Five years ago, we ran an editorial that said Pluto's demotion was a good thing. With Pluto back in the news, all the old arguments are new again, so we're running the editorial again. You can judge for yourself whether anything has changed.

I come to both bury Pluto, and to praise it—it being the decision to demote Pluto from the pantheon of planets. In 2006, the International Astronomical Union was faced with a growing collection of small, icy bodies similar in size to Pluto orbiting at the fringes of the Solar System. It settled on a new way of defining what constitutes a planet, one that left Pluto and its peers demoted to dwarf status. The decision triggered outrage that continues unabated, and some have argued that the demotion given science a black eye from the public's perspective.

I'm going to argue that Pluto's demotion has actually created a fantastic opportunity to help the public understand the very nature of science itself, and connect it with a new era of discovery for the planetary sciences. But it's only an opportunity; unless the scientific community takes advantage of the chance to communicate, then we risk letting the event be portrayed as the arbitrary killing of a childhood friend.

Discovery is a good thing

The astronomy community felt compelled to come to terms with Pluto's status as a direct result of its own successes. Driven by digital imaging, automated survey telescopes, and space-based observatories, we have become much more aware of the full diversity of objects orbiting the Sun. Ceres, once dismissed as an asteroid, turns out to have been sufficiently massive for its own gravity to reshape it into a sphere. Eris, the most distant dwarf planet yet found, appears to be both larger and heavier than Pluto. Estimates are that there are dozens to hundreds of additional bodies of this class that we've yet to discover.

And those are just the oddballs of our own Solar System. Planet-hunting probes have spotted gas giants heated up to extremely low densities and rocky bodies that are pelted with vanadium oxide rains. As instruments continue to improve and more results roll in, who knows what we might end up finding? We've also gotten a better look at some of the Solar System's large moons, many of which are significantly larger and more complex than any of the dwarf planets, with atmospheres, lakes, volcanism, and, quite possibly, liquid oceans hidden underneath their frozen crusts.

All of these discoveries have radically changed our perspective on the bodies that inhabit this Solar System and the apparent multitude of systems beyond it. And that's a great thing—it's the sign of exciting times and a field that's progressing rapidly, which means that definitions that date from 50 years ago have simply become obsolete. But instead of focusing on the excitement, many who covered the decision and aftermath presented these discoveries as a problem for Pluto, rather than a benefit for the field.

Science is rarely a question of right or wrong

The fact that our understanding of what constitutes a planet needs revision is no surprise, really. Science, by its very nature, is tentative, and our understanding gets updated as new information comes in. That's not to say our old definition was wrong, per se, simply that it didn't account for everything that we now know about the bodies in our Solar System. This pattern, where a scientific understanding isn't wrong, but is found to be limited, is central to what we hope is the progress of science. Scientific ideas rarely are completely wrong, but they are often more or less accurate.

This is a facet of science that the public doesn't understand well. A new finding that completely overturns our present understanding makes for a nice narrative in the popular press, but it's one that rarely reflects the evolutionary process of science. Old ideas are rarely out-and-out wrong, just not as accurate as newer ones; the newer ones are (hopefully) a bit more accurate, but they will eventually need revision as well.

But the public gets fed a steady diet of science stories that distill down to "everything we knew was wrong—meet the new right answer." It's no surprise that many in the public now cynically expect every new right answer will be wrong five to ten years down the line.

That expectation creates problems. Even if its conclusions are uncertain and tentative, science can produce reasonably accurate answers to important questions. These answers may be limited at the time, and revised in the future, but that doesn't make them "wrong," nor does it mean that science is a poor basis for decision making.

Nature abhors a binary

If the public is confused about the fact that science doesn't rely on binary, "right or wrong" distinctions, it's equally confused by the fact that nature doesn't rely on these either. In the end, the Solar System is comprised of whatever bodies happened to form due to its starting conditions and physics. We can try to define planets and dwarf planets as distinct categories within those bodies, but there's no reason for physics to necessarily obey our definitions. There's still a chance that we'll find something at the fringes or our Solar System that awkwardly straddles the divisions we're trying to impose on the natural world, and an excellent chance we'll find an exoplanet that makes a mockery of them.

And this is true in all areas of science. Pick anything we'd like to place in a neat category—a gene, a species, a galaxy—and the Universe will almost always throw us a curve ball, something that's neither here nor there, or a little bit of both. It may be convenient to try to constrain the natural world with definitions, but the world has a nice way of blurring sharp boundaries and making a mockery of our attempts to pin it down.

In the end, another lesson from Pluto is that we can't get too hung up on definitions. They're convenient and useful, but they're only a mental construct that we're attempting to impose on a Universe that isn't necessarily interested in what we find useful and convenient. Recognizing the difference between human terminology and the underlying reality can be essential.

The death of ideas we cherish

The debate about Pluto brought up all kinds of arguments in favor of retaining its planetary status. It's in the textbooks, in mnemonics, people had an emotional attachment to it, its demotion would (and has) unleash a popular outcry, we've been talking about nine planets for years... In short, there's an intuitive, emotional pull towards maintaining the status quo.

When it comes to science, none of that should matter. Scientists get emotionally attached to their pet ideas all the time, and have trouble letting them go. But, ultimately, the most beautiful theories can be slain by simple, ugly facts, and scientists have little choice but to let them die, or slide off the end of the crackpot scale—and a crackpot is, in effect, someone who has given up on science as a process in order to keep a cherished idea alive.

The fact that many scientists defended Pluto's status was a great opportunity to show what a passionate, thoroughly human activity science is. The fact that they lost shows us something even more important: no matter how much you care, you have to let the natural world have the final say. Pluto is qualitatively different from the remaining eight planets, which left it on the wrong side of the planetary divide.

In the end, the story of Pluto's demotion has everything: exciting discoveries, emotional appeal, and important lessons about the very nature of a process, science, that's essential for understanding the modern world. It may not have been an especially popular decision, but it can play an important role in helping the public understand the scientific process. In that sense, it's an incredibly valuable opportunity.

But it's an opportunity that will be wasted if the scientific community and press simply present it as a controversial and arbitrary decision, instead of bringing the bigger picture into focus.