But there is reason to believe that comets are what the Kuiper belt mostly consists of.
Renu Malhotra, at the Lunar and Planetary Institute in Houston, Texas, theorises that Pluto was and for that matter, still is part of the Kuiper belt.
The researchers say this dust must be coming from collisions among small bodies similar to the comets or icy bodies that make up today's Kuiper Belt objects in our solar system.
This would make sense if short-period comets started off in the Kuiper belt and were nudged into closer orbits around the sun by the gravity of the most distant of the giant planets, Neptune.
But the forces exerted by objects in the Kuiper belt (a collection of small bodies orbiting the sun beyond the orbit of Pluto), or by the galaxy as a whole, were also far too weak to be responsible.
In 1995 a team led by Anita Cochran of the University of Texas at Austin used the Hubble space telescope to find what it thinks were 30 comet-sized fragments, at a distance that would put them inside the Kuiper belt.
But most of the history of the solar system--this is why the comet is so important--most of the history of this comet was spent out beyond Neptune, out where Pluto is at the very edge of the solar system, what we call the Kuiper belt.
Mr. STERN: So a few weeks after the Pluto encounter when we've chosen our target, we'll fire the engines of the spacecraft, but we'll have to fly a distance something like from here to the sun or twice that far to reach these Kuiper Belt objects.
Long-period comets can arrive from any direction in space and are believed to originate in the Oort cloud (a reservoir of comets encircling the sun, the existence of which was worked out at the same time as the Kuiper belt by Jan Oort, a Dutch astronomer).
Alan Stern expects that after the Pluto flyby in 2015 or so, there will be plenty of power left, both electricity from the plutonium generator and fuel, so New Horizons will keep on going to explore other Plutolike objects in this region of the solar system which is known as the Kuiper Belt.
The Kuiper-belt bodies Dr Jewitt and Dr Luu have found range from 100km (60 miles) to 500km across.
Unlike the larger Kuiper-belt objects, comet-sized ones would be too faint to show up in a single photograph even one taken by the mighty Hubble.
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