How I Killed Pluto and Why It Had It Coming

"Mike Brown is a planetary astronomer. He studies ice balls in the outer solar system. Earth is a rocky, warm planet near the sun. Jupiter is a gas giant farther from the sun. And, out past Pluto, there are gigantic balls of ice that were discovered in the 1990s. They range from very small to hundreds of miles across, and we think there are hundreds of thousands of them. It turns out Pluto looks very similar to them, so in the 1990s astronomers began to ask: is Pluto the smallest example of a planet in the solar system, or the biggest example of an ice ball? Mike Brown was finding objects that were as big or possibly bigger than Pluto in the outer solar system. His discoveries triggered this debate about how we define what is a planet. How I Killed Pluto and Why It Had It Coming is a wonderful first-person account about his journey, and his personal life as well as his professional life. It’s a great story. Mike’s a friend of mine but the real reason I love this book is because I had wrestled with this idea of defining a planet. There was an official definition of what a planet is and I didn’t like it. No matter how people define the word planet I can always find an example of an astronomical object that looks like a planet but doesn’t fit their definition. It occurred to me as I was reading his book—maybe it’s not a good idea to have a definition of a planet. It’s a better idea to just have a concept of what one is. In the book, Mike talks about how a planet is something big and important in the solar system. In other words, the solar system would be substantially different if you removed a planet. That is certainly true of the Earth and of Jupiter. If you deleted either, it would change the configuration of the solar system. On the other hand, if you removed Pluto nothing much would change. There are other objects the same size as Pluto out there and there are thousands of objects that are somewhat smaller. It’s a society of professional astronomers from all over the world that gets together every few years at symposiums and presents new scientific findings. They’re the ones who answer the question of what is a planet. They literally voted on the definition. There was a small committee who argued back and forth. People who observed the skies favoured one definition, dynamicists who work out the physics of motion of the planets favoured another. If you ask me, the very fact that all of these people were arguing should indicate that the definition isn’t cut and dried, and maybe it’s not a good idea to issue a rigid one. As we’ve learned over and over again, nature loves putting things on the boundary between categories. There are objects out there that are sort of like planets but sort of like stars. People have argued about what to call these things. Is it a star or a super planet? Nature doesn’t care what you call it, and I don’t think we should either. We should let the object’s physical characteristics tell us what it’s doing, not some name that we decide to call it. We’ve tried to define what a star is and it’s not easy. When we look up in the sky, all of the stars that we see are gigantic balls of gas. The pressure in the centre has compressed that gas to the point where the very nuclei of the atoms themselves – hydrogen, helium, carbon and heavier elements – are all squeezed together. A star is defined as an object that does this, or has done it in the past. “We’ve tried to define what a star is and it’s not easy” There are all kinds of stars. There are stars like the sun. There are dead stars. There are proto stars. A black hole used to be a star. So, just as with planets, I think a star is more of a concept than something you can rigorously define. A solar flare is an explosion on the sun, caused when the magnetic field lines of the sun get all tangled up and then suddenly burst. A flare releases huge amounts of energy, but rarely causes trouble on Earth. They launch a big wave of subatomic particles into space. When those hit the Earth they can cause aurorae, and sometimes damage satellites or even cause power grid overloads, which happened in 1989 in Quebec. That doesn’t happen often, but I wish power companies would take the risk more seriously and plan better for such an event. That’s really the only serious risk to us on the ground – the Earth’s air stops the particles completely so they can’t reach us."