How Pluto Lost it All

October 6, 2015 | Sarah Tse

High-resolution MVIC image of Pluto in enhanced colour to bring out differences in surface composition.
Photo credit: NASA / JHUAPL / SWRI

Many of us still feel bitter about Pluto’s demotion to dwarf planet status in 2006. If you want to know exactly how if went down and why, read on.

We all remember that fateful, heartbreaking day when we learned that Pluto had lost its status as a proper planet. Those of us who cherish our childhood memories of learning about nine planets, not eight, felt like our confidence in the solar system had crumbled away.

The Battle Lines Are Drawn

Long before the International Astronomical Union officially demoted Pluto to a mere dwarf planet in 2006, discoveries at the edge of the solar system sowed the seeds of dissent over Pluto’s status. While many people might imagine the solar system to end at Pluto’s orbit, it actually extends much farther and includes a large band of tiny, icy bodies called the Kuiper Belt. The detection of the first Kuiper Belt Object (KBO), called 1992 QBI, caused astronomers to question where these objects fit into our existing perception of the solar system.

SEE ALSO: Massive Shadowy Planet May Lurk Beyond Pluto

Neil deGrasse Tyson, director of the Hayden Planetarium, dealt the first blow against Pluto in 2000 when he unveiled an exhibit with only eight planets. In the following years, discoveries of larger and larger KBOs provided mounting evidence questioning Pluto’s status. In particular, the KBO named Eris, discovered in 2005, appeared to be even larger than Pluto. Some even called it the solar system’s “tenth planet,” igniting a controversy that suited the KBO’s namesake from the Greek goddess of strife. Mike Brown, the Caltech professor who discovered Eris and self-proclaimed “Pluto-killer,” later determined the mass of Eris to be a full 27 percent larger than Pluto. In fact, Eris is so large that it could comfortably contain all the asteroids in the entire asteroid belt between Mars and Jupiter.

The Armies Clash

By 2006, with more and more discoveries of KBOs piling up, the International Astronomical Union finally initiated a committee to reevaluate the definition of a planet. At first, the committee considered leaving Pluto alone and instead expanding the club of official planets to include Eris and Ceres, another large body located in the asteroid belt. But some astronomers felt this would dilute the integrity of planethood. Iwan Williams, then president of IAU’s planetary systems science division, said at the time, “By the end of the decade, we would have had 100 planets, and I think people would have said ‘my goodness, what a mess they made back in 2006’.”

Pluto’s fate was sealed on August 24, the last day of the assembly. The committee resolved that “a planet is a celestial body that is in orbit around the sun, has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and has cleared the neighborhood around its orbit.”

Pluto fits the first two stipulations — it obviously orbits the sun, and is large enough that its gravity has forced it into a rough sphere. But on that last point, Pluto just doesn’t cut it. It shares its orbital neighborhood with plenty of other KBOs. In one day, Pluto lost the prestige it had enjoyed for the 76 years since its discovery. Instead, it joined the ranks of other “dwarf planets,” which include Ceres, Eris, and two other KBOs called Quaoar and Sedna.

The Aftermath

Of course, diehard Pluto devotees continue to argue against the IAU’s reclassification. For example, Earth’s orbital neighborhood is crowded with over 12,000 asteroids, so by the new definition our home world isn’t a proper planet either. Alan Stern called the outcome “an awful decision” and “internally inconsistent,” a stance he maintains to this day as chief scientist of the New Horizons mission. In fact, in 2014 he even challenged Tyson to a debate, although Tyson declined: “I don’t have opinions that I require other people to have.”

Meanwhile, astronomers have turned to the other dwarf planets that cost Pluto its planethood to see how they compare to their more well-known comrade. NASA’s Dawn spacecraft recently brought back fresh data on a cluster of startlingly bright spots on Ceres, a mystery yet to be solved. Perhaps after all this time, we should move on from Pluto’s downgrade and focus on what we can learn from all the other inhabitants of the solar system.

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