Everyone knows there are nine planets in our Solar System. Most grade-school students I speak to can name the planets and a sizeable fraction can put them in the correct order from the Sun: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto. Easy, right? Interestingly, it's not that simple, and even some of the grade-schoolers I meet are on to it. A few of the true fourth-grade show-offs reverse the two outer planets when they recite the order of the solar system. First comes Pluto, then Neptune. Despite making life difficult for their teachers, who often aren't astronomy experts, the kids are right.
Or at least they were until last year. The problem is that sometimes Pluto is the farthest planet from the Sun, and sometimes it isn't (that's a lot to ask grade-schoolers to keep track of). And that's just the beginning of Pluto's strangeness. Pluto is so unusual and so unlike the rest of the planets, astronomers have been forced to consider a grade-school heresy: maybe there aren't nine planets in our solar system after all. Maybe Pluto doesn't qualify.
What makes this idea so compelling is that while Pluto is an awfully strange planet, it's actually quite typical for a Kuiper Belt object. The Kuiper (pronounced COY-per) Belt is a loose collection of dust, cometary nuclei (dirty ice-balls a few tens of miles across), and some larger icy bodies that orbit the Sun out beyond Neptune.
This dark, sparse disk of material is quite intriguing to astronomers because it is literally the leftovers from the process that formed our solar system. Once just a rotating disk of cold gas, the solar system really began to take shape when the sun turned on. The heat and pressure from our young star blew huge amounts of light material (like water) out into the far reaches of space. Some of the material was attracted together by gravity, eventually forming giant planets such as Jupiter and Saturn.
But a lot of the stuff was never caught by a growing planet, and escaped farther out into space. Unable to completely break free of the sun's gravity, Kuiper Belt objects drift around the sun in loose orbits, starting right outside of Neptune and gradually blending into the most distant and diffuse part of our solar system, the Oort Cloud. The Oort Cloud is composed of dark, cold chunks of ice that drift around the sun in huge, randomly oriented orbits that may extend as much as two light-years away, half-way out to the nearest star.
The Kuiper Belt and the Oort Cloud are both sources of comets, which usually fall into the inner part of our solar system as the result of some random gravitational pull from one of the outer planets. For the most part, we've ignored the outer reaches of our solar system. We're getting a lot more interested now that we suspect the Earth gets re-surfaced every hundred million years or so by one of these big dirty ice-balls. Something to watch out for.
So why are we beginning to suspect that Pluto should be grouped with the Kuiper Belt instead of with the other planets? For starters, it helps explain Pluto's weird orbit. Most of the time, Pluto is indeed the farthest planet from the Sun, but for 20 years of its 248-year orbit, it slips inside the orbit of Neptune. We're just coming out of Pluto's most recent close encounter, which began in 1979 and ended in 2000.
Pluto's distance from the sun varies quite a bit during its orbit, from 30 astronomical units (an astronomical unit is the distance from the Earth to the sun, about 93 million miles) to 50. That's a change of 60 percent! How does a planet manage to change its distance from the sun by such a sizeable percentage?
In astronomy terms, Pluto's orbit is highly eccentric. Social judgments aside, what "eccentric" means is that instead of being shaped like a circle, Pluto's orbit looks more like an elongated ellipse. Now, I'm sure you've heard that all the planets move around the sun in elliptical orbits, but for all the planets except Pluto, the orbits are so circular that your eyes would not be able to tell the difference between the orbit and a perfect circle. But it's not just the shape of Pluto's orbit that's out of whack.
At first, the prospect of Pluto moving inside the orbit of Neptune might seem a bit alarming. Could the planets possibly collide if they were both at the same place in their orbits at the same time? The chances of this happening are vanishingly small, and not just because Pluto and Neptune take so long to orbit the sun (248 and 165 years, respectively). Pluto's orbit is also highly inclined, compared to the other planets. Instead of orbiting the sun in the same plane as the rest of the planets, Pluto's orbit is tilted by about 17 degrees (the other planets' orbits are all inclined less than two degrees). Not only does Pluto move closer and farther away from the sun, it also moves above and below the rest of the planets, safely out of collision range.
Now, Pluto's orbit may be highly unusual for a planet, but there are thousands of smaller members of our solar system that follow nearly identical orbits. The Kuiper Belt really starts with the Plutinos. The name should give it away; these are objects very much like Pluto, just a bit smaller. Plutinos tend to be on the order of 100 miles across, and there could very well be several thousand of them orbiting in almost exactly the same orbit as Pluto. It's hard to get an exact count, as these objects are so distant, small (by astronomical standards) and dark.
Plutinos' orbits are so similar to Pluto's that many scientists think it can't be coincidence. When the solar system was just forming, lots of half-formed planets were banging into each other, slowly coalescing into the familiar solar system to today. Plutinos may be large ice chunks thrown off during a collision of two giant ice-balls, which eventually formed Pluto and its moon Charon.
Pluto is also much more similar in size and composition to Kuiper Belt objects than to the other planets. Barely 1500 miles across, Pluto is less than half the size of Mercury (the next smallest planet), and considerably smaller than several major moons, including our own. Not only is Pluto small, it's not very dense either. At just 1.1 grams per cubic centimeter, almost exactly the density of water, Pluto is far less dense than any other planet. And you guessed it- here's where the Kuiper Belt objects come in again. We know of several objects in the Kuiper Belt that have very similar sizes and densities to Pluto.
Does it surprise you to hear that we know of other nearly Pluto-sized objects, but we haven't announced the discovery of a new planet? Currently, we know of about 10 objects in the Kuiper Belt that are several hundred miles in diameter. One of them, 2001KX76 (lovely name, eh?), is nearly 800 miles across, more than half the size of Pluto. We haven't found a Kuiper Belt object as big as Pluto yet, but I wouldn't be at all surprised if we find one in the next few years. Right now, it's just a question of building more sensitive telescopes and looking in the right place.
If we find something as big as Pluto (or bigger) in the Kuiper Belt, then will we announce the discovery of the 10th planet? In the end, it all comes down to definitions. Kuiper Belt objects are a distinct group compared to planets. They formed in different orbits, are made of different materials, and inhabit a different location in space. But definitions are always a little fuzzy. You can either think of Pluto as a very unusual planet, or just a slightly larger than normal Kuiper Belt object. In the same manner, you can claim that our solar system has gained several new planets in the last few years, or maybe it just lost one. You be the judge.