The sun's family of planets, including the Earth, as well as its other companions like comets, all formed from a disk of material that was present in the early days of the solar system. Remnants of this disk can be found today in the outer parts of the solar system, in particular among the so-called "trans-Neptunian objects" (TNOs) -- bodies that orbit the sun farther (on average) than Neptune, which itself is thirty times farther from the sun than the Earth (i.e, thirty astronomical units, AU).
The first TNO to be found was Pluto, in 1930. The second TNO discovered (not counting Pluto's moon, Charon) was found in 1992, and is called "1992 QB1". Since 1992 another approximately 1000 TNOs have been found, out to a distance of about 60 AU. The largest of these is Eris, a TNO that, because it is larger than Pluto, helped prompt the debate that changed Pluto's status as a planet. TNOs do not emit visible light, but are seen in reflected sunlight. Because they are small and distant, it is difficult to see much farther away than about 60AU, and few are known. However, some astronomers think that there just may not be many that far or further away. Another star, they suggest, may have passed close to the sun in the early days of the solar system, for example helping to trigger a barrage of comets that ultimately passed near the earth. If so, this encounter might also have truncated the size of the disk, and so there just may not be many TNOs further away. Searching for more distant TNOs, or other evidence, could help resolve this puzzle about the early solar system. Indeed, one recent result from the Hubble Space Telescope suggests that there are very few more distant TNOs; unfortunately that result was of limited scope.
CfA astronomer Matt Holman and his student Cesar Fuentes used very sensitive optical images to search for TNOs in a relatively large region of the sky. Their survey, which was able to detect objects nearly ten thousand times fainter than Pluto, is the first with both the sensitivity and coverage to reach significant general conclusions. They found 82 new TNOs, and model their properties to report clear evidence for a drop in the number of TNOs farther away from the sun than about 60 AU, as previously speculated. In particular, they find that there are many fewer small objects (100 kilometer diameters or less) than would be expected if there were no such drop. These results not only help to supplement the growing catalog of TNOs, they help astronomers piece together the cosmic environment of the early solar system, when the earth was in its infancy.