The New Horizons spacecraft recorded images of craters that imply an unexpected dearth of small objects in the Kuiper Belt.

In the far reaches of the solar system, the Sun feebly illuminates millions of icy, rocky bodies known as Kuiper Belt objects orbiting the Sun beyond Neptune. But surprisingly little is known about these worlds—the most advanced Earth-based telescopes, even the Hubble Space Telescope, have trouble resolving any smaller than about 100 kilometers in diameter. Now, by studying craters on Pluto and its largest moon, researchers have determined that the Kuiper Belt likely contains far fewer small objects measuring a few kilometers in size than previously thought. This finding sheds light on how the solar system formed billions of years ago, the researchers suggest.

A Trove of Craters

“We definitely didn’t see any craters before New Horizons.” Kelsi Singer, a planetary scientist at the Southwest Research Institute in Boulder, Colo., and her team analyzed images returned by the New Horizons spacecraft during its flyby of the Pluto system in July 2015. The mission revealed mountainous, cratered worlds, said Singer. “We definitely didn’t see any craters before New Horizons.”

Singer and her colleagues were doing more than crater counting—they wanted to learn about the Kuiper Belt objects that produced these features. Identifying smaller craters would help the scientists learn something about these small objects we can’t otherwise observe.

A Lack of the Littles

The scientists applied a well-tested scaling law relating the size of an impactor—in this case, a Kuiper Belt object—and the size of the resulting crater. When they calculated the size distribution of Kuiper Belt objects, they found a relative dearth of the smallest bodies measuring 1–2 kilometers in diameter.

This deficit is probably real, the researchers suggest, as opposed to being an outcome of tectonic or landslide activity destroying or covering up the smallest craters. That’s because the trend persisted on a part of Charon known as Vulcan Planitia, which is believed to have been geologically inactive for roughly the past 4 billion years. These findings were published today in Science.

This result helps to constrain models of how the solar system formed, the researchers suggest. Rather than the solar system assembling from countless numbers of kilometer-scale and smaller bodies, as has been proposed by theory, these new results suggest it may have developed from comparatively larger objects, roughly 100 kilometers in diameter.

Singer and her colleagues are looking forward to studying the craters on yet another faraway object revealed by New Horizons: 2014 MU69, also known as Ultima Thule, a Kuiper Belt object the spacecraft zipped past on 1 January 2019. That’s in-progress work, said Singer. “We’re just getting more images back now.”

—Katherine Kornei ([email protected]; @katherinekornei), Freelance Science Journalist