The idea of free floating planets is not new. In this case, free floating means any planet that is not attached to a parent star, but instead drifts through space all by itself, doing who knows what.
It is though that our own solar system may have had another gas giant many years ago. Eventually, gravitational tides formed by the other gas giants conspired to usurp this planet, and eventually evict it from the solar system. We can't say where it is now, and we will probably never find it.
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While it is possible and even likely that there are swarms of wayward planets flying around out there, the idea that they may be spreading DNA or microbial life is not as certain. Statistically, it is more likely that life evolved once and has been spreading, rather than evolving separately several times. This presupposes that life exists elsewhere, of course.
The only way to know for certain is to find some extraterrestrial life and examine its DNA. If the genes are very similar to ours, then it is quite likely that life evolved once and spread. If the DNA is very different, then life probably evolved separately.
Therefore, in the name of science, I call on everyone to send in any alien life forms they may have captured. For example, I once found a beetle in my garage that I'm certain was not of this earth.