The frozen worlds orbiting beyond Neptune include not only dwarf planets like Pluto and Ceres, but also a tiny, icy toehold just one-third of a mile wide.
The discovery, made by a team of astronomers scouring Hubble Space Telescope observations, sets a new record for the smallest Kuiper Belt object found. Previously, the smallest known Pluto sibling was a 30-mile-wide Kuiper Belt object.
The Kuiper Belt region, located about 4.6 billion miles away, is filled with objects believed to be left over from the solar system's formation. It is similar to the asteroid belt, located between Mars and Jupiter, but much bigger. Unlike the asteroids that contain rock and metals, Kuiper Belt objects have icy bodies of methane, ammonia, water and other volatiles.
The Kuiper Belt is particularly interesting to scientists looking for planetary systems beyond our solar system. Planets are believed to form from collapsing disks of gas and dust orbiting stars.
"The dusty particles begin to stick together and eventually build up larger objects. Not all make it into planets. It's the leftover ones are what we're seeing when we look at Kuiper Belt objects and asteroids," University of Arizona astronomer John Stansberry told Discovery News.
The finding of a very small Kuiper Belt object links our solar system's debris disk to those observed around other stars, added University of Toronto's Hilke Schlichting, who led the team that made the discovery.
"We can observe micron-sized particles (in extrasolar debris disks), which are thought to be induced by collisions, from grinding down larger objects," Schlichting told Discovery News. "By finding this evidence for collision grinding in the Kuiper Belt, it seems to be the missing link between our Kuiper Belt and extrasolar debris disks."
When it comes to Kuiper Belt objects, size matters. Scientists can use this information to determine an object's density and what it is made from. In larger bodies, gravity plays the dominant role in shaping objects. In smaller ones, it is the strength of its materials that matters.
"The discovery of just one small object is probably not going to lead to great advances. But if we started to discover statistically significant numbers of them, then we can compare the number of large and small bodies, and you can start to get a handle on the material strength of the objects. It also might tell you about the violence of the collisions," said Stansberry.
"Potentially, it might be a new field if we can make more discoveries like this," he added.
Schlichting and colleagues combed through 4.5 years of Hubble data to find the tiny Kuiper Belt Object, discovered as it passed in front of a background star, momentarily dimming its light.
"These tiny objects are much rarer than you would expect," Schlichting told Discovery News.
Based on the number of known objects in the Kuiper Belt, scientists would have expected to find between 30 and 100 tiny bodies in their analysis of 50,000 guide stars observed by Hubble.
So far, the team has only looked at 30 percent of the available Hubble data.
"We only found one," Schlichting said. "It shows that there's kind of a break in the size of objects in the Kuiper Belt from large objects, meaning bigger than 50 kilometers (31 miles), and smaller ones."
The dearth of small bodies may be evidence that objects in the Kuiper Belt are crashing and grinding down, she added.
The research was published last month in Nature.