Dr Alan Boss of the Carnegie Institution of Science said many of these worlds could be inhabited by simple lifeforms.
He was speaking at the annual meeting of the American Association for the Advancement of Science in Chicago.
So far, telescopes have been able to detect just over 300 planets outside our Solar System.
Very few of these would be capable of supporting life, however. Most are gas giants like our Jupiter; and many orbit so close to their parent stars that any microbes would have to survive roasting temperatures.
But, based on the limited numbers of planets found so far, Dr Boss has estimated that each Sun-like star has on average one “Earth-like” planet.
This simple calculation means there would be huge numbers capable of supporting life.
“Not only are they probably habitable but they probably are also going to be inhabited,” Dr Boss told BBC News. “But I think that most likely the nearby ‘Earths’ are going to be inhabited with things which are perhaps more common to what Earth was like three or four billion years ago.” That means bacterial lifeforms.
Dr Boss estimates that Nasa’s Kepler mission, due for launch in March, should begin finding some of these Earth-like planets within the next few years.
Recent work at Edinburgh University tried to quantify how many intelligent civilisations might be out there. The research suggested there could be thousands of them.