(Read this for another viewpoint on the status of Pluto)

Pluto: To Be or Not to Be a Planet

Howard L. Cohen

Emeritus Assoc. Professor
Dept. of Astronomy, Univ. of Florida, Gainesville, FL 32611
September 1, 2006

At its 2006 August Prague meeting, the IAU voted to define the word "planet"
and classify Pluto as a "dwarf planet." This vote remains controversial.
Is it just another take on a silly matter that is more culture than science?

To be or not to be a planet? That is apparently up to the IAU (International Astronomical Union). Or is it? Is it better to accept a potentially unsatisfying and argumentative definition, or to take issue against an unnecessary attempt to settle a cultural problem? And, by opposing, do we continue to emphasize an embarrassing question that has little scientific value but is a vain attempt to redefine nature? (See Appendix for the full IAU resolutions.)

If you want to ignore these issues, there is a rub; we forgo a chance to bring science and astronomy back into everyday conversations. By showing the good and bad of a ridiculous issue, we have the opportunity to get people back to looking at the sky and our solar system.

So, is it not better to jump into this tempest and even bear the scorn of rivals, than to wish the problem away? No recount of votes is necessary to define a planet and the status of Pluto. Let Pluto stand historically as a planet and let its status raise a thousand questions.

As an example, the IAU resolved that a planet is in "orbit around the Sun." Does this exclude all such objects that orbit other "suns" (stars)? About 200 are now known and the number is increasing. What if a Jupiter-like object is found roaming the vast space between stars? Is it not a planet?

Planets, too, the IAU said, must have sufficient mass for its self-gravity to overcome rigid body forces so it assumes a nearly round shape ("hydrostatic equilibrium"). Nevertheless, what criterion exists for deciding how far a planet can deviate from roundness?

Furthermore, the IAU resolved that planets must have "cleared its neighborhood" around its orbit. However, what is a planet's neighborhood? Has Earth "not cleared its neighborhood" since the Moon orbits with us? Is then, the Earth not a planet? Pluto swings inside Neptune's orbit as this icy world orbits the Sun. Is Neptune also, then, not a planet? Many planets have multitudes of smaller objects ("moons") around them as do Jupiter and Saturn. Are they not planets? On the other hand, Mercury and Venus have no naturally occurring objects orbiting around them. Therefore, are these two bodies our solar system's only "true" planets!

The IAU likewise defines a "dwarf planet" as having similar properties to other planets but has "not cleared its neighborhood" around its orbit, and is not a satellite. Yet, what is a satellite?

The IAU also resolved that Pluto is, therefore, a "dwarf planet" and is recognized as the prototype of a "new category" of trans-Neptunian objects. This puts Pluto into a "new class" rather than a subclass of planets.

Yet, we categorize Earth and Jupiter as both planets but they also fit into two very different, distinct and unlike groups. The first group, the Terrestrial planets (Earth, Venus, Mars and Mercury), contains objects much smaller and rocky; the second group, the Jovian planets or "gas giants" (Jupiter, Saturn, Uranus and Neptune), is much larger and have most of their material as light gases (hydrogen and helium). Interestingly, each group, too, is ordered by distance from the Sun—first the Terrestrial, then the Jovian.

For years we have also recognized that Pluto belongs to a third group of objects quite distinct from the others; they are small but round and have large amounts of ice and rock. Like the other two groups, this third class is also ordered by distance from the Sun, coming after the Jovian planets. So is this group really new?

If we consider that both Terrestrial and Jovian objects are two different categories of planets, then why not also make the Pluto group into a third catagory of planets? (Call this group what you wish—Plutons, Plutonians, etc., for it has no universally accepted name.)

Each of these groups have different properties. One can thus argue that Pluto is so unlike Earth that this ice world should not be a "planet." However, one could also argue that the Terrestrial planets are actually more similar to Pluto than Jupiter. Consequently, if Earth is so different from Jupiter, should we not also demote the Earth-like objects from planetary status? Does the Solar System, therefore, really have just four "planets" (Jupiter, Saturn, Uranus and Neptune)?

In addition, what should we do with Ceres, a spherical object like Pluto and largest body that orbits the Sun in the so-called "asteroid belt" between Mars and Jupiter? If Ceres has not cleared its neighborhood, is it not then a "dwarf planet" like Pluto? However, Ceres is a small, rocky world like the Terrestrial planets. Thus, categorizing rocky Ceres as a dwarf planet puts this object into the same group as icy Pluto.

The purpose of classification is to help us understand the origin, evolution, structure and properties of objects. Do the IAU resolutions aid this cause? You decide.

Finally, what of our Moon? This rocky globe orbits with Earth and we call it a "satellite." However, the Moon is physically much like Earth and has terrestrial properties. Is the Moon not a planet from a morphological point of view?

If this makes sense or not, if you "grunt and sweat by it all," have solace in that a planet by any other name could still be a planet.

Bill Helms, AAC president (2006) recently wrote:

"Pluto was originally a planet and as far as I know Pluto's still where it was before."

Besides, Pluto, has always been a center of controversy ever since its discovery in 1930 based on calculations of Percival Lowell. Although astronomers have long debated Lowell's work and beliefs about Pluto and other plants as Mars, Lowell was a giant in his profession for his visionary ideas, his perseverance in his work, and his enthusiasm and inspiration that he brought to both the public and scientific sectors.

For this alone, Pluto should stand monumental as a testament to Lowell's achievements that helped open new vistas in his field, a tenacity to inspire that would help make astronomy flourish in the twentieth century.

Pluto is physically no different now than it was when discovered about seventy-six years ago, still a significant historical and triumphant discovery in the annals of astronomy. Does demoting Pluto really change its status? Or does Pluto belong to the ages, now and always a member of the "classical nine?"

Consequently, trying to define this group of objects is a foolish attempt to find meanings that relate more to culture than science. In fact, the IAU first proposed a definition that would not only have included Pluto but also increased the number of planets to twelve. Then a week later the IAU reversed its stance and adopted a resolution that excluded Pluto, reducing the number to eight. Their final resolution raises many questions that astronomers will continue to debate for years.

After all, what ultimately matters is the science not the name.
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(The views expressed here are that of the author and not necessarily the AAC)


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