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So recently it was announced that there might be a ninth planet out there.

Some people (from NASA) say the odds are 50% that it is a real planet, since it hasn't been discovered by telescopes yet.

Let's say there is no ninth planet. Is there another explanation for those orbits that require a big mass (8 times earth mass) out there?

Could it even be a very small black hole (a few centimeters in radius) or maybe a clump of dark matter?

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    $\begingroup$ I'm not sure enough to state this as an answer, but the perturbations needed to affect orbits might be so small that it could be caused by things like out-gassing, non-uniformities in the heliosphere environment, or hitherto unstudied effects from proximity to interstellar space (i.e., shifts in stellar wind, etc.). The simple fact is that there needs to be proof that there is a planet out there -- Neptune was predicted similarly but not accepted until confirmed by observation! $\endgroup$ – Brian Lynch Jan 22 '16 at 2:19
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What else could it be? Very simply, it could all be a coincidence...

What they have done is noted that objects in the outer solar system have their orbits grouped in interesting ways. Someone suggested this might indicate there was a heavy planet (or core of a giant protoplanet) that influenced their orbits in this way; the recent announcement is the result of examining this idea in detail. It is likely to be caused by gravitational influence of such a body.

Of course the TV and newspapers have picked this up and said a planet HAS been discovered. But there might be nothing at all; the loose clustering could all be a coincidence - though that seems, statistically, unlikely at the moment.

Here is the paper for anyone who wants to check. (Beware, lots of mathematical terminology.)

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The introduction of the paper mentions some alternative interpretations put forward in earlier papers. It seems as if this "problem" has been noted earlier. This new solution to the problem beats earlier solutions. But maybe it is an evolving discovery process which again will invent new better explanations?

In addition to those mentioned in the paper, I suggest the following candidate alternatives:

-) Too few observations in order to hold up in a soon dramatically increased discovery rate. 4 or 5 or maybe even 8 TNOs which, as they claim, give a 0.007% significance level is certainly resting on some assumptions which will be challenged if thousands of similar objects are found in the next decade. So, as Andy has answered: possibly a coincidence due to small sample. (The paper's 0.007% probability is, well, we'll see)

-) Weird unthought-of science bias which, in math more than in lens, tends to select inadvertently among the (candidate) observations made and their characteristics. Since the authors are at the top in their fields, and Michael Brown has discovered loads of distant objects, including Sedna, because of that maybe one could suspect that some one thought has created a bias somehow.

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