The news last week that Eris might actually be a tiny bit smaller than Pluto led to the inevitable question: doesn’t this mean that Pluto should be a planet, after all? [update: the final analysis suggests that to the best of our ability to measure Pluto and Eris are the same size. No way to know which is bigger or smaller.]
The simple obvious answer to this question is no. Pluto was not demoted in 2006 simply because it was no longer the largest known object beyond Neptune, but because it was one of many many such small objects beyond Neptune. The fact that it might still be the largest gives it some bragging rights at the next dwarf planet convention, but – just like we never considered Eris a planet when we thought it larger than Pluto – being the largest known thing beyond Neptune doesn’t get you an invitation to the planet ball.
The solar system is a beautifully and profoundly ordered place, a systematic product of the seemingly chaotic processes that formed it 4.5 billion years ago. Back before there was a sun at all, there was just a slowly spinning cloud of gas and dust gradually collapsing under its own weight. As the cloud collapsed more and more it began spinning faster and faster and flattened itself out into a pancake-shaped disk. The vast majority of the material fell towards the center and created the centerpiece of the solar system: the sun itself. But the material still left in the disk slowly and randomly began to stick together and accumulate into a smaller number of ever larger bodies. The largest body in any area began to get so big that it had enough gravity to pull in all of the smaller bodies and all of the remnant gas in its vicinity. The more material that these largest bodies pulled in, the more circular and disk-like their orbits became, until, over millions of years, there were only eight such things like them left. Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune are each the product of this long-term accumulation process, and each preserves the memory of these earliest times. After having pulled in so much material, they all are on nearly circular orbits in nearly the same disk, and each is so large compared to anything else in their immediate vicinity that nothing exists which can kick them out of their stunningly regular journeys around the sun.
But the eight planets are not the only things out there. In some spots in the solar system this accumulation process never had time to finish. Just outside Mars, Jupiter was so nearby and so big that it kept trying to tug the material of this region towards it. Jupiter never quite succeeded in grabbing all of that material, but it did enough damage to make sure no planet ever got to form there. Rather than a single regular body in an orbit circling the sun in the same disk as the planets, the solar system at this spot now has an asteroid belt consisting of countless tiny bodies on tilted elongated overlapping orbits flying everywhere through this region. Jupiter continues harassing these asteroids to this day. If an asteroid finds itself in the wrong spot at the wrong time it could be flung onto a new orbit causing it to smash into a rocky planet or burn up in the sun or even get ejected from the solar system entirely. Really, Jupiter does not do this harassing intentionally. It’s just a planet. And planets are generally too big to notice all of those other tiny bodies that they are pushing around the solar system. The asteroids are as different from planets as the krill moving around the ocean currents are from a pod of migrating whales.
Beyond the planets lies another region of disorder. At the far edge of the solar system Neptune grew too big too fast and had the same effect on the small bodies growing out there as Jupiter did in the inner solar system. Beyond Neptune these small bodies never got much of a chance to coagulate into larger bodies at all. Instead, they were pushed and tugged and kicked around by Neptune until they, like the asteroids, were on bizarrely tilted elongated overlapping orbits, only more so. Pluto’s once bizarre-seeming orbit, which is tilted by nearly 20 degrees and is so non-circular that the most distant spot in its orbit is nearly twice as distant from the sun as its closest spot, is thoroughly typical of the other objects out in this region. Eris is tilted by 45 degrees and goes almost twice as far from the Sun as Pluto! In this region – what we now call the Kuiper belt – the disorderly small bodies, which were never allowed to accumulate into a dominant planet, swarm around and hope they never get too close to Neptune, which could either kick them out of the solar system entirely, or shove them into the inner part of the solar system to eventually burn up as quickly melting comets.
Our solar system has a lattice of profound order with a background of swarming seething chaos. Saying that there are eight planets is simply a short-hand way of recognizing the intense difference between the eight objects which make up this methodical lattice and the millions upon millions in the swarming background.
Pluto, whether it is a bit larger or a bit smaller than Eris, will always be part of the swarm. It will always be different from the eight planets. If you have a desperate nostalgia for the days when Pluto was still called a planet, it is possible to come up with scientific-sounding ways of categorizing objects in the solar system that would lump together the 8 planets with Pluto and Eris and perhaps a hundred other objects and call them all planets, but only if you close your eyes and shut your ears and ignore what the solar system itself is telling us. There is beauty in eight planets. There is order in eight planets. The solar system speaks its profundity in this regularity and stateliness. Letting go of Pluto has finally allowed us to be able to sit back and hear not only the quiet grandeur of the eight planets around us, but also, finally, the continuous buzzing of the seething background all around.