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After Pluto's demotion as a planet, we have currently eight planets in our solar system. But Sun's gravitational pull can be felt well beyond Pluto, so is it possible to have a ninth planet beyond Pluto? I asked this question because we have Kuiper belt beyond Pluto, so there might be some Kuiper belt object which may qualified to be a ninth planet, isn't it possible?

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There could well be, since when we had nine planets we were looking for planet X (planet number 10). However, the modern day designation of a planet is: "is massive enough for its own gravity to make it roughly spherical (slight "oblate-ness"), and has "cleared its neighbourhood" of smaller objects around its orbit". Hence, we would more than likely discard anything far out such as Pluto because it would belong to either the Kuiper belt or the Oort cloud. Anything further out than Neptune would likely be considered a Dwarf planet or proto-planet.

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    $\begingroup$ Addendum: There are 5 officially recognised dwarf planets in our solar system, they are Ceres, Pluto, Haumea, Makemake and Eris. So there are plenty of interesting objects in our Solar System other than the 8 planets. $\endgroup$ – MichaelJRoberts Nov 2 '15 at 15:40
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    $\begingroup$ If there were a body the size of, say, Neptune or larger in the far outer solar system that hadn't "cleared its neighborhood", I suspect it would prompt another update to the definition of "planet". $\endgroup$ – Keith Thompson Nov 2 '15 at 19:54
  • $\begingroup$ I'd imagine so too. But it was a nice way of excluding objects within the asteroid belt (i.e., Ceres) that could be considered to be spheroidal in shape. However, I would state that Ceres is a proto-planet in these sense that at some point very far in the future it would probably coalesce with its neighbourhood through multiple collisions. $\endgroup$ – MichaelJRoberts Nov 2 '15 at 20:34
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According to Caltech there is possibly a planet 10 times the mass of earth in a 15-20 thousand year orbit at an odd angle. It hasn't been observed yet. There's a search for it ongoing.

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    $\begingroup$ They've found evidence of it, but they haven't confirmed it. $\endgroup$ – Keith Thompson Jan 21 '16 at 2:46
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    $\begingroup$ There's potentially a planet. It is not certain yet. $\endgroup$ – Ellesedil Jan 21 '16 at 4:03

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