Solar System Uncertainties

Just a few years ago our solar system endured a major blow, as its sum of planets was reduced from nine to eight. What was for nearly a century regarded as our most outlying planet from the sun—Pluto—abruptly became downgraded to a dwarf planet. This was a controversial move on the part of the International Astronomical Union in 2006, as an immediate argument arose as to what constitutes a real planet and what is a dwarf. Pluto had many supporters who strongly disagreed with its demotion.

There are countless rocky bodies orbiting our sun—everything from grains of dust to small collections of rocks (like asteroids) to the monster planet Jupiter. When our solar system whirled into existence, nearly five billion years ago, from a collection of gases and rocks, most of that gas collapsed into a ball at the center and ignited into our sun. The rocky pieces circled around that solar center and gradually formed planets, as the stony chunks of material bumped into each other, coalesced, and slowly grew larger.

That process continued for a billion years or so, as our solar system settled into its present form. But what is the difference between a planet, a dwarf planet, a moon, a “Kuiper Belt object,” a big asteroid, a meteoroid, and a comet? (A “what” belt?) How do we differentiate the largest of these, a planet, from its lesser companions? The definitions have evolved, as we learn more about the nature of our solar system. Recent discoveries of thousands of planets orbiting other stars have added to the quandary. We are discovering that there are a wide variety of solar systems in our galaxy—some of them quite different from our own, and that knowledge alters our perspective of our own system even more.

But back to Pluto and its demotion: It was downgraded, primarily because it is so small, but also due to the fact that it orbits within the Kuiper Belt—a region beyond the orbit of Neptune (the eighth and last “real” planet)—which contains countless objects like Pluto; many of them are so-called "minor planets.” We are just beginning to detect some of these rocky aggregates, which are too small to be designated as planets, but large enough that gravity has shaped them into potato-shaped lumps and even spheres. Should we call them proto-planets? Wee planets? How about dwarf planets?

In fact, two other dwarf planets have recently been discovered out there: Sedna and 2012VP₁₁₃ (this one is so new that it has yet to be named). How many more minor planets may be discovered out in the Kuiper Belt? Are there even a few more major planets lurking out there? Is our solar system still waiting a more precise definition?

The space probe New Horizons is now approaching Pluto and will fly by the new dwarf planet, this coming July. It will undoubtedly vastly increase our understanding of what once was designated our ninth planet. As the probe continues on out into the Kuiper Belt, it likely will add much to our understanding of our planetary system. Many astronomers' breaths are being held in anticipation. I think I'll breathe now.