Planet Nine (a variant of the old X-Planet and Tyche-Thelistos hypothesis) proposes a super-earth planet at the edges of the solar system with a mass of around ten Earths. However, given the interest in exotic objects and gravitational variants, maybe it's interesting to study "exotic" scenarios:

  1. Tiny black hole. Using the famous online calculator http://xaonon.dyndns.org/hawking/, for ten earth masses, we get a 89 mm black hole. Temperature about 2 mK! How can we put bounds on this scenario?

  2. An exotic compact object. Of course, we have no evidence so far of general relativity modifications of gravity, but if not a black hole but another "thing" (a gravastar, a quark star, a preon star,...), I find hard to estimate how could we be sure to give up experimentally these examples.

Of course, the real thing is to think something less exotic (I am not sure how to differentiate the P9 hypothesis from a swarm of thousands of ETNO dwarf planet-like or planetesimal-like, but the real issue is how to test these alternative models and if these could affect what we know about the origin and formation of the solar system and its actual current structure, any hint to this in the answer to above exotic cases would also welcome).

  • $\begingroup$ I am no longer confident that such a source would definitely have been picked up in all sky X-ray surveys. It depends inversely on the velocity (cubed) with respect to any accreted medium. The orbital speed is not the right number if outside the heliopause. $\endgroup$ – Rob Jeffries Oct 12 '18 at 12:34
  • $\begingroup$ What bounds? Size, orbit, temperature, luminosity .... $\endgroup$ – user1569 Oct 12 '18 at 12:56
  • $\begingroup$ Mass, temperature, orbit,...How do we know from all-sky searches? And mostly, have we excluded any exotic stuff out there up to 10 earth masses? $\endgroup$ – riemannium Oct 12 '18 at 13:19
  • $\begingroup$ The material is irrelevant - we detect it by means of observing its gravitational effect on other objects, and that just depends on total mass. $\endgroup$ – Carl Witthoft Oct 12 '18 at 15:49
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    $\begingroup$ @CarlWitthoft If it were that easy it would have been ruled out or found by now $\endgroup$ – Rob Jeffries Oct 12 '18 at 20:37

I think the answer is included in my answer to Could a black hole pass quiescently through the Oort cloud? The mini black hole would be detected via its accretion from the interplanetary medium, probably at X-ray wavelengths.

A ten Earth mass black hole is a ten thousandth (or so) less massive than the Sun, but if it were in a distant orbit, it could be moving slower with respect to the accreting material than the 10 km/s I assumed in the answer cited above.

The net outcome is that this might still be an X-ray source well above the detection limits of all-sky surveys. What's more, it would move with time, so if followed up would show appreciable proper motion.

The variables here are the distance to the black hole and the speed at which is travels with respect to any medium it is travelling through.

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  • $\begingroup$ Thus, what are the known bounds of this tiny black hole hypothesis? X-ray surveys have searched for it and they discarded this thing? I think so, but I don't know a reference. By the other hand, I find harder to give up exotic compact objects not emitting X-rays...Don't you? $\endgroup$ – riemannium Oct 12 '18 at 12:04
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    $\begingroup$ @riemannium I don't see why there could be any difference between accretion onto a small black hole via an accretion disk or onto any other compact object. (I'm also unaware of any suggestion that a stable "compact object" of mass only 10 Earth masses can exist, unless you include planets in that definition). $\endgroup$ – Rob Jeffries Oct 12 '18 at 12:22
  • $\begingroup$ May be ECO don't emit R-X but other kind of radiation... $\endgroup$ – riemannium Oct 12 '18 at 13:18
  • $\begingroup$ That's cool research, however it would be wrong for planet 9... smaller than a melon at 400AU distance, it would generate nearly no X-Rays. At the OORT region, the voyager shuttles didn't react with very much, they had enough sensors to detect the density of material there. It would be cool to design a mission that would measure the OORT cloud, cos it's currently undetected. $\endgroup$ – com.prehensible Oct 14 '19 at 7:24
  • $\begingroup$ @com.prehensible please show your working as to what you mean by "nearly no X-rays". The linked answer shows my working - and it might be detectable. The Voyager probes have almost no chance of detecting a planet and they aren't anywhere near the Oort cloud. $\endgroup$ – Rob Jeffries Oct 14 '19 at 7:40

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