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Other than the hypothetical Planet IX, is it likely if there's a (dwarf) planet more massive than Eris orbiting between Eris and Planet IX? Michael Brown seems to have found it likely. As far as we know about the many Kuiper-belt-and-beyond objects now, can we exclude a more massive body beyond Eris or is it possible that there's such massive body at, let's say, 150 au from the Sun?

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  • $\begingroup$ There is a significant amount of debate on this issue and a lot of very smart people on either side in the planetary science community. Numerous people on those sides also have certain personal biases, which make it really hard even for those of us (like me) in planetary science to really figure out where most evidence is right now. In my personal opinion (hence a comment rather than an answer), there are way too many potential biases in astronomical surveys for the kind of evidence that Mike Brown points to, to be definitive. I don't think it's likely, I don't think it's unlikely. $\endgroup$ – Stuart Robbins Aug 17 '20 at 3:58
  • $\begingroup$ @StuartRobbins In the meantime we found many more TNOs like FarOut and FarFarOut, so based on these discoveries respectively the bodies' orbital parameters, I wonder whether it has become more likely or more unlikely for a body at, let's say, twice the mass of Eris to exist beyond Eris' aphelion. I think we can exclude the existence of a Pluto-/Eris-like body within Eris' orbit, right? $\endgroup$ – Giovanni Aug 17 '20 at 5:44
  • $\begingroup$ Yes, we can pretty certainly exclude large Eris/Pluto-sized bodies within their orbits at this point, again due to surveys and probably orbital mechanics (not entirely sure on that one). The "Planet 9" proposed by Brown & colleagues is proposed to be MUCH farther out. $\endgroup$ – Stuart Robbins Aug 17 '20 at 19:54
  • $\begingroup$ @StuartRobbins Right, its perihelion is estimated at 200 au, more than twice as far as Eris' aphelion at 97 au. So the question now is whether there might be a Pluto-/Eris-mass body between 97 and 200 au from the Sun. $\endgroup$ – Giovanni Aug 18 '20 at 6:14

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