Some astronomers have labeled these objects “rogue planets”, but the International Astronomers’ Union, (or the IAU) they’re the one’s who kicked Pluto out of being a planet by the way, have a much more conservative definition of a planet. They don’t consider anything that doesn’t orbit the sun, not any sun, specifically our sun, a planet. According to the IAU, if it has planet like characteristics and orbits another star then the body is and exo planet, not a planet, but if it doesn’t orbit any star, then they call that body a sub-brown dwarf, not because they’re star-like, just because they’re smaller than brown dwarfs.
Anyway, these rogue bodies, I’m going to call them rogue planets, (Take that, IAU!) are out there, and the more astronomers study them, the more they’re beginning to think that there are probably far more of these rogue planets out there than there are planets that orbit stars. So, if there are so many of these things, what are they, and where did they all come from?
Well, they’re planets, like the ones we have here in our solar system. The rogue planets astronomers have actually found have been gas giants, in the size range of Jupiter, or in some cases several times bigger, but astronomers think it’s pretty likely that there are lots of smaller, rocky, Earth-sized rogues out there too. They’re just much harder to spot, because they are comparatively so much smaller. As to where they come from, astronomers think there are two likely sources: One is that, apparently, as solar systems form it is quite common for them to throw off at least one planet. It seems that things are much more chaotic during these periods, and as planets form they sort of jockey for position, or to be more precise, orbit, and usually, no all of the newly formed planets make the cut. The ones that don’t are hurled off into space, to live out their existences as starless wanderers. The other way they occur, astronomers think, is that a solar system begins to form, but just, doesn’t have quite enough gravity or mass to complete the process. Some planets are formed, but there isn’t enough gravity to hold on to them, and so they begin their own journeys into the cosmos, alone.
(Source: Sciencenews.com)