31st March 1998


UK astronomers have discovered two of the faintest objects ever seen orbiting our Sun. One is estimated to be 150 km (90 miles) across and the other 110 km (70 miles). Both are about 45 times farther from the Sun than Earth (4,200 million miles or 6,750 million km), and more remote than the planet Pluto, which is currently 30 times farther from the Sun than Earth. The objects belong to the so-called Kuiper Belt in the region of space beyond the planet Neptune. Since 1992, 61 Kuiper Belt objects ('KBOs') have been discovered, including 7 found by the same UK team. Most are over 200 km across.

But rather like the curious incident of the dog that famously did nothing in the night-time [in the Memoirs of Sherlock Holmes] it is the fact that the team DID NOT find more objects that may prove to be a highly significant clue as astronomers probe the mysteries of these outer reaches of the solar system. Based on present ideas about how Kuiper Belt objects formed, astronomers expected to be finding these faint objects at even greater distances. Since they did not, those ideas may need to be revised. It may be that the average size of the KBOs is smaller the farther away they are, so the most distant ones were too faint even for this survey. Or it might be that the objects actually discovered mark the outer edge of the Kuiper Belt.

These latest results will be presented at the National Astronomy Meeting at the University of St Andrews on Tuesday 31st March by Dr Alan Fitzsimmons of Queen's University Belfast on behalf the team, which also includes Miss Edel Fletcher (Queen's University of Belfast), Dr Mike Irwin (Royal Greenwich Observatory) and Professor Iwan Williams (Queen Mary & Westfield College London).

Hardly any previous searches have been targeted on objects as faint and as small as those the UK team were looking for when they made the observations in November 1997. They used the 2.5-metre Isaac Newton Telescope on La Palma to image the sky for 7 nights, searching a total area slightly smaller than that covered by the full Moon. On the telescope they had a new, sensitive, wide-field camera built by the Royal Greenwich Observatory. The new camera can see five times as much sky as the one previously used.

During each night they stared continuously at different patches of sky for up to four hours at a time. In each patch of sky several thousand distant stars and galaxies could be seen. However even these images were not sensitive enough to record the solar system objects the team were seeking. So they combined the images by computer in a way that eliminated all stars, galaxies and nearby asteroids and revealed only faint solar-system objects at large distances from the Sun.

Two new objects were discovered. One is roughly 150 km in diameter, while the other is only 110 km across. Both appear to be around 45 times further from the Sun than the Earth. At this distance, the smallest object was forty million times fainter than the faintest stars that can be seen by eye on a dark night. They are so faint that it is not worth taking up all the telescope time needed to track them in the future, and it is very unlikely they will ever be seen again.

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