By Dr David Whitehouse
BBC News Online science editor
The Hubble Space Telescope (HST) has detected the smallest objects ever seen orbiting beyond the distant planet Neptune.
Small, distant and cold
The three objects, which are just a few tens of miles across, are relics from the formation of the Solar System.
The icy bodies become comets if they approach the Sun where its heat turns their ice into a billowing tail of gas.
But astronomers are puzzled as there seem too few distant chunks of ice and rock to account for the number of comets seen orbiting the Sun today.
Born of ice and dust
The planets formed over four billion years ago from a cloud of gas and dust that surrounded the nascent Sun.
Tugged by gravity, the fragments of ice and dust stuck together to form lumps that grew from pebbles to boulders to city- or continent-sized so-called planetesimals.
Around 1950, astronomers Gerard Kuiper and Kenneth Edgeworth proposed that in the region beyond Neptune there are no planets capable of dispersing leftover planetesimals.
They postulated that there should be a zone - now called the Kuiper Belt - filled with small, icy bodies.
Despite many years of searching, the first such object was not found until 1992. Since then, astronomers have discovered nearly 1,000 from ground-based telescopes.
Astronomers estimate that if collected together into a single planet, the resulting mass would only be a few times bigger than Pluto, the Solar System's tiny outermost world.
'Difficult to understand'
The most recent search detected three small objects named 2003 BF91, 2003 BG91, and 2003 BH91, which range in size from 15-28 miles (25-45 km) across.
The study's big surprise is that so few Kuiper Belt objects were discovered. Given Hubble's abilities, astronomers had expected to find at least 60 Kuiper Belt members as small as 10 miles (15 km) in diameter - but only three were discovered.
"Discovering many fewer Kuiper Belt objects than was predicted makes it difficult to understand how so many comets appear near Earth, since many comets were thought to originate in the Kuiper Belt," says Gary Bernstein of the University of Pennsylvania, US.
"This is a sign that perhaps the smaller planetesimals have been shattered into dust by colliding with each other over the past few billion years."
The new Hubble observations, combined with ground-based Kuiper Belt surveys, reinforce the view that Pluto and its moon Charon are just large Kuiper Belt members.
Why the Kuiper Belt planetesimals did not form a larger planet, and why there are fewer small planetesimals than expected, are questions that will be addressed by future studies.