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Is it possible that this planet changed other planet orbits (from which we assume that it exists) but then was thrown out of the Solar System in past when Sun was close to some other star? And it doesn't exist in our system anymore and we can't find it because of it.

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  • $\begingroup$ @ProfRob Nope. That question about moving Planet 9 from close to sun to far to sun. My question: "Is it possible that Planet 9 not exist anymore in Solar System?" $\endgroup$
    – Robotex
    Feb 15 at 14:36
  • $\begingroup$ @Robotex and did you read the answer? Where's the significant difference to you between "far out and very loosely bound" and "ejected"? $\endgroup$ Feb 15 at 14:50
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    $\begingroup$ There is almost no difference at all between very far out and ejected. Indeed the problem would be to fine tune a mechanism that just avoided ejection. $\endgroup$
    – ProfRob
    Feb 15 at 20:12
  • $\begingroup$ @ProfRob So, we can never to find it bacause it can be ejected, right? $\endgroup$
    – Robotex
    Feb 18 at 13:16

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By definition Planet 9 is in the Solar System. Planet 9 is a hypothetical body, and we suspect that it might exist because of the motion of other bodies in the Solar System.

If a planet used to be in the Solar System but then got ejected, then that planet will no longer interact with the other objects in the Solar System. So it won't explain what we see happening in the Solar System.

It is not impossible that other planets existed in the Solar System and got ejected, but those hypothetical planets are different from the hypothetical object that is generally called "Planet 9". If it's not gravitationally bound, then it's not planet 9.

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  • $\begingroup$ The ejection of another planet from the Solar System almost certainly would have had consequences for the present-day solar system configuration. Planet 9 could refer to an ejected planet. $\endgroup$
    – ProfRob
    Feb 16 at 8:34
  • $\begingroup$ "but then got ejected, then that planet will no longer interact with the other objects in the Solar System. So it won't explain what we see happening in the Solar System." -- but we can see the result of interactions in past $\endgroup$
    – Robotex
    Feb 18 at 13:16
  • $\begingroup$ @Robotex "Planet 9" is the name we give to a hypothetical body that might explain a given set of things we observe in our solar system. If an object doesn't explain these things, then it is not "Planet 9". Other hypothetical bodies have been given other names. $\endgroup$
    – usernumber
    Feb 22 at 16:42
  • $\begingroup$ @usernumber I mean that that object can explain these thing but not exist anymore. Imagine, you walked on sand on beach and then go out, but you steps still on the sand $\endgroup$
    – Robotex
    Feb 23 at 17:23
  • $\begingroup$ @Robotex We're talking about dynamical effects here. If the orbit of a planet isn't explained by general relativity, that can mean one of two things: either there is something that is still tugging at it (so not a body that was ejected), or we need a new model to describe gravity (so not a body at all). $\endgroup$
    – usernumber
    Feb 24 at 9:54

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