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This article suggests that there might be a tennis ball-sized black hole orbiting the Sun somewhere beyond the orbit of Neptune. According to the article, observational anomalies of trans-Neptunian objects suggest that there is an object out there about 10 times the size of Earth (I assume it to mean about 10 Earth mass). Since searches have failed to find such an object, an alternative hypothesis claims that there may be a primordial miniature black hole out there. The article goes on to state that a black hole has not be found there because searches have been for planets and the methods used to find planets are different than those used to find black holes.

My question is: how would such an object (mini black hole) be detected, to either confirm or rule out its existence?

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  • $\begingroup$ Is it ten times the size of the earth or the size of a tennis ball? The difference is quite important. I guess you mean ten times the mass of the Earth. $\endgroup$ – Rob Jeffries Sep 29 '19 at 22:30
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    $\begingroup$ @RobJeffries I was quoting the article; I would infer it to mean 10 Earth mass, and diameter about the size of a tennis ball. $\endgroup$ – Anthony X Sep 29 '19 at 23:19
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I would say that the only way this can be done is making ever more careful observations of the TNOs that are claimed to be affected by this putative black hole.

Ultimately such observations should narrow down the location of the object. Even then, confirming it's existence will be very difficult. It will have a small lensing/magnification effect on background stars, but it would have to get very close (in terms of angular separation) to a background star for this to be observable. It's unlikely that the position could be pinpointed accurately enough to predict when such a microlensing event would take place.

Perhaps a better bet would be to send a probe, like New Horizons, to its likely location, where the gravitational effects could be studied in situ.

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  • $\begingroup$ Leads me to wonder if the evidence to-date could justify the budget for such a probe mission. If there is nothing there, it would seem an awful waste of money. On the other hand, discovering a black hole in our own cosmic back yard would be well worth the cost of such a mission. $\endgroup$ – Anthony X Sep 29 '19 at 23:25
  • $\begingroup$ It seems to me you can never be sure it's a black hole. You can verify it's a compact mass by orbiting it, and ruling out white dwarf or neutron star, black hole may be only alternative known to physics. Maybe dropping something in can be more convincing? $\endgroup$ – Bit Chaser Sep 29 '19 at 23:28
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    $\begingroup$ @bitchaser 10 Earth mass white dwarfs are called planets and can be observed. 10 Earth mass neutron stars can't exist. $\endgroup$ – Rob Jeffries Sep 30 '19 at 5:46
  • $\begingroup$ @AnthonyX Accumulating lots of dynamical evidence for a 10 Earth mass object that cannot be seen may be sufficient evidence for a black hole, if it can be localised. This is after all how we know about the BH at the centre of our Galaxy (and others). I am not suggesting sending a probe until we know where it is $\endgroup$ – Rob Jeffries Sep 30 '19 at 5:50
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With increasing dynamical evidence, we should be able to get every better localization and mass for the hypothetical object. With an accurate enough read it should become possible to rule out a conventional planet-like object by lack of direct observations at the predicted location.

However, this will not necessarily tell us that the object in question is a black hole. For example, a planet made of dark matter would present in almost the same way (and would also be exciting!).

In situ observations by an orbiting probe would be needed to further determine the nature of the object.

Just by orbiting the object we would be able to learn a lot. For example, the radius of the orbit will allow us to get a lower bound on the density of the object. By steadily shrinking the orbit one could, rule out many alternatives. However, there will be a practical limit to how small we can make the orbit before a probe is torn apart by tidal forces (this would happen long before reaching the innermost stable).

However, once you are this close to the hypothetical black hole there are other experiments you could do. For example, black holes have characteristic frequencies (quasinormal modes) with which they oscillate if you excite them, for example, by scattering off a wave. For a black hole of several Earth masses, the electromagnetic quasinormal modes fall in the microwave regime. By scattering microwaves off the object and looking for the quasinormal modes we can learn a lot about about the nature of the object. (Observation of the qnm spectrum will tell us that it is a black hole, if it is an ultra compact object we would expect to see echoing bursts, etc.)

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