Free-flying planets, not anchored to a star, could outnumber stars in the Milky Way by 100,000 to one.
According to researchers at the Kavli Institute for Particle Astrophysics and Cosmology (KIPAC), the discovery could affect current theories of planet formation and could change our understanding of the origin and abundance of life.
"If any of these nomad planets are big enough to have a thick atmosphere, they could have trapped enough heat for bacterial life to exist," says team leader Louis Strigari.
Almost all of the more than 500 planets detected outside our solar system are orbiting stars. But a very few nomad planets have already been discovered using gravitational microlensing, leading astronomers to hypothesize that there were around twice as many nomad planets in our galaxy as stars.
The Kavli researchers, though, believe that nomads may actually be as much as 50,000 times more common than that.
They took into account the known gravitational pull of the Milky Way galaxy, the amount of matter available to make such objects and how that matter might be distributed.
There's room for error, to say the least, as there's still great uncertainty as to how nomad planets form. Some were probably ejected from solar systems, but research indicates that not all of them could have formed in this way.
But the Kavli team hopes that the space-based Wide-Field Infrared Survey Telescope and the ground-based Large Synoptic Survey Telescope, both set to begin operation in the early 2020s, could help to find the answer.
If there really are that many nomad planets, says the team, there's a distict possbility that they could be spreading the seeds of life through collisions.
"Few areas of science have excited as much popular and professional interest in recent times as the prevalence of life in the universe," says KIPAC director Roger Blandford.
"What is wonderful is that we can now start to address this question quantitatively by seeking more of these erstwhile planets and asteroids wandering through interstellar space, and then speculate about hitchhiking bugs."