Chandra Wickramasinghe, director of the Buckingham Center for Astrobiology at the University of Buckingham, led an international team of researchers who argue a few hundred thousand billion free-floating Earth-sized planets may exist in the space between stars.
Writing in the journal Astrophysics and Space Science, they propose the planets originated in the early universe within a few million years of the big bang and make up most of the so-called missing mass of galaxies.
Around 750 exoplanets have been discovered since 1995, but all are planets orbiting stars, and very few, if any, have been deemed potential candidates for life, the researchers said.
Earlier studies in which the effects of gravitational "lensing" -- or disturbance of light from distant quasars by intervening planet-sized bodies were measured -- suggest the possibility of a much larger number of planets in our galaxy, they said.
Such planetary bodies could have crossed our inner solar system every 25 million years or so and during each transit picked up dust with components of the solar system's living cells, the scientists propose.
The free-floating planets could have the property of mixing the products of local biological evolution on a galaxy-wide scale, they argue.