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I got some interesting answers for What would happen if a rogue planet hit one of the planets in our Solar System?

But I have seen some documentaries that state that rogue planets from other planetary systems (which are ejected from the planetary systems in which they formed) have become part of the other planetary systems and orbit around those new stars as planets.

For example, please check this link:

The latest research suggests that sometimes, these rogue, nomadic worlds can find a new home by being captured into orbit around other stars.

How is this possible? Stars' gravity can pull the rogue planet into them. How do rogue planets escape this and orbit around the new stars. This seems to be a bit mysterious.

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    $\begingroup$ Here is an accessible explanation of how the Solar system might have ejected a rouge planet. Basically, if Jupiter moves inwards, some other mass must move outwards. And Sedna is actually a captured rouge planet. $\endgroup$
    – LocalFluff
    Oct 21, 2015 at 3:46
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    $\begingroup$ @LocalFluff - That Sedna is a captured rogue planet is a conjecture; you stated it as if it is fact. Don't take everything published scientifically as truth. The scientific peer reviewed media is where science starts, not ends. $\endgroup$
    – David Hammen
    Oct 21, 2015 at 7:17
  • $\begingroup$ Ejecting a planet can happen via gravity assist. We use gravity assists to accelerate our space crafts but they can happen naturally and with larger objects. Generally it takes one large planet (Jupiter sized) and a smaller planet can get caught in Jupiter's gravity and fall towards it, but basically fly around it, and if the angle is just right the smaller can get ejected from the solar system. It's rare, but young solar systems can have lots of young planets during formation so it can and probably does happen, on occasion, mostly with young solar-systems. $\endgroup$
    – userLTK
    Oct 21, 2015 at 8:37
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    $\begingroup$ @LocalFluff, that's a tough one. Too many variables for me to guess with any accuracy. It would depend on how large the nearby stars were, how close and angle and velocity of escape for the Rogue planets. If stars are a few light years apart, the rogue would need to fly awfully close to just the right angle for capture to happen so My guess is that only a small percentage of Rogue planets would be captured by nearby stars forming around the same time, but that's just a guess. $\endgroup$
    – userLTK
    Oct 21, 2015 at 9:10
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    $\begingroup$ @userLTK Few percents sounds big. Sounds like a fun topic in vogue for someone to research, I suppose no one has. Young stars playing ping pong and trading planets with each other. $\endgroup$
    – LocalFluff
    Oct 21, 2015 at 11:56

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Capture of any object is never common. A rogue planet that passes a star will be accelerated by the star's gravity, and provided that it doesn't hit anything will pass the star in a hyperbolic path.

For capture to occur, the rogue planet has to lose momentum, and there are a few ways in which this can happen. The most general way is for the rogue planet to interact with one of the stars existing planets. If the rogue planet passes close one of the star's orbiting planets, it can interact gravitationally, and transfer momentum to the planet, and slow down in the process. This is rather like the reverse of a gravitational slingshot.

It is also possible that a double planet can be captured, if one of the bodies transfers momentum to the other. One of the double planet will escape, the other is captured by the star's gravity.

Capture of rogue planets is very rare. There is no evidence that it has occurred in our solar system (never in 4.7 billion years is a big Never) though some have speculated that Sedna might be a captured object. But we know that captures like this are possible, as Jupiter and Saturn have a collection of moons, some of which seem to have been captured from the asteroid belt.

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There are a few ways in which this can happen.

One of them is by a supernova explosion. A star explodes pushing out the planets who were once orbiting this into free space, going too fast for recapture by other stars. Some planets closer to the star would obviously be decimated but the outer ones could survive the blast(as said here by UserLTK).

Another is that a space object with enough velocity and mass to knock out a planet from its orbit into space. Such as if the originally solar system has too much planets, planet to planet interaction could send one if not both out of the system.

A rare occasion for this way of separating planets by their host star is by a black hole. There is current 16 (as far as I could remember) stars travelling at hyper speeds through the galaxy because it came too close to the black hole at the center of the milky way, then spinning outwards at incredible speeds and then the velocity finally comes over the blackhole's gravitational pull and shoots out. The speeds in which they come out is about 7-10 million miles per hour and the top speed achievable is at 30 million miles per hour. That's about crossing earth's diameter in about 10 seconds.

As LocalFluff said and UserLTK's contribution to it in the comment above, that's one of the ways too for a planet to be ejected out of the system. Link for that: http://blogs.discovermagazine.com/badastronomy/2011/11/16/did-jupiter-toss-a-giant-planet-out-of-the-solar-system/#.Vicx5aQx5mo

Here's some good sites for answering your question:

http://www.space.com/15308-rogue-alien-planets-billions-stars.html http://www.space.com/15023-warp-speed-planets-light-speed.html

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    $\begingroup$ Thank you, i went thru the links. They explain how the star is ejected from its own system. But it doesn’t answer my basic question, How they orbit around alien star once it is ejected from its Parent star? $\endgroup$
    – user3278897
    Oct 22, 2015 at 0:46

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