How I Killed Pluto And Why It Had It Coming by Mike Brown

I’ve always wondered what went into deciding Pluto’s planetary status. Brown’s book sounds like a fun and interesting read. I love learning little anecdotes. Great review!

Actually, scientific agreement about Pluto has not changed. Its planet status has been disputed since its discovery in 1930 and remains a matter of debate. School books and museums do not need to make a change so much as to make it clear that there are two camps of astronomers when it comes to Pluto’s planet status. Dynamicists, who focus on the way celestial bodies perturb other celestial bodies, believe an object has to be the dominant one in its orbit to be a planet. In contrast, geophysicists, who focus on the individual bodies themselves, believe that any object large enough to be rounded by its own gravity and in orbit around a star is a planet. In fact, the leading opponent of the IAU decision, which was made by only four percent of its members, most of whom are not planetary scientists, is the Principal Investigator of NASA’s New Horizons mission to Pluto and the person who first coined the term “dwarf planet” back in 1991. His intention was to create a third class of planets in addition to terrestrials and jovians, small planets large enough to be rounded by their own gravity, a state known as hydrostatic equilibrium, but not large enough to gravitationally dominate their orbits. He never intended for dwarf planets to not be considered planets at all.

Objects in hydrostatic equilibrium have geology and weather and are often geologically differentiated into core, mantle, and crust, just like Earth is. They are complex like the bigger planets and very different from shapeless asteroids and comets. So while four percent of the IAU say dwarf planets are not planets, just as many astronomers disagree with this. Significantly, in astronomy, dwarf stars are still stars, and dwarf galaxies are still galaxies.

To the author: I’m glad you’re never stopping at Neptune when reciting the planets. You shouldn’t even stop at Pluto. Eris, which is much further from the Sun, is a planet too, as are Ceres, Haumea, and Makemake, and probably many more small spherical objects in the outer solar system.

Interestingly, hundreds of professional astronomers led by Stern signed a formal petition opposing the demotion of Pluto and the IAU planet definition. One of these was Dr. David Rabinowitz, a member of the team of three that discovered Eris (the other is Dr. Chad Trujillo, who has not weighed in publicly on either side of this debate).

I was personally inspired to become an expert on Pluto and went back to school to study astronomy. You can find out more about the position that dwarf planets are a subclass of planets from my Pluto Blog at http://laurelsplutoblog.blogspot.com . There are also some great books advocating for Pluto’s planet status (and the planet status of all dwarf planets). The best are “The Case for Pluto” by MSNBC correspondent Alan Boyle and “Is Pluto A Planet?” by Dr. David Weintraub, a signatory to Stern’s petition. I am still working on a book of my own, “The Little Planet That Would Not Die: Pluto’s Story.”