New York: April 30, 2009 -  Hundreds of gigantic black holes, leftovers from the galaxy-building days of the early universe, may wander into the Milky Way. These black holes may threaten to swallow anything that gets too close, according to the latest study.

But the good news is that our planet is safe. The closest rogue black hole should reside thousands of light-years away. Astronomers are eager to locate them though, for the clues they will provide to the formation of the Milky Way.

"These black holes are relics of the Milky Way's past," said Avi Loeb, Harvard-Smithsonian Centre for Astrophysics, the study co-author. "You could say that we are archaeologists studying those relics to learn about our galaxy's history and the formation history of black holes in the early universe."

According to theory, rogue black holes originally lurked at the centres of tiny, low-mass galaxies. Over billions of years, those dwarf galaxies smashed together to form full-sized galaxies like the Milky Way.

Each time two proto-galaxies with central black holes collided, their black holes merged to form a single, "relic" black hole. During the merger, directional emission of gravitational radiation would cause the black hole to recoil.

A typical kick would send the black hole speeding outward fast enough to escape its host dwarf galaxy, but not fast enough to leave the galactic neighbourhood completely. As a result, such black holes would still be around today in the outer reaches of the Milky Way halo.

Hundreds of rogue black holes should be travelling the Milky Way's outskirts, each having a mass of 1,000 to 100,000 suns. They would be difficult to spot on their own because a black hole is visible only when it is swallowing matter.

These findings will be published in the Monthly Notices of the Royal Astronomical Society.