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Since 2016 there are hints that a (probably) ice giant planet may exist far beyond the Kuiper belt. There are speculations that it may be a rogue planet captured by the Sun. But is it likely that there are even more undiscovered planets farther beyond, which also may be captured rogue planets or something?

I must clarify that the planet may be no captured rogue planet, it could also be a planet that was pushed into the outer system by the gas giants or so.

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    $\begingroup$ Yes of course it's "possible": why shouldn't it be? Perhaps you might like to edit your question to articulate what you think might prevent this possibility and ask what astronomical evidence might support your suspicion. Alternatively, you could ask what the current technical limitations are that prevent us from observing such objects if they do exist, and whether there are any technological advances on the horizon (e.g. new telescopes, etc) that might enable such observations in the future... :-) $\endgroup$ Feb 11, 2020 at 0:13
  • $\begingroup$ @ChappoSaysSEDuddedMonica In the meantime I've found another article in media which also states that there might be more planets beyond the currently postulated one. From my POV it is rather not so possible because it is so far from the Sun, so the Sun's gravity is much weaker out there. $\endgroup$
    – user30007
    Feb 11, 2020 at 11:44
  • $\begingroup$ Since one of the definitions of a planet is to clean out it's orbital region, one could argue that past a certain distance that criteria stops making sense because the orbit is too long and slow. At half a light year a single orbit takes over 5 million years. At about 1 light year the orbit becomes unstable and perhaps less than that if you factor in the occasional close pass by other stars. There's room for several planet sized objects in that vast range from the estimated several hundred AU for planet 9 to perhaps many thousands of AU, but room for doesn't mean planets are there. $\endgroup$
    – userLTK
    Feb 12, 2020 at 8:30
  • $\begingroup$ @userLTK The definition you're referring to is the one from 2006, selected by ~4% of the IAU. As you see, the definition is nonsense. If Mercury or Mars were in the Kuiper belt, the IAU couldn't accept them as planets either. However, since no planet actually "cleared its orbit", there would be no planets at all. Jupiter has to be a "dwarf planet" in their definition. So let's forget about their definition and hold to the facts; the postulated planet would be a planet (if it's no primordial black hole, in that case it would be a star). $\endgroup$
    – user30007
    Feb 12, 2020 at 11:24
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    $\begingroup$ @userLTK I hope that if the postulated planet is discovered (if it exists and if it is one) the debate on what is a planet will be renewed. $\endgroup$
    – user30007
    Feb 12, 2020 at 16:41

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As with so many things, it's complicated. Depending on your definition of a planet (and for the sake of argument, we'll just assume that the definition of a planet is a stellar body that roughly circularizes under it's own mass and has an orbit around a star). That being the case, the odds are good that there are many objects beyond the Kuiper belt that fit that criteria.

The gravitational influence of Sol extends very, very far into space. Much farther than most people think. The Oort cloud is between 50,000 and 125,000 AUs away from Sol, and some estimates suggest that the entire sphere of influence may be as large as 2 light years. This represents a huge, mostly unmapped area of space, and planets within this space would be very, very difficult to detect with current, planetary instruments (or even instruments in Earth's SOI).

Remember, both Neptune and Pluto were first posited from inferences suggested by mathematics and it took decades to confirm their presence. If there is a "Planet 9" beyond the KBO, then the gravitational perturbations in the Kuiper Belt would most likely be our first indications of its' presence, and since we don't really have a good map of the Kuiper Belt to begin with, it will still be quite awhile before this is a settled question.

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