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While we are struggling to find an hypothetical planet 9 to explain the unusual orbit of transnettunian objects, I was wondering if it could be just dark matter.

We know that the universe is mostly made of dark matter, since it interacts only with gravitational force and there is no evidence of another planet in our solar system, can't we just assume as more probable that these effects are caused by a cluster of dark matter?

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    $\begingroup$ we are struggling to find an hypothetical planet 9 to explain the unusual orbit of transnettunian objects Is this true? Can you point us to a source of information to support this claim? For evidence to the contrary, see en.wikipedia.org/wiki/… $\endgroup$ – Ben Crowell Sep 29 '19 at 20:20
  • $\begingroup$ en.wikipedia.org/wiki/Planet_Nine#Ongoing_searches. Search is still ongoing, but since we have already excluded the presence of another planed in most of sky, and it's very unlikely to find it due to new constraints we need to consider alternative hypothesis. I continue to read many articles, with any kind of theory, but I wonder why no one has ever consider to just explain with dark matter. $\endgroup$ – Stefano Balzarotti Sep 29 '19 at 20:39
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    $\begingroup$ I think you're misunderstanding the Wikipedia article. Batygin's analysis does not involve anomalous accelerations, it concerns statistical facts about the orbital parameters. The relevant paper seems to be arxiv.org/abs/1601.05438 $\endgroup$ – Ben Crowell Sep 29 '19 at 22:01
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Here is a link to an article from "News from Science" that conjectures that planet nine may actually be a small black hole with a large dark matter halo: https://www.sciencemag.org/news/2019/09/planet-nine-may-actually-be-black-hole?utm_campaign=news_daily_2019-09-27&et_rid=17519419&et_cid=3006922

So, yes that is a possibility. The link goes on to describe experimental searches underway for such an objct.

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