Meeting the Neighbors



A century ago, our solar system seemed split into two tidy groups: the terrestrial inner planets — Mercury, Venus, Earth, Mars — and the giant, gaseous outer worlds. Then Clyde Tombaugh found Pluto in 1930. It was small and seemingly alone at the ragged edge of the solar system.

Scientists are suspicious of such oddballs, and soon some predicted it was part of a new third planetary realm. But Tombaugh searched for years and never found another Pluto.

Hard evidence arrived half a century later, when scientists discovered Pluto’s large moon, Charon. They soon realized the pair formed when two dwarf planets collided in the outer solar system. The odds of that happening are slight, unless Pluto was part of a third group.

In 1991, Stern did the math. If only two Pluto-like worlds ever existed, the chances they’d collide are less than 1 in a million. The odds get even greater when you add in other observed coincidences in the neighborhood. It would be like plunging your hand into 100,000 tons of beach sand and snagging the only purple grain, Stern wrote in Astronomy magazine at the time. Pluto and Charon make sense only if hundreds or thousands of dwarf planets once roamed the outer solar system.

By 1992, astronomers had the tools to prove their hunches correct. A telescope in Hawaii equipped with a state-of-the-art digital camera found a 100-mile-wide world they dubbed 1992 QB1 — or “Smiley.” Scientists have since turned up more than 1,000 objects in this region, now called the Kuiper Belt, a disk-shaped realm of icy objects out past Neptune. Its most famous member is Pluto, but another is about to take the spotlight.

Cold Isolation



Astronomers suspect 2014 MU 69 — or just MU 69 (pronounced “mew”) for short — is a “cold classical Kuiper Belt object.” This group of worlds has stayed in flat, circular orbits right where they were formed, unperturbed by the goings-on closer to the sun.

Even asteroids, which are also excellent time capsules, were stirred up by Jupiter’s gravity and bombarded by solar radiation, altering their surfaces. That doesn’t happen in the cold classical Kuiper Belt. No major planets are anywhere close, and our sun’s pale light — which shines like a dim star at that distance — has barely changed the surface of these far-flung worlds.