The Kuiper Belt, the zone in space immediately beyond Neptune, has fewer small objects than we would expect, a study of the surfaces of Pluto and Charon suggests. Explaining this partial absence may overturn models of how the Solar System formed and evolved.
Advances in telescope production have allowed us to detect numerous Kuiper Belt Objects in recent years. For decades, Pluto and the occasional comet with an elongated orbit were the only known inhabitants of this space. Now, however, we are discovering them so fast we can't keep up with naming them. However, most of those we have found are more than 100 kilometers (60 miles) in diameter, simply because anything much smaller would be too faint to detect, even with giant modern telescopes.
We expect smaller space objects to be more common than larger ones, but Dr Kelsi Singer of the SouthWest Research Institute thought this theory worth testing. Dr Singer headed a large team that looked at the impact craters on Pluto and Charon, as photographed by New Horizons, and used well-established relationships between the size of an object and the crater it produces to see what has been striking the pair.
Parts of Pluto, such as the famous heart, are constantly resurfaced by geological processes, with small craters particularly easily lost. However, other parts of both planet and moon are thought to preserve billions of years of craters intact.
The outskirts of the Solar System are so much less busy than the bustling uptown that it was anticipated even ancient parts of Pluto and Charon would be much less cratered than the Moon. Nevertheless, Singer reports in Science these areas have even fewer small craters than models suggest. Charon in particular experiences a sharp drop below 13 kilometers (8 miles) across, the size of a crater formed by objects 2 kilometers (1.2 miles) wide or smaller.
The predictions for small craters were based on those seen on the well-studied moons of Jupiter and Saturn, and on theories about the way objects formed in the Kuiper Belt in the Solar System's early era.
Although the Kuiper Belt is vast and largely empty, such a long time has elapsed since its formation that scientists expected many of the objects circling within it to have been involved in collisions. These should have either broken smaller objects off them or smashed them entirely into smaller pieces, leaving a trail of smaller objects behind. Why these smaller objects appear to be so much rarer than we thought will take some explaining.
Whatever answer is found, Singer notes, will influence how we imagine the history of comets like 67P/Churyumov-Gerasimenko that have spent the vast bulk of their time in the Kuiper Belt, before plunging inwards for us to study.