How I Killed Pluto

Well, no, not me.  It's the title of an interesting book I just read by Caltech astronomer Dr. Michael Brown.

How I Killed Pluto and Why It Had It Coming (Spiegel & Grau, 2010) is an easy read about the human side of astronomy and the events that lead up to the demotion of Pluto as a planet - an event in which Brown played a critical role.

Brown's passion as an astronomer is in hunting for trans-Neptunian Kuiper Belt objects.  The Kuiper Belt is the region out beyond the orbit of Neptune, discovered in the early 1990s, where a large number (tens of thousands we believe) of icy bodies orbit the Sun.

Brown, along with his colleagues, have discovered numerous objects in this part of the solar system but the Holy Grail for them was to discover something larger than Pluto - a tenth planet.  In 2005, Brown believed he did - an object nicknamed Xena (after the TV character).  This discovery, however, created a problem.  There was no formal definition for the word "planet" in astronomy.  Was Xena a planet?  What about other objects discovered which were slightly smaller than Pluto.  Were they planets?  Why or why not?

The International Astronomical Union (IAU) took up this question in 2006, muddled around for a bit, and then came up with this definition which settled the issue.  A planet has three characteristics:

1.  A planet orbits the Sun.  Jupiter's moon Ganymede is larger than Mercury but it orbits Jupiter so it's not a planet, it's a natural satellite.

2.  A planet is large enough (has sufficient mass) to have formed a spherical shape.  That leaves off all those potato-shaped asteroids.

3.  A planet has cleared the neighborhood around its orbit.  This omits Pluto as a planet.  Objects which fail this condition are now called dwarf planets.

There are now five official dwarf planets - Pluto (some consider Pluto a binary dwarf planet because the center of mass with its moon Charon lies between the two bodies), Ceres (a spherical asteroid in the asteroid belt), Haumea (discovered by Mike Brown but with some controversy since a Spanish group of astronomers may have unethically tried to claim credit - an event Brown discusses in his book), Makemake (also discovered by Mike Brown), and of course Eris (Xena's official name - discovered by Mike Brown).  Brown and his colleagues have also discovered other trans-Neptunian objects (Orcus, Quaoar, etc) that are likely to be dwarf planets (hard to tell their shape when they're so far away and relatively small).  There are likely hundreds of dwarf planets out there.

Any astronomer will tell you that Pluto is fundamentally different from the four terrestrial, rocky planets (Mercury, Venus, Earth, and Mars) and the four Jovian, gas giant planets (Jupiter, Saturn, Uranus, and Neptune).  Pluto, as a small icy body with a highly elliptical, inclined orbit, is much more like the Kuiper Belt objects we've been discovering over the past two decades than it is like the traditional eight planets.  I think most astronomers agree that Pluto deserved its demotion - it is a fundamentally different type of object.  That's scientifically interesting and studying objects like Pluto will teach us much about the formation of the solar system.

Mike Brown interweaves all of this information with a personal account of his life and work.  He spends a lot of time in the book talking about his wife and new daughter, born while he was making his important discoveries, but it works with his narrative.  Reading this book will give you insight into the life of a planet-hunting astronomer - the long nights, hatred of the Moon's light and cloudy nights, and days and weeks of examining raw data and telescope images for little moving dots of light.

It's a great read and I highly recommend it.  By the way, here's Mike Brown's Planets blog if you'd like to learn more.