When astronomers discovered a reservoir of icy bodies in the nether-space beyond the orbit of Neptune, they thought they had identified the main source of comets that periodically swing into the inner solar system. But now, a meticulous search for small objects in this so-called Kuiper belt has turned up fewer than 4% of the expected number. The puzzling find may shed new light on the early evolution of the solar system.
The Kuiper belt is made up of lumps of dirty ice--probably leftovers from the birth of the solar system--known as trans-Neptunian objects (TNOs). Most astronomers agree that the planet Pluto and its moon, Charon, would be better classified as the largest TNOs. So far, more than 800 TNOs have been found, most of them more than 100 kilometers across.
Using the eagle-eyed Advanced Camera for Surveys on board the Hubble Space Telescope, a team of astronomers led by Gary Bernstein and David Trilling of the University of Pennsylvania in Philadelphia has carried out the first sensitive search for very faint (and hence relatively small) TNOs in a tiny but typical patch of sky in the constellation Virgo. Based on the known numbers of large bodies, the scientists had expected the search to turn up some 85 TNOs as small as 20 kilometers across. Instead, they found three.
In a paper submitted to the Astrophysical Journal, the researchers say their results are "wildly inconsistent" with the observed number of short-period comets--comets with orbital periods of less than 200 years--that are believed to be small TNO escapees.
"This is very exciting work," says small planetary body specialist Dan Durda of the Southwest Research Institute in Boulder, Colorado. The dearth of small bodies suggests that the Kuiper belt experienced a bout of intense collisions in which icy bodies a few tens of kilometers in diameter were smashed to smithereens. "This new find is a valuable clue to the early collisional history of the outer solar system," Durda says. As for the short-period comet problem, Durda thinks it's possible the comet precursors are not missing at all, but just too small to be seen by Hubble's prying eyes. At the same time, he says, it's also possible the observed number of comets could be generated by fewer TNOs than researchers had assumed.