"How I Killed Pluto and Why It Had It Coming" made Mike Brown a lot of enemies

Cal Tech astronomer and author Mike Brown helped to demote Pluto from planethood – and got a lot of hate mail as a result.

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    Astronomer Mike Brown discovered the dwarf planet Eris, which helped seal Pluto's fate, with this 48-inch telescope at Palomar Observatory.
    Courtesy of Palomar Observatory
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Never mind that he's a nice guy who spends his time studying the also-rans of the solar system, the little pieces of debris that get overshadowed by the big kahunas like Jupiter, Saturn and that big yellow thing in the sky. To those who have left him angry messages and emails, Cal Tech astronomer Mike Brown is a traitor to underdogs.

Why? Because he played a crucial role in Pluto's demotion from planethood. Five years ago, he made a discovery that raised questions about Pluto's status as the ninth planet. Pluto is now officially known as a "dwarf planet," disappointing hordes of kids – and those who are kids at heart – who feel a connection to the cold and lonely orb.

Some astronomers say he's wrong, but Brown is sticking to his guns. And how! His new book is called How I Killed Pluto and Why It Had It Coming.

I asked Brown about what makes a planet a planet and how his discovery changed the way that we see the solar system. And, of course, we talked about that other Pluto: the Disney cartoon dog, which is said to have been named after the planet. Er, make that ex-planet. (It got its name from the Roman god of the underworld.)

Q: Did you like Pluto as a kid like so many people do?
I think my experience was typical for a kid who loved planets. I had a poster of the solar system on my wall that showed all of the planets and pictures of them. Pluto was always an incredibly appealing place: the little artist's pictures were crazy, and it was fabulous.

Q: How much influence do you think the Disney dog has on people's perceptions of Pluto?
That's part of the appeal of the word Pluto. As a kid, I'm sure that I heard "Pluto" as much from the cartoon from the planet, if not more from the cartoon. It becomes an even more integral part of your life.

Q: Pluto was discovered back in 1930. Why did astronomers think it was a big planet, not a tiny rock?
It couldn't be seen very well, and even with the biggest telescopes, you can't make out of the disc of it.

At the time, astronomers were convinced that it was massive, and it was hugging Neptune and had to be a planet. Once you get the idea that it's a planet, even for the wrong reasons, it's difficult to come back and fix that.

Q: By the 1970s, astronomers had begun to doubt that Pluto was very big. Then came the discovery of its moon, Charon. What did that mean for Pluto?
You could very immediately weigh the thing that's pulling the moon around. That's when its mass suddenly dropped to a tiny fraction of what it was originally thought to be.

Q: All of the planets can be seen with the naked eye or binoculars. What about Pluto?
You could easily see it with a backyard telescope and a digital camera. It's difficult to find if you just look through a telescope. It looks like an other star, and it's sitting in the middle of the Milky Way galaxy.

Q: So how did Pluto's status as a planet begin to deteriorate?
Once it got discovered and settled into the consciousness of people as a planet, no one really discussed its status at all until after 1992. That's when a first object outside Neptune was discovered. It was 100 miles across. But once you discover the second new object outside of Neptune, your mind can easily extrapolate that this region is going to be filled with objects, and some will be just like Pluto. Pluto is looking a little precarious.

Q: You found one of those objects in 2005 while looking at images of the sky taken at Southern California's Palomar Observatory. The leading astronomy body had to decide what to call it, and it decided it was a "dwarf planet," not a planet. That led to Pluto getting the same designation. What's a planet in your mind?
For at least 40 years, planets meant the big things that go around the sun. Early on, we decided that asteroids weren't planets because they weren't big enough to make the grade. Here, we basically reaffirmed that definition and reminded everyone that Pluto isn't one of those big things that goes around the sun.

Q: The object you found is officially named Eris, but you initially named it after something else, right?
We always gave nicknames to objects so we could know what we were talking about. We had named things Santa and Easter Bunny, and we'd always been saving a special name for something bigger than Pluto. That was Xena. We wanted a mythological name, and while Xena is only TV mythology, we figured we could use it.

Q: Did you expect the uproar over Pluto's demotion?
It was always pretty clear to me that no matter what happened, it would be a big deal and it would cause someone to be irritated. If I said Eris should be a planet, people would say, "That Mike Brown, what a jerk." They'd demonize me. So I really couldn't win.

The only way to win was to do the right thing. Rather than sit around and argue that all these things I found are planets, I'd rather point out how the real solar system is arranged.

Randy Dotinga reviews books regularly for the Monitor.

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