Is Pluto Really A Kuiper Belt Object?

Since the discovery of the first Kuiper Belt Objects in 1992, some astronomers have argued that Pluto is just one of many objects in the Kuiper Belt, now referred to as Kuiper Belt Objects, or KBOs.  The Kuiper Belt is named for Gerard Kuiper, who in the 1950s postulated the existence of a second belt of asteroids beyond the orbit of Neptune.

Of course, most KBOs are small, shapeless asteroids too small to have attained hydrostatic equilibrium, the condition at which differentiation and geophysical processes begin to occur on such bodies.  The largest ones such as Eris, Pluto, and Makemake are different from the majority in that they are in hydrostatic equilibrium, which is why simply grouping them with the KBOs without distinguishing them for their roundness is not an accurate portrayal.

The argument made by supporters of the geophysical definition of planet, which states that the only criteria for planethood are that an object be non-self luminous, in hydrostatic equilibrium and orbiting a star, is that these round objects have a sort of dual citizenship as both KBOs and planets (of the dwarf planet subcategory), as does Ceres, which is unique as a round object in the asteroid belt between Mars and Jupiter.

Now, the latest research into the area beyond Neptune's orbit suggests that the term Kuiper Belt may have been used too broadly to describe the large region beyond Neptune.  That region, it turns out, is actually a composite of several distinct sub-regions.  Its central area, the Kuiper Belt proper, where most KBOs are located, is at quite a distance beyond Pluto and the small objects that along with it orbit in a 3:2 resonance with Neptune, known as the plutinos (literally meaning little Plutos).

In a diagram presented on Saturday at the educators' workshop of the Great Planet Debate, the division of what is commonly described as the Kuiper Belt into three separate areas was obvious.  The first area, where Pluto and the plutinos are located, is at the very edge of this region.  Significantly further is the area most densely populated with objects while even further is an area of objects scattered at various orbital inclinations.  These objects in the third region, which include the round Sedna, are known as Scattered Disk Objects, or SDOs.

This means that while all objects in this area can be accurately termed Trans-Neptunian Objects (TNOs), that broad term encompasses objects in three separate regions, which in each case have characteristics like other objects in their regions but not necessarily like TNOs in other regions. The question arises, should this entire huge area, which is also the source of short term comets, be classified as the Kuiper Belt, or should that term be reserved for the central region where most TNOs are clumped, a region of which Pluto and the plutinos are clearly not a part.

Our understanding of this far-removed region is constantly evolving as more discoveries are made.  These discoveries are now coming in at a very rapid pace.  Only this week, a new object called 2006 SQ372, made of rock and ice and estimated to be only 50-100 kilometers across, was detected by the Sloan Digital Sky Survey.  The object is comet-like but has no tail as it never comes near the sun. Its orbit is extremely elliptical, taking it as far as 150 billion miles away (compare that with Pluto, which is 3.6 billion miles from the sun) at its furthest, and within the orbit of Neptune, where it is now, at its closest.  It takes a whopping 22,500 years to orbit the sun (compare that with Sedna, which takes 10,000 years, Eris, which takes 550 years, and Pluto, which takes 248 years).

What is this object?  We have no category in which to place it.  It has an orbit like that of a comet and a composition like that of an asteroid, yet it is different from both of these. Astronomers theorize that it originates in the inner Oort Cloud, a still theoretical region that is believed to be the source of long period comets. Yet even at the most distant point in its orbit, 2006 SQ372 is ten times closer to the sun than the main Oort Cloud area is estimated to be.  This object and Sedna are the only ones we know of that appear to have originated from the Oort Cloud.

Clearly, new discoveries will present the need for new categories and new classifications.  A great deal of the controversy over these objects stems from the fact that there is too much about them and their regions that we simply don't know.  What has been viewed as the Kuiper Belt may actually be several separate regions, and there may yet be more regions beyond that. In these cases, the best option for scientists, educators, textbooks, and web sites is to present what we do know while explaining that there is far more we don't know, which will likely inform future classifications.  That is a far better option than to leap to conclusions when major pieces of the puzzle are still unknown to us. Even children can understand that there is still a lot that even the best minds and experts in the world do not yet know.

As for Pluto, not only is its status as a KBO in question; its classification as a Plutoid is clearly problematic.  The suffix "oid," when added to a word, means a thing like the original word; hence, "humanoid" means a life form akin to humans in body shape, composition, etc.  By calling Pluto a Plutoid, the IAU is saying what--that Pluto is like itself??? Also, Plutoids are defined by the IAU as objects with a semi-major axis greater than the orbit of Neptune, meaning they orbit beyond Neptune.  But for 20 of its 248-year orbital period, Pluto's eccentric orbit actually takes it closer to the sun than Neptune.  Does that mean that Pluto is a Plutoid for 228 years but not a Plutoid for the other 20? Again, we have a definition that makes very little sense. 

Also discussed at the Great Planet Debate were the asteroids Vesta and Pallas. These objects are not round but a look at images of them illustrate they are far closer to being round than the other, many shapeless asteroids between Mars and Jupiter.  In fact, Vesta appears to have been round at one time only to have been hit by an asteroid that lopped off its south pole.  Does that mean it was a dwarf planet once but is so no longer?  Do Vesta and Pallas have geophysical properties like the planets, and if they do, doesn't classifying them as asteroids blur that distinction?  We will learn some answers when Dawn gets to Vesta in 2011.  In the meantime, we have yet another gray category, with objects that do not clearly fit into any of the classifications we have created.

What makes something a planet or a comet or a KBO; what makes an entire area part of a belt as opposed to a separate region with its own characteristics?  If there is one thing these questions bring to bear, it is that there is far more that we don't know than what we do know. In light of that, some definitions and classifications will have to remain in a state of flux until we learn more.  This is another important lesson the IAU needs to take into account.  Better than endorse the false perception of a dichotomy (planet vs. not planet), they should keep the subject open with the recognition that only with time and research will we have enough data to make these determinations. Between New Horizons, Dawn, and new discoveries from earth and space-based telescopes, there is no reason to rush to judgment.