It seems that some people maintain the existence of a "planet X" is all but science fiction, while others claim that its existence is a possibility, even if that's relatively a remote one. Others take this further and make the argument that its existence is likely. Is there a compelling case that such a planet could exist, and if so how come it hasn't yet been discovered - is it a simple case of it being too far away, or is there more to it?

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    $\begingroup$ Are you asking for speculation on a 10th planet? I think the question would be better if you presented something that suggested a 10th planet (such as gravitation forces, or astronomical observations) and then maybe asked if there was merit in pursuing a further search. $\endgroup$ Jul 16, 2013 at 21:58
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    $\begingroup$ The sun's SOI is something like a light-year across, so it's possible to have a gas giant in orbit half a light-year away. I'm not absolutely sure how stable that is though. $\endgroup$
    – Joshua
    Oct 2, 2015 at 20:37
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    $\begingroup$ If anyone stumbles by this old question, closed as off topic though perhaps it could have been migrated, here is a pointer to new discussion elsewhere on stack exchange: astronomy.stackexchange.com/questions/13282/… $\endgroup$
    – Puffin
    Jan 21, 2016 at 21:25
  • $\begingroup$ First we need to find a ninth one. $\endgroup$ May 15, 2019 at 23:18

3 Answers 3


This topic is actually pretty nicely covered in the Planets beyond Neptune Wikipedia page, so I recommend reading it, if for nothing else then for a convenient collection of references. But to quote the most relevant part on Harrington's Planet X (beyond Pluto) of that Wiki page:

Planet X disproved
Harrington died in January 1993, without having found Planet X. Six months before, E. Myles Standish had used data from Voyager 2's 1989 flyby of Neptune, which had revised the planet's total mass downward by 0.5% — an amount comparable to the mass of Mars — to recalculate its gravitational effect on Uranus. When Neptune's newly determined mass was used in the Jet Propulsion Laboratory Developmental Ephemeris (JPL DE), the supposed discrepancies in the Uranian orbit, and with them the need for a Planet X, vanished. There are no discrepancies in the trajectories of any space probes such as Pioneer 10, Pioneer 11, Voyager 1 and Voyager 2 that can be attributed to the gravitational pull of a large undiscovered object in the outer Solar System. Today, most astronomers agree that Planet X, as Lowell defined it, does not exist.

  • $\begingroup$ It's crazy that you can spend the last moments of your life believing something that will be disproved in 6 months... time truly is against us. How unfortunate he didn't live to get the new data :(. $\endgroup$ Aug 18, 2018 at 4:50

No. A planet of significant mass would cause detectable perturbations in the orbits of the other planets.

This is how Neptune's existence was postulated. Deviations from predictions in Uranus' orbit suggested where an 8th planet must be located. A focused search in the projected area led to its discovery.

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    $\begingroup$ But Neptune was observed several times before its existence was postulated, see en.wikipedia.org/wiki/Discovery_of_Neptune $\endgroup$
    – Uwe
    Dec 19, 2016 at 15:18
  • $\begingroup$ With all due respect, 20 minutes watching he Kepler Orrery on YouTube will clearly show that this answer is completely false. We have found hundreds of exoplanets. They come in every possible arrangement. SuperJovian planets closer to their stars than Mercury are quite common. It is a marvel, watch it as soon as possible. In fact, small rocky worlds seem to be a bit scarce, although it's possible that we miss a few. youtube.com/watch?v=gnZVvYm6KKM $\endgroup$
    – chiggsy
    Jan 4, 2019 at 3:58

From basic physics if there is one it cannot be massive, or must be incredibly far away, as we would see the perturbations in the orbits of the other planets.

Additionally, if it was massive but very low density, for some reason, so we couldn't see it's effect on the orbits of the other planets, we would expect to see something with a regular orbit from astronomical observations.

From what we know of planetary distribution, massive planets are expected, well, about where Jupiter and Saturn sit, with smaller ones closer to the sun, and also smaller ones further out, with Uranus and Neptune the last of the gas giants.

  • $\begingroup$ What density has to do with that ? Only mass and distance matter $\endgroup$
    – Antzi
    Sep 19, 2020 at 13:22

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