After spending an eternity rocketing through the interstellar void, rogue planets can sometimes end their travels and find new solar systems to call home. But these adoptive parent stars aren't all that welcoming of their new planets.
As illustrated in the artist's conception up top, these planets would likely take up orbit at distance that are at least 100 or even 1000 times that of the distance between Earth and the Sun (which is known as the astronomical unit or AU). All our solar system's planets are within about 30 AU, while the far reaches of the Kuiper Belt aren't much more than about 50 AU. These recaptured planets would then be frozen worlds, their new stars barely more than a speck in the dark.
Recent estimates have suggested there are as many rogue planets in the galaxy as there are stars, with these runaway planets the result of chaotic interaction between the planets of a newly formed solar system. Astrophysicists Hagai Perets of the Harvard-Smithsonian Center for Astrophysics and Thijs Kouwenhoven of Peking University ran with this estimate and built a computer simulation full of these free agent planets. They found that 3 to 6% of such planets ended up finding a new home, with more massive stars the most likely to adopt a planet.
We haven't yet found clear evidence of such a captured planet, in part because it's much easier to detect planets orbiting close around their star than it is right at the fringes. Indeed, it's still conceivably possible that there's a rogue planet right at the end of our own solar system that we just haven't seen yet, according to Perets, who says a large planet is impossible but there's still some chance a small planet is out there. These planets likely would have orbits tilted relative to the orbital plane of the other planets, with some potentially even orbiting their stars backwards.
Via CFA. Image by Christine Pulliam (CfA).