges of the solar system?

The question as to whether Pluto is indeed a planet has been the subject of much debate over the past few years. In order to clearly illustrate the difficulties in very simply defining what we mean by "planet", the author, a professor of astronomy, takes the reader on a spellbinding historical overview of how the planets and other objects that reside in our solar system were discovered - including the evolution of human thought on the solar system's structure. Near the end of each chapter the author revisits the question of what is a planet in light of what has been discovered thus far in the book. As astronomers' tools improved over time, it eventually became clear how complicated the answer to this seemingly simple question can be. A postscript includes the thoughts/opinions/reasoning of various astronomers on this issue and an appendix provides a synopsis of what is known about Pluto and how this compares with the other planets.

The writing style is very clear, friendly, lively, authoritative and tremendously captivating. It is also accessible to a very broad readership; any technical terms that are introduced are very well explained as they occur. With plenty of photos and diagrams, this is a book that can be enjoyed by anyone, especially those who are interested in the issues that pertain to the "Is Pluto a Planet?" debate.

As a professional astronomer, one thing I find students of all ages to be extremely interested in is the question of little Pluto's planetary status. Maybe Pluto evokes images of the adorable Disney character or maybe people simply have an affinity for the underdog, for whatever reason the smallest and most remote planet of our solar system holds a special place in the hearts of many school children, even grown up school children. As a result, when Pluto's membership among the ranks of other planets in the solar system was questioned and eventually withdrawn, students of all ages had a very emotional reaction to poor Pluto's demise. David Weintraub, in anticipation of the most recent furor over little Pluto, has compiled an excellent account of the development of our understanding of the solar system. In language accessible to all lay readers, Weintraub recreates the discoveries of a variety of solar system objects, including several of which you have not heard much. Eventually, he turns to the question of Pluto and argues very convincingly that this is indeed a scientific question and should not be answered by a majority vote of the International Astronomical Union. So, if you are a science teacher at any level or are just interested in learning more about the confusion over Pluto, don't think twice about getting this book. In the pages of this book, there is much to be learned about the nature of science, scientific discovery, and what it means to reach a consensus. Oh yeah, there is also an answer to the question...