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Given that we've discovered hundreds of intergalactic stars (IGs), and that most stars have planets, what do we know about the likelihood of IGs retaining their planets after being ejected from their galaxy?

Related to that, what are the chances of rogue planets being intergalactic as well? If there's a planet floating out in intergalactic space, is it more likely to be orbiting a star, or on it's own?

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  • $\begingroup$ Whatever processes could fling a star out of the galaxy could probably do the same to a planet. I'd be curious to see if the answer differs between a rogue planet or an ejected star with a planet. $\endgroup$ – user10106 Mar 13 '18 at 8:33
  • $\begingroup$ Why would you think otherwise? "fling" is relative; the gravitational force between star and planet is much greater than the external force which ejected the star +planet(s) from a galaxy. $\endgroup$ – Carl Witthoft Mar 13 '18 at 14:17
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While near encounters with supermassive black holes are bad for the stability of solar systems [citation needed?], many intergalactic stars become intergalactic without a close encounter. They are part of streams and tidal tails that emerge when galaxies or globular clusters get tidally disrupted by another galaxy.

Still, even while encounters with black holes are usually disruptive simulations suggest that some planets may remain around the star. This is most likely for planets real close to the star, like hot jupiters.

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