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In 2016, Caltech astronomers began testing a Planet Nine hypothesis to explain the eccentric and inclined orbits of distant dwarf planets such as Sedna, 2015 TG387, Niku, and 2012 VP113. The hypothesis stated that these objects are being shepherded by an undiscovered ice giant planet called "Planet 9" about 3.5 times larger and 10 times the mass of Earth.

Recently, an alternative hypothesis has surfaced suggesting that the Planet Nine object does not exist and that the orbits of said dwarf planets are being tugged by the collective mass of other, low-eccentricity objects. But this hypothesis also has it's downsides. The combined mass of the entire Kuiper belt of dwarf planets and asteroids is positively puny compared to the mass of Planet Nine, so it wouldn't be able to effect dwarf planets like Sedna.

But could it be both ways? The majority of these high-eccentricity plutoids are suspected to be in the inner Oort Cloud, a theorized ring of comets and possibly minor planets in the outskirts of the solar system. The combined mass of the inner Oort Cloud (or Hills Cloud) would be roughly 2.37 Earth masses if an estimate of 5 Earth masses is taken for the entire Oort Cloud. If the theorized Planet 9 was only about 7.63 times the mass of Earth, the gravitational pull of it and the Hills Cloud combined would be enough to change the orbits of Sedna-like objects.

This hypothesis more or less worked out in a Universe Sandbox 2 simulation I ran to test the theory, but would it actually be true, considering the inner Oort Cloud extends far beyond the orbit of Sedna?

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  • $\begingroup$ There is an update post on findplanetnine.com from early 2019 that suggests a mass correction if the hypothetical planet towards 5 earth masses. $\endgroup$
    – user31179
    Commented Dec 15, 2019 at 10:41
  • $\begingroup$ @ebv Considering the largest terrestrial planets clock in at around 7.5 earth masses, could this mean that the hypothetical planet might be a terrestrial planet and perhaps something like Barnard's Star b? $\endgroup$
    – Nirvana
    Commented Dec 16, 2019 at 0:53
  • $\begingroup$ Well, what unites planet 9 and barnard b is lack of confirmation of their existence :-) But if i understand the authors correctly, a super earth type planet is the candidate for planet 9. If it exists. $\endgroup$
    – user31179
    Commented Dec 16, 2019 at 15:07

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It could. As posted by the original authors on their blog pages findplanetnine.com it might be ~5 earth masses

https://arxiv.org/abs/1902.10103 and the answer to this thread

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