An ambitious hunt for small, faint objects in the outer solar system is set
to begin in the next few weeks. The project could shed light on the shadowy region and reveal the forces that shaped the early solar system.
The project will target the Kuiper Belt, a ring of objects beyond Neptune left over from the formation of the planets about 4.5 billion years ago. Most of the 1000 Kuiper Belt Objects (KBOs) discovered since 1992 orbit the Sun at a distance 30 to 50 times further than the Earth.
Based on their brightness, they appear to range from 100 to 1000 kilometres in width. Astronomers expect to see many more KBOs of smaller size - which probably formed through collisions - but these are difficult to detect because they reflect so little light. Only the Hubble Space Telescope has managed to turn up any - a few objects tens of kilometres wide.
"Progress in this area is achingly slow," says Charles Alcock, director of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, US. "Most of the volume of the solar system is inaccessible to direct surveys."
So, he and an international team of astronomers have devised an alternative scheme to search for small KBOs. Their plan is to look not for the objects' reflected light but for their shadows. They will use four 50-centimetre telescopes in Taiwan to study 3000 stars simultaneously in the hopes that one will be dimmed by the passing of an intervening small KBO.
This method could detect objects as small as 3 km across at distances of 100 astronomical units - one AU being the distance between the Earth and the Sun, about 150 million km. But a KBO transit event would last less than a second, and any detection would be dependent on the star, KBO, and telescopes being perfectly aligned.
"It's like the shadow of a cloud - you have to be in the right place to see it," explains Lawrence Wasserman, an astronomer at Lowell Observatory in Flagstaff, Arizona, US and a member of another survey team that has found about half of all known KBOs.
Alcock acknowledges that the project, called the Taiwanese American Occultation Survey (TAOS), is likely to turn up just 10 events per year - what he calls a "painfully low rate". And he says the automated observation programme is vulnerable to false positives from, among other things, birds and aircraft. "It's an act of desperation," he admits. "One should only turn to this technique when other techniques are not going to work."
But astronomers say the project, which will start observations later in November, will nevertheless reveal interesting science even if it does not turn up any occultations. "If nothing happens, then we'll know there are fewer objects than we thought," Wasserman told New Scientist.
That could mean most small KBOs have already been ground into dust by collisions, he says. "The question is, what is the relationship between the Kuiper Belt and the dust rings we see around other stars?"
The Kuiper Belt should also hold clues to how the solar system formed, because its shape and size must have been determined by these processes. "But first we've got to understand what the Kuiper Belt is now," says Wasserman.
All comments should respect the New Scientist House Rules. If you think a particular comment breaks these rules then please use the "Report" link in that comment to report it to us.
If you are having a technical problem posting a comment, please contact technical support.
