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So how come calling Pluto a planet is so goofy?

May 16, 2010 12:37 am


Pluto, imagined here with its moon Charon, was demoted recently to dwarf planet. But there is more to the Plutonic 'neighborhood' than meets the eye.


--How many planets are there in our solar system? This seemingly simple question has plagued astrono-mers for decades but by the mid-1990s had grown into a glaring deficiency that could no longer be overlooked. The problem astron-omers faced was that in science one desires clear and well-defined terminology, and such a definition of a planet was lacking.

The definition crafted by the International Astronomical Union in 2006 essentially classified Pluto as one of the larger members of a belt of icy objects that lies more distant from the Sun than Neptune. Known as the Kuiper Belt, this group is analogous to the belt of rocky asteroids located between the orbits of Mars and Jupiter. Both collections are thought to be remnants, leftover bodies that did not get incorporated into the larger planets, and understanding them is key to understanding the solar system's formation.

The original meaning of the word "planet" was simply a point of light in the sky that moved relative to the fixed stars. Over the centuries we have learned more about their nature, and included the Earth as one of them. The question of Pluto's status was forced when, in the 1990s, astronomers began to discover other objects beyond Neptune, many similar to Pluto in size and appearance. One of these, now called Eris, was especially problematic because it is larger than Pluto and was dubbed the 10th planet when it was discovered in 2005. It raised the concern of how to deal with future discoveries of similar objects. Could we end up with dozens or even hundreds of planets?

In order to classify anything one needs not only to know the object's properties but also the criteria for the classification. Is the tomato a fruit or vegetable? To answer, we need to know what constitutes a fruit and a vegetable as well as the relevant properties of the tomato. Neither category is better or worse than the other. Such is the same case with planets. "Is Pluto a planet?" is only half the question, the rest being: "What is a planet?"

Obvious criteria such as setting a minimum size for a planet turn out to be quite arbitrary. When a glass or coffee mug is dropped, one knows that it shatters into many pieces. There may be just a couple of large pieces, many smaller ones, and many more of the smallest fragments. The formation of the solar system might be viewed in much the same way with a distribution of different-size objects orbiting the sun. We call the largest ones the planets, but there is really no clear demarcation between what we'd call big versus small, making that cutoff somewhat arbitrary.

The International Astronomical Union has members from 90 countries and among its purposes is to oversee the official naming of celestial objects. For example, the naming of Pluto must be agreed upon simply to ensure that everyone is talking about the same object. There were several proposals for planet definitions, including grandfathering Pluto's planet status, or expanding the definition to include 12 planets or possibly even more that might be discovered. In a conference held in Prague in 2006, these proposals were debated and eventually the final version was passed by a vote of attending members. Science does not typically operate by public vote in this manner, but at issue was not the terms of a scientific theory but rather nomenclature.


A definition of a planet would preferably mirror the practical usage of the term. Everyone agrees that the Earth is a planet, and so are Jupiter, Saturn, etc. But the moon isn't, and comets aren't. So any definition should preserve those common-sense notions also.

The first part of the approved definition is perhaps the simplest requirement: A planet must orbit the sun. However, that is not sufficient as other objects, such as comets, asteroids, and even dust, orbit the sun.

Secondly, a planet must be large enough to be round or nearly so. Small objects, such as most comets and asteroids, don't have enough gravity to pull them into a round shape and are thus disqualified from planet status, with a few hundred miles being roughly the minimum size needed to become roundish. Some argue this is not a test since nothing, not even the large planets, are perfectly round, so there still may be some gray area in deciding how round is round enough.

The third criterion of the final definition represents the sense that a planet should be the largest object in its part of the solar system. This is stated as "clearing the neighborhood around its orbit" in reference to the formation of the planet and its accumulation or ejection of all other material near it. There are still small bodies orbiting near the large planets, especially near Jupiter, but those are so much smaller that essentially the large planets dominate.

How does Pluto fit into this recipe? It does orbit the sun and is large enough to be roundish. However, it fails the third requirement: It has not cleared its neighborhood and is just one of many objects in that region of the solar system. It is not even the largest of those. The IAU defined a new class for this situation, "dwarf planet." A dwarf planet orbits the sun, is nearly round, but has not cleared its orbit. Depending on who is counting, there are five to 15 dwarf planets currently known. The largest three are Eris, Pluto, and Ceres. Objects not large enough to assume a rounded shape are simply grouped as small solar system bodies.


History offers us an interesting comparison. The first and largest asteroid, Ceres, was discovered on the first day of 1801. Many at first referred to this as a newly discovered planet between Mars and Jupiter. Roughly a year later a second asteroid was found, and then a third, then a fourth. In contrast, Pluto was discovered in 1930 and over 60 years passed before the second object in its region of the solar system was discovered. Almost 100 asteroids had been discovered in the 60 years after the first one was, and today we know of hundreds of thousands. But the lack of a second discovery soon after Pluto allowed enough time for its planet status to sink into our consciousness.

Until our understanding of planet formation improves and a solid scientific definition takes shape, the boundary between planets and dwarf planets will likely keep the official planet tally at eight. More discoveries await us in the distant parts of the solar system, and so the definition may need to be revised again.

As Juliet asked of Romeo, "What's in a name? That which we call a rose by any other name would smell as sweet." Scientists will continue to study Pluto, Ceres, and the other dwarf planets regardless of their official titles.


Gregory Black is a planetary scientist at the University of Virginia.

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