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Is it possible for a planet to be ejected from one star system and end up in the habitable zone of another? Or do captured planets always end up in wide, eccentric orbits? I guess in the vast expanse of the Universe, it seems like this should be possible but maybe very unlikely?

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A captured planet would enter the system with at least escape velocity.

Circularizing its orbit would require a series of carefully calibrated swing-byes.

So, yes, theoretically it's possible, but very difficult to happen.

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    $\begingroup$ Why would a rouge planet require more than escape velocity for our system? Of course, it needed the escape velocity for the system it came. $\endgroup$
    – NomadMaker
    Sep 9, 2023 at 23:08
  • $\begingroup$ Thanks for the answer! $\endgroup$
    – Elhammo
    Sep 10, 2023 at 0:55
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    $\begingroup$ @NomadMaker: The planet's trajectory relative to the new star can be hyperbolic, parabolic, or elliptic. It can't be elliptic since it doesn't start out gravitationally bound to the new star. Therefore the trajectory must be hyperbolic or parabolic, either of which implies it enters the system with at least escape velocity. (To be captured, it has to change this trajectory by dumping some of its momentum -- typically on another planet, but very minor changes could be made via tidal forces and/or interactions with more distant stars.) $\endgroup$ Sep 10, 2023 at 1:27
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For reference, there is an hypothesis according to which Venus is a captured planet. Evidence for this is her weird angular momentum (spins so that the Sun rises on the west, one sol lasts 224 Earth days). Do notice that current scientif consensus is that Venus was not captured, but rather spins retrograde due to her strong winds acting on her surface over geological eras. Her orbit is less eccentric than Earth's and she's almost in the habitable zone.

It may have been just luck of the draw that put Earth and Mars in the goldilock zone around the Sun but not venus. There is even another idea that every planet in our solar system formed in a different orbit and got moved to some other orbit, so...

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    $\begingroup$ Velikovsky is probably the first to propose the Venus capture idea. Gripping readng though it may turn out not to be true. You can read it online at the Internet Archive Lending Library. >>>> archive . org/details/worldsincollisio0000imma_b5d5 $\endgroup$
    – KalleMP
    Sep 10, 2023 at 14:02

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