The discovery of exoplanets in very strange orbits—for example, gas giants orbiting their host stars at distances much closer than Mercury is from our Sun—has shown that planets often migrate from the orbits where they formed. That means, in turn, that planets can often be flung out of their planetary systems by gravitational interactions with other migrating planets, as if they were in an interplanetary game of billiards. Although this has not happened in our own solar system for billions of years, the Copernican Principle suggests that, someday, our solar system could undergo such an upheaval as well.

If this planet-flinging scenario proves to be true, then there could be billions of rogue planets flying through interstellar space, free from the gravitational wells of the stars that birthed them. If such a planet had a thick crust and an underground liquid ocean, then tidal or geothermal processes deep in those planetary cores may be warming that ocean, creating a teeming ecosystem that is hurtling unfettered through the galaxy. Could such a planet fly through our own solar system someday? The odds are slim to none, but it is not impossible.