We now estimate that every star in the galaxy has at least one planet, but that is leaving aside the potentially billions more planets that were ejected from their solar system and are now hurtling through the universe all alone.
We know that these rogue planets exist - indeed, they could outnumber all the other planets in the galaxy by a factor of two or three to one, and our own solar system possibly once had a fifth gas planet that went walkabout. The question, then, is why all these planets form around stars and then up and leave their home solar systems. The most common explanation had been that their orbits became gravitationally unstable, and while that's likely still a part of the story, some rather more unusual possibilities are now being considered, thanks to some nifty new computer simulations by researchers at Cambridge and the University of Bordeaux. ScienceNOW has the story:
One possibility is stars literally pushing the planets into interstellar space after the suns reach the end of their normal hydrogen-burning lives and begin expanding into red giants. Other scenarios involve gravitational perturbations, either caused by passing stars, a solar system entering and exiting our galaxy's gravitationally dense spiral arms, or even via interactions with dense molecular clouds.
For more, check out ScienceNOW. The original paper, which will appear in Monthly Notices of the Royal Astronomical Society, is available here.
Artist's conception of rogue planet via NASA/JPL.