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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... :-) $\endgroup$ – Chappo Hasn't Forgotten Monica Feb 11 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 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 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 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 at 16:41

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