A threatening presence from planet remains

 

Astronomers at Queen's University Belfast and Armagh Observatory are undertaking an international study of a belt of planetary leftovers, enormous objects up to 800 km across which orbit the sun beyond Neptune.

New "Trans-Neptunian Objects", TNOs, were being discovered every month, said Dr Alan Fitzsimmons, reader in observational astrophysics at Queen's, although the first was identified only in September 1992. The most recent, the 113th, was announced on February 16th, he said, and his group had discovered eight. He said the objects were found in what is known as the Kuiper Belt, a band of material in orbit between 30 and 50 astronomical units (AU) from the sun. An AU is equivalent to the distance of the Earth from the sun, about 93 million miles.

The first astronomer to theorise about their presence was an Irishman from Streete, Co Westmeath, Kenneth Edgeworth, an accomplished amateur who published two papers in the 1940s. These remained virtually unknown but the idea persisted, culminating in a paper in 1951 by a Dutch astronomer, Gerard Kuiper, whose name now describes their place in the solar system.

TNOs are remarkably difficult to spot, Dr Fitzsimmons explained, because of their small size relative to their distance from us and because they don't reflect much light.

"These objects are darker than coal," he said, "and you might expect to find no more than one in an area of sky about the size of a full moon."

This accounted for the long delay before the first TNO was identified seven years ago. It requires a very sensitive camera, but also their discovery was very much a matter of "believing that they were there", he said.

It is only in recent years that this has become possible, using a combination of wide field CCD (charge-coupled device) cameras and large 2.5-metre telescopes. The current generation of CCD cameras can take images of remarkably faint objects.

"We can see objects that are 15 million times fainter than the faintest star you could see on a dark night in Connemara," he said.

Observers use a trick of the ancient Greeks to distinguish the planets from the stars. It involves taking two images a given period of time apart and looking for near objects that have moved relative to the background of more distant stars. Knowing the time delay and the distance travelled gives astronomers estimates of how far away the object is.

TNOs are of great interest to researchers. They are assumed to be material left behind after the proto-planetary disc of matter that must have originally surrounded our sun condensed into planets. "What we believe we are looking at are the remnant building blocks of the planets. Samples would tell us much about the stuff from which planets are made."

Researchers are also interested because the Kuiper Belt is believed to be the source from which short period comets arise. These include Comet Temple Tuttle, dust from which produces the annual November Leonid meteor showers.

Visitors from the Kuiper Belt could eventually become Earth impactors. TNOs drop out of the belt and move into the solar system proper once every 100,000 years or so and there are eight or nine known objects moving between Neptune and Jupiter, he said. "We believe these are slowly moving into the inner solar system."

The impactor thought to have wiped out the dinosaurs 65 million years ago was estimated to have been between one and two kilometres across, but TNOs range from 50 km up to 800 km, with most falling between 100 km and 400 km. It is thought that any TNO making it past Jupiter's gravitational pull would be broken up into smaller pieces, but these would still represent a serious threat if they drifted towards us.

There is a move afoot to have the Kuiper Belt renamed the Edgeworth-Kuiper Belt and to re-christen TNOs as Edgeworth-Kuiper Objects, said Dr Mark Bailey, director of the Armagh Observatory. While it does seem that Kuiper did not rely on Edgeworth's work, his earlier papers are documented and they do predate Kuiper. It shows that Ireland's size does not militate against its position in international research.