“My Daddy Killed Pluto”


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A year or so ago, I was lecturing in my graduate seminar in epistemology. To illustrate a point, I brought up the recent fate of the ninth planet—Pluto. This was spontaneous. I hadn’t thought about the example before. But I knew that astronomers/astrophysicists had determined that Pluto was not, after all, a planet.

“How was it decided that Pluto was not a planet?” I wondered out loud. One possibility is that our scientists, working with a definite conception of what makes a planet a planet, had discovered that the stellar object we call “Pluto” does not satisfy the conditions for being a planet. Hence, it had to be demoted from planetary status to something else—an “ice ball,” perhaps.

Alternatively, our scientists may have known of Pluto’s properties and recently decided that the concept of a planet should be refined. With a refined conception of planethood, it would turn out that Pluto could no longer be considered a planet.

Which of these is the actual story? Did scientists discover something about Pluto that violated the standard conception of planethood, or did scientists revise their concept of planethood, knowing that Pluto’s claim to planethood would thereby be precluded?

I didn’t know the answer to the question at the time, my knowledge of the demoted status of Pluto being embarrassingly anecdotal. I was less embarrassed, though, when I asked a physicist acquaintance if he knew the answer. He didn’t.

So what is the answer?

Mike Brown, astronomer at the California Institute of Technology, narrates the answer in his recent book How I Killed Pluto and Why It Had It Coming. The Amazon page for this book includes “A Letter from Author Mike Brown” that recounts the author’s PR problem with his own daughter. “Daddy,” she said to him, “I know you had to kill Pluto, but will you promise me one thing?” He told her that he would. To find out what she asked him to promise, visit this page.



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