Writing in Nature, they say they have found 10 Jupiter-sized objects which they could not connect to any solar system. They also believe such objects could be as common as stars are throughout the Milky Way.
The objects revealed themselves by bending the light of more distant stars, an effect called "gravitational microlensing".
Objects of large enough mass can bend light, as Albert Einstein predicted. If a large object passes in front of a more distant background star, it may act as a lens, bending and distorting the light of that star so that it may appear to brighten significantly.
The researchers examined data collected from microlensing surveys of what is called the Galactic Bulge, the central area of our own Milky Way.
They detected evidence of 10 Jupiter-sized objects with no parent star found within 10 Astronomical Units (AU). One AU is equivalent to the distance between our Earth and Sun. Further analysis led them to the conclusion that most of these objects did not have parent stars.
'Common' objectsBased on the number of such bodies in the area surveyed, the astronomers then extrapolated that such objects could be extremely common.
They calculated that they could be almost twice as common as "main-sequence stars" - such as our own Sun - which are still burning through their hydrogen fuel stock.
Co-author Takahiro Sumi, an associate professor at Osaka University in Japan, said these free-floating planets were "very common, as common as a regular star".
The "rogue" planets act as lenses, bending the light from distant stars "The existence of free-floating planets like this is expected from planetary formation theory. What is surprising is how common they seem to be."
According to astronomical convention, planets orbit a star or stellar remnant, so if these objects do not have a host star, then they are not technically planets, even if they may have formed in the same way as what we call planets.
Indeed, the researchers hypothesise these objects were formed in a planetary disc, like the planets in our own Solar System, before gravitational forces ejected them from these systems.
Professor Joachim Wambsganss of the University of Heidelberg in Germany, who reviewed the study for Nature, said this was the "most plausible theory". However, he added there was a minority view that planets could form the same way that stars do, but fail to reach the critical point of thermonuclear ignition.
He too agreed the most "shocking" element of the data was the projected frequency of such objects.
Dr Martin Dominik of the University of St Andrews in Scotland agreed, and said he would be "a bit cautious" about the results.
"There is this theory that planets formed around a star and due to the gravitational effects between planets, one of them gets ejected from the system, so people have predicted that there are planets out there that are no longer bound to stars," he said.
"But they don't predict this number of them."
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