<p>I think the answer is included in my answer to <a href="https://astronomy.stackexchange.com/questions/27925/could-a-black-hole-pass-quiescently-through-the-oort-cloud">Could a black hole pass quiescently through the Oort cloud?</a> The mini black hole would be detected via its accretion from the interplanetary medium, probably at X-ray wavelengths.</p>

<p>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 <em>could</em> be moving slower with respect to the accreting material than the 10 km/s I assumed in the answer cited above.</p>

<p>The net outcome is that this <em>might</em> 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.</p>

<p>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.</p>

CC BY-SA 4.0
2018-10-12T12:04:21.550Z
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?
riemannium
CC BY-SA 4.0
2018-10-12T12:22:46.100Z
@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).
ProfRob
CC BY-SA 4.0
2018-10-12T13:18:18.010Z
May be ECO don't emit R-X but other kind of radiation...
riemannium
CC BY-SA 4.0
2019-10-14T07:24:45.090Z
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.
bandybabboon
CC BY-SA 4.0
2019-10-14T07:40:40.530Z
@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.
ProfRob
CC BY-SA 4.0
2019-10-14T08:34:39.580Z
I found an escape velocity calculator here https://www.omnicalculator.com/physics/escape-velocity it suggests that a P9 BH would indeed catch plenty of material. Then it gets a bit complicated.
bandybabboon
CC BY-SA 4.0
2018-10-12T11:31:32.433Z
2018-10-12T12:32:15.787Z
2018-10-12T12:32:15.787Z
ProfRob
<p>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: </p>

<ol>
<li><p>Tiny black hole. Using the famous online calculator <a href="http://xaonon.dyndns.org/hawking/" rel="nofollow noreferrer">http://xaonon.dyndns.org/hawking/</a>, for ten earth masses, we get a 89 mm black hole. Temperature about 2 mK! How can we put bounds on this scenario?</p></li>
<li><p>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. </p></li>
</ol>

<p>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).</p>

CC BY-SA 4.0
2018-10-12T12:34:04.263Z
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.
ProfRob
CC BY-SA 4.0
2018-10-12T13:19:34.660Z
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?
riemannium
CC BY-SA 4.0
2018-10-12T15:49:11.293Z
The material is irrelevant - we detect it by means of observing its gravitational effect on other objects, and that just depends on total mass.
Carl Witthoft
CC BY-SA 4.0
2018-10-12T20:37:54.643Z
@CarlWitthoft If it were that easy it would have been ruled out or found by now
ProfRob
CC BY-SA 4.0
2019-10-14T07:08:36.470Z
The topic was heavily covered 10 year after you wrote about it here, and there is some information on Astronomy.com http://www.astronomy.com/news/2019/10/planet-nine-may-be-a-black-hole-the-size-of-a-baseball
bandybabboon
CC BY-SA 4.0
2021-06-05T14:43:46.273Z
@DeltaEnfieldWaid I realized it! Unfortunately, the publishers did not quote this questions or source as trigger of the problem. Sad.
riemannium
<black-hole><solar-system><earth-like-planet>
Could Planet Nine be a tiny black hole or an exotic compact object such that it escaped detection? What would its bounds be?
riemannium
# Could Planet Nine be a tiny black hole or an exotic compact object such that it escaped detection? What would its bounds be?

(Asked by: username_0 on 2018-10-12)

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).

**Tags:** black-hole, solar-system, earth-like-planet

> **Comments:**
> **username_1**: 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.  
> **username_0**: 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?  
> **username_2**: The material is irrelevant - we detect it by means of observing its gravitational effect on other objects, and that just depends on total mass.  
> **username_1**: @CarlWitthoft If it were that easy it would have been ruled out or found by now  
> **username_3**: The topic was heavily covered 10 year after you wrote about it here, and there is some information on Astronomy.com http://www.astronomy.com/news/2019/10/planet-nine-may-be-a-black-hole-the-size-of-a-baseball  
> **username_0**: @DeltaEnfieldWaid I realized it! Unfortunately, the publishers did not quote this questions or source as trigger of the problem. Sad.  

---

## 1 answers

### Answer by username_1 on 2018-10-12. Score: 4

I think the answer is included in my answer to [Could a black hole pass quiescently through the Oort cloud?](https://astronomy.stackexchange.com/questions/27925/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.

> **Comments:**
> **username_0**: 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?  
> **username_1**: @username_0 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).  
> **username_0**: May be ECO don't emit R-X but other kind of radiation...  
> **username_3**: 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.  
> **username_1**: @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.  
> **username_3**: I found an escape velocity calculator here https://www.omnicalculator.com/physics/escape-velocity it suggests that a P9 BH would indeed catch plenty of material. Then it gets a bit complicated.  

---

