Astronomers in Colombia say they've observed a comet graveyard where some, inactive for millions of years, can return to life as "Lazarus comets."

In the last decade, 12 active comets have been discovered in the main asteroid belt region between Mars and Jupiter, leading a team at the University of Anitoquia in Medellin to set out to investigate their origin.

Millions of years ago, the main belt was populated by thousands of active comets, the researchers said, but this population aged and the activity subsided.

"We found a graveyard of comets," astronomer Ignacio Ferrin said.

"Imagine all these asteroids going around the sun for eons, with no hint of activity. We have found that some of these are not dead rocks after all, but are dormant comets that may yet come back to life if the energy that they receive from the sun increases by a few percent."

Twelve of them are true comets that were rejuvenated after their minimum distance from the sun was reduced a little after they were nudged by the gravity of Jupiter.

The little extra energy they received from the sun was sufficient to revive them from the graveyard, the researchers said.

"These objects are the 'Lazarus comets,' returning to life after being dormant for thousands or even millions of years," Ferrin said.

"Potentially any one of the many thousands of their quiet neighbors could do the same thing."