Monday, September 22, 2008

And Now There Are Thirteen

Last Wednesday, September 17, the IAU officially bestowed dwarf planet status on 2003 EL61, an egg-shaped Kuiper Belt Object now given the Hawaiian name Haumea.  This brings the total number of dwarf planets in our solar system to five and the overall planet count around our sun to 13.

That number results from using what should be the correct definition of "dwarf planet" as a subclass of planets, specifically referring to small planets that do not dominate their orbits but have attained the crucial state of hydrostatic equilibrium and therefore have the geophysical properties that make it appropriate to designate them as planets.

In order of distance from the Sun, the planets are: Mercury, Venus, Earth, Mars, Ceres, Jupiter, Saturn, Uranus, Neptune, Pluto, Haumea, MakeMake, and Eris.

Being that Haumea is so oddly shaped, I was initially surprised at its being granted dwarf planet status at all. However, according to Dr. Alan Stern, premier expert on Pluto and the Kuiper Belt, hydrostatic equilibrium doesn't mean an object has to actually be spherical. "It just means in fluid equilibrium, which can and does include centripital forces...EL61 is certainly massive enough to be in hydrostatic equilibrium where (it) is spherical. That it is not is just a detail due to the high spin rate, but spherical is not the real central issue in the dwarf planet or planetary definition, it's hydrostatic equilibrium." It turns out EL61's odd shape is due to its being elongated due to a very high spin (rotation) rate.

Discoverer Dr. Mike Brown, in responding to a comment on his blog, notes that there are likely many additional Kuiper Belt Objects in hydrostatic equilibrium that should be given dwarf planet status. This is undoubtedly true. In some cases, these objects' tremendous distance from us combined with their small sizes makes it difficult for us to determine whether any specific individual one has attained hydrostatic equilibrium.

What this means is that for a time, either years or decades, we will have to live with a degree of uncertainty about these objects. For now, it is useful to refer to them as "dwarf planet candidates" until we have sophisticated enough tools to determine whether they actually meet the criteria for consideration as dwarf planets.

Daniel Fischer, a supporter of the IAU planet definition, claims that the fact that we will likely end up having several hundred dwarf planets in the Kuiper Belt "devalues" the term planet because there will be so many of them. What exactly does he mean by "devalue?"  Does being "special" mean the number has to be limited and the occurrence of a phenomenon scarce?  Until the 1920s, astronomers believed there was only one galaxy, the Milky Way.  Now we know there are billions.  Does that diminish the significance of the term galaxy?  Does the fact that there are billions of stars diminish the value of the word "star?" Fischer's logic is hard to understand because it is so subjective.

In the early days of civilization, humans believed the Earth to be the center of everything. Over subsequent centuries, we came to realize we are one planet in one solar system in one galaxy in a universe with a multitude of galaxies, solar systems, and planets.  We're not inherently "special"; we're one planet of many. Similarly, the term planet does not somehow lose its value because we have discovered that instead of there being nine in existence, there are hundreds, both in our solar system and in others.

Fischer also argues the need for a fixed number of planets in our solar system. This too is unnecessary. We all grew up in a world with a fixed number of nine planets; the real change we must acknowledge is not that that number is now reduced to eight; it is that that number is no longer going to remain fixed.  Even looking only at our own solar system, the rapidity of new discoveries makes it clear that the number of planets orbiting our sun--or rather, the number of which we are aware--will continue to be in flux for a long time. There is no logical reason why textbooks and teachers cannot explain this fact. Not only will doing this save the need to publish new textbook editions every time another planet or dwarf planet is found; it will also will bring home the excitement that discovery is ongoing and that we do not need to artificially set the number of planets orbiting around our sun at any particular number.

Brown states that his main concern is distinguishing the eight gravitationally dominant objects in our solar system from those that are not gravitationally dominant.  That can in fact be accomplished while still acknowledging that dwarf planets are planets due to their differentiation and geophysical characteristics.

In 2000, Stern and Dr. Hal Levison published a paper explaining the existence and distinction of two classes of planets. That article can be found here:
http://www.boulder.swri.edu/~hal/planet_def.html .  Stern and Levison argue the following:

Because such smaller bodies [KBOs] clearly play a dynamically different role in the solar system than the large bodies that architecturally shape the system, distinguishing between the bodies on some dynamical basis is both useful and desirable.
Hence, we define an ├╝berplanet [higher-planet] as a planetary body in orbit about a star that is dynamically important enough to have cleared its neighboring planetesimals in a Hubble time. And we define an unterplanet [under-planet] as one that has not been able to do so.From a dynamical standpoint, our solar system clearly contains 8 ├╝berplanets and a far larger number of unterplanets, the largest of which are Pluto and Ceres.”

While this description clearly distinguishes those objects that dominate their orbits from those that do not, it nevertheless recognizes that both categories, “uberplanets” and “unterplanets,” which we can view as “classical planets” and “dwarf planets,” still fall under the broader category of planets.

The key here is balance. We need a planet definition that encompasses both the dynamical and the geophysical characteristics of these objects. Using only the dynamical, as the IAU did, leaves us with the absurd situation of having the same object be a planet in one place and not a planet in another. For example, if Earth were located in Pluto’s orbit, it would not be considered a planet. Neither would a Mars-sized object if one is found there.

Conversely, if we use only the geophysical definition, we ignore the importance of an object’s location and the role it plays within its orbit. This too leaves us with a very incomplete picture of the solar system as a system.

All these issues were discussed extensively last month at the Great Planet Debate. Some of these discussions are now online at http://gpd.jhuapl.edu/debate/index.php . I urge anyone interested in this issue to take the time to listen to some of the most eminent planetary scientists in the world discuss all these issues and more.

I would also like to make reference to education, a subject that came up many times at the Great Planet Debate. Ideally, this issue can be used as a “teachable moment” to help young people and those of all ages learn that there are many unresolved issues still open to debate.  Yet during the last few weeks, I have read about or received communication about children being taught only one view as fact, specifically the view that our solar system has eight planets. One parent in the Philippines was baffled when after learning the planets in school, her daughter responded to a question about Pluto with, “there is no Pluto.”

These parents responded very positively when referred to Internet sources, especially the petition signed by Stern and 300 professional astronomers saying they will not use the new planet definition, and are often very excited to learn that there is an ongoing debate.

At the same time, in many places around the world, Pluto is still being taught as a planet. Anecdotally, I have heard from people all over the world, in places as diverse as South Africa and Egypt, that Pluto remains part of the curriculum on planets of our solar system.

One interesting anecdote comes from a Facebook group, where a mother relates how her nine-year-old and five-year-old nearly came to blows over this issue, with the older one stating Pluto is a planet, and the younger one maintaining it is not.  What is significant about all this is that in most cases, children’s pronouncements on either side are determined by the fact that “my teacher said so.”   No matter which view teachers convey, if all they do is teach one view as “the way it is,” they are doing a tremendous disservice to their students by promoting an authoritarian world view in which something is a certain way because an authority says so rather than because the student has reviewed the evidence and come to his or her own interpretation.

If I were the parent of these children—which, if they are reading this, they likely are very thankful I’m not—I would send both of them to books and Internet sites with the assignment of defending their positions with real evidence, not just a statement that “someone said so.” Whether they changed their views or not, at least they would have a learning experience that encouraged them to question and think for themselves.

Isn’t that kind of thinking the goal of science? Isn’t it the goal of education in general?