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How ‘killing’ Pluto redefined the solar system

“Pluto is dead.”

Mike Brown, California Institute of Technology astronomer and self-proclaimed Pluto killer, uttered these words exactly nine years ago today after the International Astronomers Union (IAU) officially revoked Pluto’s planetary status. Since then, we’ve continued to ponder Pluto, with our obsession culminating this summer when the New Horizons spacecraft cruised by the celestial object on July 14. So on Pluto’s day of demotion, we celebrate how the lonely dwarf planet has forever changed our view of our solar system.

The “killing” of the planet was actually more of an involuntary manslaughter. Pluto’s planetary status came into serious jeopardy in 2005, when Brown and his colleagues discovered a bright object beyond Pluto. The object, now known as Eris, turned out to be similar to Pluto in many ways. It was round. It orbited the sun. It had a moon. And while Eris was smaller in volume than Pluto, it was more massive.

Eris’ existence, and the likelihood that there were hundreds of undiscovered objects in the solar system just like it, raised a serious question for the IAU — how should these objects be classified?

“It wasn’t clear whether they were going to be considered in the same category as planets, or would just be asteroids,” said Owen Gingerich, a Harvard University astronomer.

Gingerich was head of the committee appointed to determine the classifications. Despite the committee’s recommendation that a new class of planets called “Plutons” be created, the IAU ultimately voted to consider Pluto, and other objects like it “dwarf planets.”

The decision is still a controversial one, even among astronomers. Pluto was a popular planet, and some argue that “demoting” it did little to clarify how we understand the solar system.

A view of Pluto and Charon as they would appear if placed slightly above Earth's surface and viewed from a great distance.  Recent measurements obtained by New Horizons indicate that Pluto has a diameter of 1,472 miles, 18.5% that of Earth's, while Charon has a diameter of 750 miles, 9.5% that of Earth's. Photo by NASA.

A view of Pluto and Charon as they would appear if placed slightly above Earth’s surface and viewed from a great distance. Recent measurements obtained by New Horizons indicate that Pluto has a diameter of 1,472 miles, 18.5% that of Earth’s, while Charon has a diameter of 750 miles, 9.5% that of Earth’s. Photo by NASA.

“I can understand it,” Brown said. “Even when I was a kid, Pluto was my favorite planet. I had a poster on my wall. Pluto was off on the edge, and it was mysterious, and we had never been there. You know there’s just something that everybody likes about it, and I think that’s always been the case.”

The debate was so heated that it came to overshadow Pluto itself — a fascinating object.

“When people ask me about my opinion on Pluto’s planethood, my opinion is that it’s the least interesting conversation to have about Pluto,” Emily Lakdawalla, a senior editor at Planetary.org said. “I would so much rather talk about the exciting things that we’ve seen on Pluto … How did it form? How did its moon form? What does it have to say about the formation of our solar system? Is it like the other worlds in the Kuiper Belt, or not? … So what word a few English-speaking people use to talk about it is really the least interesting conversation I can think of.”

Recently, the conversation has been dominated by the findings from the New Horizons mission.

Pluto nearly fills the frame in this image from the Long Range Reconnaissance Imager (LORRI) aboard NASA’s New Horizons spacecraft, taken on July 13, 2015 when the spacecraft was 476,000 miles (768,000 kilometers) from the surface. This is the last and most detailed image sent to Earth before the spacecraft’s closest approach to Pluto on July 14.  Image by NASA/APL/SwRI

Pluto nearly fills the frame in this image from the Long Range Reconnaissance Imager (LORRI) aboard NASA’s New Horizons spacecraft, taken on July 13, 2015 when the spacecraft was 476,000 miles (768,000 kilometers) from the surface. This is the last and most detailed image sent to Earth before the spacecraft’s closest approach to Pluto on July 14.
Image by NASA/APL/SwRI

The space probe’s data showed a variety of terrain on Pluto, including possible glaciers made from frozen nitrogen, a red-colored surface like Mars, a geologically active center and Rocky Mountain-sized peaks. Sensors also detected that Pluto’s atmosphere extends high above the planet’s surface and is blown off by solar wind, creating a cometlike tail of nitrogen gas.

The public has loved New Horizons, following the mission with unprecedented interest. The voyage made international headlines and inspired a number of artistic tributes, like Haikus on twitter.

And this is only the beginning, as it will take 16 months to download all of the data from New Horizons.

“I think this really makes people interested in what else might be out there in the Kuiper Belt,” Cathy Olkin, a deputy project scientist on the New Horizons mission, said. “This is the first example we’ve looked at, and it’s really fascinating and interesting. We’re going to be learning new things about processes in the solar system and planets and what’s going on, and this is just the first one we’ve looked at. I think it’s hard to imagine that there wouldn’t be other fascinating bodies out there to study as well.”

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