In the summer of 1992, two researchers were atop a Hawaiian mountain when they saw a pinpoint of light slowly moving through the constellation Pisces. Located more than a billion kilometers behind Neptune, that seemingly little asteroid would force us to rethink our position in the solar system.
Outside the planetary systems’ orbits, there wasn’t nothing but a wide assortment of objects waiting to be discovered. The Kuiper Belt was found by scientists, and it is a doughnut-shaped band of icy debris that was left behind after the solar system’s birth.
Understanding the Kuiper Belt better helps scientists pin down the solar system’s beginnings and development. Recent discoveries of icy worlds in the Kuiper Belt have provided new insights on the origins of our solar system and others like it. And scans of this region have uncovered thousands of similar items, termed Kuiper Belt objects, suggesting that pinballing planets were once a common occurrence in the early solar system. The modest originator is a lump of ice and rock with a diameter of about 250 kilometers. This month marks the 30th anniversary of its discovery.
During the 1980s, MIT astronomer Jane Luu and planetary scientist David Jewitt were several years into a peculiar quest. In Arizona, the two had been using telescopes to snap random pictures of the night sky. “We were literally just staring off into space looking for something,” says Jewitt, who is now a professor at UCLA.
Researchers were prompted by a seemingly inexplicable phenomenon: Aside from the gas giant planets and tiny, cold Pluto, the inner solar system is quite packed with rocky planets, asteroids, and comets. Luu, who currently divides his time between the universities of Oslo and Boston, speculates that “maybe there were things in the outer solar system.” To investigate it seemed like a good idea.
Jewitt and Luu spent countless hours poring over photographic plates made of glass and digital photographs of the night sky, searching for objects whose extraordinarily slow motion indicated a considerable distance from Earth. Despite their best efforts, however, the two had no luck. Luu says, “Years passed, and we saw nothing.” “I had my doubts that this would work out.”
By 1992, things had turned around. Jewitt and Luu were using a telescope at the University of Hawaii on the Big Island on the evening of August 30. They used their standard procedure for finding faraway targets: Start by snapping a picture of the night sky, then wait an hour or so before snapping another picture of the exact same area of the sky. Due mostly to Earth’s orbital motion, the position of an object on the other side of the solar system would vary significantly from one image to the next. If the object is real, as Luu claims, it would move in a predictable manner and at a consistent speed.
It was 9:14 p.m. by the time Jewitt and Luu had taken two pictures of the same part of Pisces in the sky. Scientists repeatedly played back the images on the computer’s bulbous cathode ray tube monitor, searching for any signs of motion. Something jumped out right away: a pinpoint of light had moved slightly to the west.
However, it was still too soon to start partying. Images of the night sky frequently show spurious signals caused by high-energy particles hurtling across space, known as cosmic rays. The researchers understood the true test would be to see if this particle appeared in more than two photographs.
Until 11 o’clock at night, Jewitt and Luu waited anxiously for the telescope’s camera to finish shooting a third image. The similar thing had appeared, but little further to the west. Midnight’s fourth image capture showed the item had moved once again. Jewitt recalled thinking, “This is the real deal.” “We couldn’t believe it.
Jewitt and Luu did some quick math based on the object’s brightness and its slow motion, concluding that it would take about a month for it to march across the breadth of the full moon as seen from Earth. Whatever it was, it had a diameter of almost 250 kilometres. That’s quite a lot, equal to nearly one-tenth of Pluto’s breadth. As far as we could tell, it was orbiting well beyond Neptune. And it probably wasn’t even the only one there.
Jewitt and Luu had spent years painstakingly searching the skies, but they had seen very little. The two determined that there were probably thousands of such things waiting to be discovered. Jewitt describes the moment he realized the outer solar system was likely filled with objects that had yet to be discovered as “mind-blowing.” “We significantly increased the volume of the solar system that is known.” This new world was made possible by the discovery of 1992 QB1 by Jewitt and Luu (SN: 9/26/92, p.
A few months later, on April 10, 1993 (SN: 4/10/93, p. 231), Jewitt and Luu discovered a second object with an orbit far beyond Neptune. Soon after that, the dam broke. In the following few years, “we uncovered forty or fifty,” Jewitt adds. Because of improvements in the size and sensitivity of digital detectors, astronomers have found many previously unknown objects. Caltech astronomer Mike Brown, who specializes in Kuiper Belt objects, exclaims, “So many interesting worlds with rich stories.”
Jewitt and Luu concluded that the discovery of these icy worlds, some of which orbit beyond Pluto, made sense in a number of contexts. Pluto has never fit in with the other gas giants; it is a cosmic runt (smaller than Earth’s moon) and has a very different appearance. Moreover, it has an extremely sweeping orbit that puts it well above and below the orbits of the other planets. Jewitt and Luu speculated that Pluto might not be a part of the planetary system at all, and instead be a part of whatever lied beyond. Suddenly, we realized why Pluto was so out of the ordinary, as Jewitt puts it. It’s simply one body—perhaps the largest—in a collection we happened upon by chance. It was agreed that Pluto would not be considered a planet for much longer. That’s right; it hit the shelves in 2006 (SN: 9/2/06, p. 149).