The KBOs Hubble found are each about 10 times larger than typical comets, but only about 1-2 percent of the size of Pluto.

The KBOs were detected through a dedicated Hubble observing programme by a New Horizons search team that was awarded telescope time for this purpose, US space agency said in a statement.

"This has been a very challenging search and it's great that in the end Hubble could accomplish a detection - one NASA mission helping another," said Alan Stern, principal investigator of the New Horizons mission.

The Kuiper Belt is a vast rim of primordial debris encircling our solar system and belongs to a unique class of solar system objects that has never been visited by spacecraft and which contains clues to the origin of our solar system.

Unlike asteroids, KBOs have not been heated by the sun and are thought to represent a pristine, well preserved deep-freeze sample of what the outer solar system was like following its birth 4.6 billion years ago.

The KBOs found in the Hubble data are thought to be the building blocks of dwarf planets such as Pluto, noted the statement.

Though the New Horizons team started to look for suitable KBOs in 2011 and found several KBOs, none was reachable within the fuel supply available aboard the New Horizons spacecraft.

"We started to get worried that we could not find anything suitable, even with Hubble, but in the end the space telescope came to the rescue," said John Spencer, a member of New Horizons science team.

The New Horizons spacecraft mission is the first in NASA's New Frontiers Programme.

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