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Of once and future planets.

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

David A. Weintraub (Princeton University Press, 2006). 254 pages. ISBN-13 978-0-691-12348-6. $27.95.

IS PLUTO A PLANET? Good question. It's not one you can answer without asking many others about what planets are, how they evolved, and how humans have historically defined the term "planet." The answers make David Weintraub's Is Pluto a Planet? a very useful and satisfying read.

In his timely historical look at planetary definitions, Weintraub (who teaches astronomy at Vanderbilt University) begins with a very broad question--"What is a planet?"--and then branches out to the others: Are planets things that orbit the Sun? Do they have moons? Are moons planets? What about comets? Should planets be only those objects that have swept their orbits clear? Should they be a certain size? A certain distance from the Sun?

To answer these questions, Weintraub lays out the successive criteria that have defined "planet" over time. Each of his carefully researched chapters brings us closer to today's definitions, which depend on complex dynamical and morphological theories.


It may surprise you to learn that the ancient Greeks considered the Sun a planet, but not Earth. It makes perfect sense in the Earth-centered descriptions of the universe that persisted until Galileo and Kepler. What they saw knocked our home planet from its theologically preferred spot.

After that, the hunt was on for more planets. At one point, as Weintraub writes in the chapter "Sixteen Planets," when Christiaan Huygens and Jean-Dominique Cassini were finished exploring Saturn, the solar system was awash in planets, including a slew of newly discovered moons. It then settled down to 13, including several large asteroids. Somewhere along the line, Pluto became the ninth planet. Today, it's officially a "dwarf planet" and the prototype of a whole new class of objects at the frontiers of the solar system: the Kuiper Belt and Oort Cloud.


In "Not Everything That Orbits the Sun Is a Planet," Weintraub explains where things stood just before Uranus and Neptune were discovered:
   Comets are not planets. Moons are not planets. Thus, with these two
   unwritten rules in place for defining objects as "not planets," by
   the middle of the eighteenth century, the number of planets had
   become firmly established as six: Mercury, Venus, Earth, Mars,
   Jupiter, and Saturn.... The criteria for determining which
   objects were planets were no longer unassailably clear, as an
   object now could satisfy all the accepted rules for being a planet
   (i.e., Kepler's laws), yet still not be a planet.

What do these changing numbers and definitions tell us? While we know more about the solar system than Aristotle, Kepler, or Huygens did, our understanding is still incomplete. With each new discovery, planetary scientists have to classify worlds by definitions they're still working to refine.

In more recent times, Pluto's size posed a challenge to planetary classification schemes as astronomers discovered Quaoar, Sedna, and Eris (2003 UB313). In the old order, they'd be planets too. But because they share many traits with the former ninth planet, including inclined, eccentric orbits, today Quaoar and Eris are Kuiper Belt objects while Sedna belongs to the Oort Cloud.

Each solar-system object has an evolutionary story to tell and must fit into an ever-more-complex hierarchy of worlds. It's an exciting time in planetary science, which makes the question that names this book almost anticlimactic. Weintraub anticipates this in chapter 1, where he jumps right into planetary classification:
   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.... 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.

Weintraub's discussions of planetary discovery and categorization put the brouhaha over Pluto's planetary status into perspective. Certainly the scientific debates will rage for some time, as they should.

However, I found the cultural response to Pluto's demotion to "dwarf planet" status by the International Astronomical Union to be quite fascinating too. What should have been a simple reclassification of a world long in the wrong category sparked a lot of misdirected outrage.

The emotional public uproar focused on this poor little world's demoted status. Few caught the bigger story: our solar system is more extensive and complex than we ever dreamed back when planets were merely "wanderers."

Instead, we heard about how disappointed children would be about Pluto not being a planet. Lost in the shouting were valid points about how we define objects in the solar system. Although this book came out before the IAU vote, Weintraub ends by taking on the "disappointed children" meme. His final paragraph is a reminder (and rebuke) to those who would confuse science and emotion and use children as an excuse to restore Pluto to the rank of full planethood:
   If we are interested in memorizing lists with the names of the
   planets, in providing simple answers to young children, we should
   remember that simple answers are usually wrong and that young
   children ... are extremely capable of learning about the wonders
   and nuances of our universe. For the youngest among us, the
   adjectives (giant, terrestrial, belt-embedded, icy, pulsar) will
   not be a problem. It is time to start teaching our youngsters
   something more complicated, with more depth of meaning, than the
   simple memorization rubric: My Very Earthly Mother Just Served Us
   Nasty Pizzas.

CAROLYN COLLINS PETERSEN is a Massachusetts-based science writer who thinks that Pluto is cool, regardless of what we call it.
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Title Annotation:Is Pluto a Planet? A Historical Journey through the Solar System
Author:Petersen, Carolyn Collins
Publication:Sky & Telescope
Article Type:Book review
Date:Apr 1, 2007
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