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The Kuiper Belt: Thousands of Icy Worlds Instead of Just One

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When people heard in 2006 that Pluto was no longer a planet, many were outraged. The beloved ninth planet, the satisfying conclusion to the list of the solar system’s members rattled off by many a schoolchild, was no longer special. Some accused celebrity astrophysicist Neil deGrasse Tyson of killing Pluto; Tyson confessed only to driving the “getaway car.” The real villain was California Institute of Technology astronomer Mike Brown, who discovered an object called Eris that’s slightly bigger than Pluto. This observation suggested that Pluto was just one among many other “dwarf planets.”

But there is another way to look at the former planet’s demotion. We can pity its downgrade, or we can reflect with wonder on the hundreds of thousands of small worlds, including Pluto, that make up the Kuiper Belt, which we now know is one of the largest features of the solar system.

This belt lies at the outer edge of our system, beyond Neptune. If the solar system were a soccer field—with the Sun at the center mark, Jupiter orbiting on the center circle, and Earth in between, just six feet from the center—then the Kuiper Belt would begin near the sideline. It would then continue for about eighteen Chicago-sized city blocks outside the stadium. That’s 2,000 times the distance of the Earth from the Sun. At this point in space, the gravity of other stars begins to exert more pull on objects than our Sun does.

The objects that circulate within this broad belt are of all sorts and sizes. Most are small balls of ice—essentially comets-to-be, waiting to get yanked out of a stable orbit by another object’s gravity. A good number are scaled-down versions of Pluto and Eris, often accompanied by “moons” nearly the same size as the “planet.” Scientists have so far seen about 1,000 objects of sizes between comets and Eris. They believe there are hundreds of thousands more to be found.

Perhaps what’s most surprising about the Kuiper Belt is how recently we found it—and how much more we have to learn. In 1951, Dutch astronomer Gerard Kuiper suggested that debris left over from the formation of the major planets once orbited in the space beyond Neptune—but surely would have wandered into deep space or the inner solar system long ago.

It took over forty years to test this idea. Since the objects are so far from the Sun, they are dim, making them hard to see with Earth-based telescopes. But in 1992, astronomers looking through the 2.2-meter University of Hawaii telescope at Mauna Kea finally observed the first Kuiper Belt object—a small comet-like body about 120 miles across.

But our understanding of what makes up the Kuiper Belt could be about to change. Mike Brown, along with Caltech colleague Konstantin Batygin, may have found a true ninth planet after all. They haven’t observed the planet directly. Instead, they recognized that the orbits of Kuiper Belt objects are yanked a little off track, possibly by a planet five to fifteen times the mass of Earth similar in size and looks to Neptune. For now, it’s just called Planet Nine. While its orbit would be huge, probably taking about 15,000 years to complete one circuit, astronomers are narrowing down where to look for the proposed cold, dark sphere.

The idea that a planet the size of a gas giant has escaped astronomers’ notice for this long suggests that we might not know what other worlds, small or large, still hide in the Kuiper Belt. Modern astronomy may have taken one planet away from us, but it could soon replace it with a new one.

 

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