Brandon T.'s Reviews > How I Killed Pluto and Why It Had It Coming

How I Killed Pluto and Why It Had It Coming by Mike Brown
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Apr 14, 2011

it was amazing

** spoiler alert ** Most of us grew up knowing that there were nine planets in our solar system: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Neptune, Uranus, and distant, frigid Pluto.

But we were wrong.

Mike Brown explains how he discovered the tenth planet, Xena, and why his discovery led to an international identity crisis that ended with the kicking of Pluto out of the planetary club.

Brown says he didn’t set out to demote Pluto. In the 1990s, hundreds of small rocky objects were discovered out at the edge of the solar system. Collectively known as the Kuiper Belt, they reminded astronomers that there were still major features of our own neighborhood that we hadn’t noticed.

The Kuiper Belt discoveries convinced Brown that there might be another planet lurking somewhere in the shadows of the stars. He knew that no systematic search had been conducted in over 70 years. Telescopes had grown much more precise since then; computers hadn’t even existed at the time.

“How could it be that if someone went and looked again for a new planet they wouldn’t find something that had been just beyond the reach of the telescopes in the 1930s?” Brown writes. “There had to be a tenth planet.”

Through the late 1990s and early 2000s, Brown devoted most of his time to proving himself correct. The journey he relates is one of contingency, surprise, and international intrigue.

Brown and his colleagues began a daunting survey of the sky. He quickly found that modern telescopes, most of which were designed to look at far-off stars and galaxies, were actually too precise for his task. So he took up residence with the largely unused Schmidt Telescope in Pasadena, Calif. The machine was so underutilized that it still employed glass photographic plates to take pictures, as it had when it was constructed in the 1950s.

Yet the Schmidt Telescope’s primitive method ended up being key to the search for planets close to home. Unlike with digital cameras, the photographs wouldn’t lose resolution when taking pictures of large swaths of the sky. As Brown points out, “to see as much sky as you could see with the photographic plate you would need a five-hundred-megapixel digital camera. Even today that is a daunting number.”

Along the way, he discovered numerous bodies that changed astronomers’ understanding of the solar system. The first of these, dubbed Quaoar, was half the size of Pluto, but otherwise similar in its properties. Brown called it “a big icy nail in the coffin of Pluto.”

Soon thereafter, Brown came across another oddball that he eventually named Sedna, which was “unlike anything else in the known universe.” It never came close to any other planets; at its closest, it didn’t even touch the outer edge of the Kuiper Belt. And for most of its orbit, Sedna was so far away that “the sun would be just an extrabright star in the sky.”

Yet Sedna was clearly not a comet, because comets have complex orbits that are influenced by passing near multiple stars. Sedna only circles the Sun today. Nevertheless, its wildly divergent path suggested that during our solar system’s earliest years the Sun had been one half of a pair of twin stars.

Although discoveries like these were exciting for Brown and his colleagues, they didn’t quite reach the threshold for claiming a new planet—something that was larger than Pluto.

Then came Eris (originally nicknamed Xena). At the time of its discovery, it was almost four times the distance from the Sun as Pluto. It took 557 years to complete one circuit around the Sun. And it was nearly twice the size of Pluto.

Brown already understood by this time that discovering something bigger than Pluto would cause questions to arise about the definition of a planet. Indeed, he writes about pondering this subject since before he began his quest. At one point, he asked a friend with a degree in philosophy, “What does a word mean when you say it?”

“‘Words mean what people think they mean,’ was his smoothly philosophical reply. ‘So when you say ‘planet’ it means what you are thinking when you say it.’”
From Brown’s personal quandary about what defines a planet, he came to conclude that Eris—and therefore Pluto—could not possibly qualify unless he also chose to include hundreds of other objects in the Asteroid Belt and the Kuiper Belt.

In August 2006, the International Astronomical Union (IAU) did something in response to Brown’s discovery that had never been done in a millennium of astronomy: it voted on a formal definition of the word “planet.”

Tensions ran high during the IAU’s meeting, with politics playing as much of a role as science. A pro-Pluto faction grew within the group that aimed to devise a definition that would keep the nine-planet system without seeming unscientific.

Brown, watching the proceedings from California with a bevy of reporters, kept explaining to them why he, who might become the only living discoverer of a planet if the IAU decided to vote a certain way, didn’t think Eris or Pluto belonged in the pantheon.

In the end, science won out. The eight planets were given their own formal category. Pluto was relegated to a new class of “dwarf planets,” along with Eris and a few other objects in the solar system.

Innocent Pluto, a bystander in the entire affair, was collateral damage in the crisis caused by Eris.

Since the IAU’s decision, Brown writes that he’s been accosted everywhere he goes, with people asking him, “What did Pluto ever do to you?”

Despite the harassment, and despite the fact that he will never find another thing he can call a planet, he says that he’s “thrilled that astronomers…chose to put a scientific definition behind what most people think they mean when they say the word planet. They don’t mean ‘everything the size of Pluto and larger,’ and they certainly don’t mean ‘everything round.’ Instead, when people say ‘planet,’ they mean, I believe, ‘one of a small number of large important things in our solar system.’”

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