So imagine, if you will, a star. Just speeding through space, like stars do. Shades on, listening to its stellar iPod (…I feel there’s a music of the spheres joke in there somewhere, but it would take a sharper wit than mine to make it happen). You know, a typical cruise through the universe.
When lo, by the side of the hyperspace bypass (sorry, Arthur Dent), there’s a figure. Dark. Mysterious. The star is intrigued, so it offers the stranger a lift.
Galleons, that star has just picked up a space rogue.
No, not a sexy space rogue. A rogue planet.
How does a planet become a rogue? Well, as a star system is forming, the matter surrounding the star often interacts. This is usually the cause of craters and planet-shattering in world formation, but sometimes, when two planets interact, one can be tossed out of the system completely.
And then it goes rogue, roaming the star cluster alone, free from the pull of a mother star.
However, a rogue planet can be picked up by a passing star going the same direction and speed. Even when in a star system again, however, the rogue planet remains a little wonky. It hangs back from the other planets in an extremely far orbit, which is often tilted. It may even revolve backward, just to fuck with everyone.
Or, at least, that’s what we theorize. See, we’ve never seen one of these rogue planets in a star system. Or, at least, we can’t definitively say we have. Because interactions between planets within a star system can cause wide, irregular orbits, giving us a slew of imposters to wade through.
However, there is a piece of evidence that points to the idea that rogue planets can be picked up and tacked onto existing star systems. And that piece of evidence is a double-planet system.
Now, hold up. It isn’t what you think. This isn’t two planets orbiting a single star… this is two planets, sans star, orbiting each other. Two rogue planets who found each other and formed their own little binary system. Like a little interstellar romance.
This double-planet system is the prime bit of evidence pointing toward rogue planet capture. And, while it is poetically sweet and scientifically interesting… is this lone binary system the only method we have of determining whether rogue planets can be captured again?
Of course not. But, as mentioned previously, it is difficult to separate real captured rogues from imposters (though exhaustive research of many, many star systems could give us a greater understanding of how to distinguish the two). A great piece of evidence would be to find a planet in a far orbit around a low-mass star. The star’s disk couldn’t have been large enough to form a planet that far out, so that distant planet would basically have to be a captured rogue.
According to recent research, the capture of rogue planets could be more common than one would think. Assuming there are as many rogue planets as there are stars (highly likely), then 3-6% of the stars would pick up a rogue over time. It would still be a rare event, but perhaps not as rare as we once believed. Particularly in young star clusters, where stars and rogue planets would be crowded into a smaller area (making pick-up interactions more likely).
Who knows- maybe one day, our own star will pull in a loner drifting through space. Then Sol would be back up to nine planets and could feel less shitty about itself (it still hasn’t recovered from the HD 10180 situation).