Why Pluto Isn’t A Planet

Mike Brown, who discovered Xena, decided he could not in good conscience allow it to be made a planet. And killed off an old favorite in so doing.

Cal Tech astronomer Mike Brown has a new book out titled How I Killed Pluto and Why it Had it Coming.  The Atlantic has an excerpt.

The solar system does not consist of twelve planets and then everything else. That is simply a fundamentally incorrect description of it. And the next day in Prague, astronomers were going to stand up and encourage the world to think of the solar system incorrectly. As someone who spends much of my life trying to be not just a scientist but an educator, trying to explain the universe and show the excitement without resorting to science fiction or trivial simplification, the idea that astronomers would actively encourage people to have the wrong view of the solar system seemed almost criminal. The idea that I was going to, overnight, become one of the most famous astronomers in the world on account of this criminal activity made me a passive accomplice. I had to do something to stop it.


I spent most of the next twelve hours, and indeed most of the next week, on the phone talking to the press about the solar system, planets, and why the IAU’s proposed definition was fatally flawed, and explaining why Pluto — and Xena — should really not be considered planets.

Germane to the story:  Brown was the man who discovered Xena and would have been permanently famous for having made the discovery that led to the change he was fighting tooth and nail.  He won:

Astronomers around the world picked up on some of the silly implications of making Charon a planet simply by virtue of the location of the center of mass of the orbit. In the middle of one phone interview, it suddenly occurred to me that the center of mass of the sun and Jupiter lies outside the sun, so by IAU logic, Jupiter should not be considered a planet since it doesn’t really go around the sun. Another astronomer sent an email showing that if a massive moon were on an elongated orbit, the center of mass could be inside the planet during part of its orbit but outside the planet during other parts of its orbit, meaning that, according to the IAU, that moon would switch back and forth between being a planet to being a nonplanet during the course of its orbit. And a few days later, courtesy of a fabulous press release by Greg Laughlin, an astronomer at the University of California Santa Cruz, the newspapers explained that because our moon is slowly moving outward, away from the earth, in a billion years or so it will have moved so far away that the center of mass of the earth-moon system will lie outside the earth. Suddenly: boom! The moon will officially be a planet. It would be a day to celebrate.


The revolting astronomers, who grew to be a sizable fraction of the astronomers present, made it quite firmly known that they would not support the secret committee proposal. The only proposal they would support would be one where Pluto was put in its logical — rather than emotional — place. Pluto, Charon, Ceres, and my own Xena would all have to go. The press, and indeed the astronomers in Prague themselves, were quite amused by the fact that one of the most vocal supporters of demoting Pluto, Charon, Ceres, and Xena was the guy who had the most to personally gain from Xena being a planet: me.

As someone who studies politics rather than astronomy, I’m somewhat bemused that they can be every bit as ideological.  But Brown’s fight strikes me as right, even if it does upend what I learned in 2nd grade science.

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James Joyner
About James Joyner
James Joyner is Professor and Department Head of Security Studies at Marine Corps University's Command and Staff College. He's a former Army officer and Desert Storm veteran. Views expressed here are his own. Follow James on Twitter @DrJJoyner.


  1. Brett says:

    I would have preferred to have simply upped the planet count, instead of creating a new category of “dwarf planets”.

  2. sam says:

    Heh, I plugged Xena into google, and got immediate hits on the warrior princess. So, had to plug in “Xena Planet”. That was better, but then I discovered that Xena as a moon, Gabrielle… However, I then found out that Xena is cold and frozen. That sure ain’t the other Xena, which may be, I thought, why they’ve renamed it Eris (after the Greek goddess of discord). The moon will now be called Dysnomia, the goddess of lawlessness and the daughter of Eris

    Actually, though, I the reason they renamed it is…well, let Brown tell it:

    “Eris is the Greek goddess of discord and strife,” explains Brown. “She stirs up jealousy and envy to cause fighting and anger among men. At the wedding of Peleus and Thetis, all the gods were invited with the exception of Eris, and, enraged at her exclusion, she spitefully caused a quarrel among the goddesses that led to the Trojan War.”

    You can see why ‘Eris’ was chosen. The guy has a sense of humor. Seems to like eristic, too.

  3. tom p says:

    Mike Brown rules. I love reading his stuff.