Astrophysicist Sean Raymond, for Aeon:

These planets don’t orbit stars. They wander the stars. They are free citizens of the galaxy. It might seem like the stuff of science fiction but several free-floating gas giants have been found in recent years. Our own gas giants, Jupiter and Saturn, are leashed to the Sun on well-behaved orbits, but this might not be the norm in our galaxy. One study, published in Nature in 2011, suggests that the Milky Way contains two rogue gas giants for every star. That particular study remains controversial, but most astronomers agree that rogue planets are common in our galactic neighbourhood. And for every rogue gas giant there are likely to be several rogue Earth-sized rocky worlds. There are likely tens to hundreds of billions of these planets in our galaxy.

It’s fascinating to consider that a so-called “icy rogue” might harbor some exotic form of dark-adapted life. We simply don’t know at this point. But the larger the set, the more that possible becomes probable — and there may be “tens to hundreds of billions of these planets in our galaxy” alone, says Raymond. But regardless, a nearby rogue might be useful to us for other reasons:

A rogue Earthlike planet could be among our closest galactic neighbours, and in that case colonisation could be worth the effort, because we could convert a rogue planet into a jumping-off point, a waystation in our larger effort to spread out into the galaxy.