Much interest has been raised about the existence of a 9th planet. One reason for this is the strange orbits of bodies in the outer solar system. Could the orbits be explained by a near collision of a passing black hole?

Recent work is looking to see if the 9th planet could be a small black hole. But my question is different. Could a passing black hole, say a million years ago, have thrown the objects in the outer solar system into the strange orbits they exhibit?


To quote from Batygin and Brown (2016)

Trujillo & Sheppard (2014) point out that the Kozai mechanism allows libration about both ω = 0 as well as ω = 180, and the lack of ω ~ 180 objects suggests that some additional process originally caused the objects to obtain ω ~ 0. To this end, they invoke a strong stellar encounter to generate the desired configuration. Recent work (Jílková et al. 2015) shows how such an encounter could, in principle, lead to initial conditions that would be compatible with this narrative. Perhaps a greater difficulty lies in that the dynamical effects of such a massive perturber might have already been visible in the inner solar system.

So the short answer is that it is unlikely a single encounter with a massive object could cause the perturbations without leaving other traces in the dynamics of the solar system. Furthermore, stellar encounters are rare enough as it is - encounters with black holes should be even rarer since stellar evolution and the critical mass requirement of black hole formation dictate there should be far fewer black holes than stars in existence.

To me that means favoring an encounter with a black hole to be responsible for the "unexpected clustering in their respective arguments of perihelion" of Kuiper belt objects over other theories would be a violation of Ockham's Razor.

All that being said, due to the chaotic nature of orbital dynamics and the relatively short Lyapunov time of Mercury in particular, it is very difficult to rule something like that out. It may be the case that an encounter occurred long enough ago that the only remnant is in the perturbations of long-period Kuiper Belt objects. And that all other traces of its influence on the inner solar system have been obscured by its characteristic emptiness - it could in fact have even been the cause of that unusual emptiness.

It should be noted of course that this would not have been a 'small black hole' (also sometimes referred to as micro black holes) as has recently been theorized in place of a 9th planet, but a stellar black hole (roughly 5 or more times as massive as our sun).

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  • $\begingroup$ i consider a black hole of size 5 times our sun to be a small black hole. $\endgroup$ – jmh Jul 13 at 14:14
  • $\begingroup$ and I don't consider this a violation of Ockham's Razor. The addition of a 9 th planet is just as difficult to explain as a past brief encounter with a black hole. $\endgroup$ – jmh Jul 13 at 14:18
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    $\begingroup$ @jmh Of course a 5 solar mass black hole would have to be a small stellar black hole because that is roughly the lower limit for stellar black holes - but smaller black holes are possible through other creation mechanisms. I made the note of the distinction because you mention the alternative theory that planet 9 could be instead "a small black hole" (one with roughly 5-10 Earth masses). Such a black hole would have the orbit of the theoretical planet 9 but be much much more difficult to detect. The encounter in question would of course have to be with a stellar black hole instead $\endgroup$ – William Miller Jul 13 at 18:54
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    $\begingroup$ Thanks for your input and answer. $\endgroup$ – jmh Jul 13 at 19:06
  • $\begingroup$ @jmh Of course, certainly an interesting question. A final note: I only consider favoring a black hole encounter as a theory to violate Ockham's razor. Since the existence of a planet has more dynamical evidence and because there is dynamical evidence missing which would bely the past encounter it is not 'the simplest answer which satisfies the evidence.' I should also note that replacing planet 9 with a 5-10 Earth mass black hole is even more of a violation because we don't even know if black holes of that mass actually still exist - they are purely theoretical. $\endgroup$ – William Miller Jul 13 at 19:13

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