<p>Maybe, Harvard scientists have proposed a way to determine, once and for all, whether Planet Nine actually could be a black hole. Specifically, the new method would scour the outer solar system for evidence of telltale flares that are emitted when a black hole devours a comet or other distant object. Such flares, they say, should be detectable by the upcoming Vera C. Rubin Observatory in Chile, which is expected to begin a 10-year survey of the southern sky within the next few years.</p>
<p>I suppose if funding is available for this it will be done when the telescope comes on-line. This may solve the problem of if it's a black hole but a large planet could still be possible but unknown.</p>
<p>Ref: <a href="https://astronomy.com/news/2020/07/is-planet-nine-a-black-hole-or-a-planet-harvard-scientists-suggest-a-way-to-find-out" rel="nofollow noreferrer">https://astronomy.com/news/2020/07/is-planet-nine-a-black-hole-or-a-planet-harvard-scientists-suggest-a-way-to-find-out</a></p>

CC BY-SA 4.0
2020-07-17T17:18:06.353Z
Very interesting article! Thanks very much.
Ioannes
CC BY-SA 4.0
2020-07-17T22:30:40.827Z
thank you!  i hope you enjoy it!
Natsfan
CC BY-SA 4.0
2020-07-18T03:29:41.950Z
additional reading available here: https://astronomy.stackexchange.com/a/36882/7982
uhoh
CC BY-SA 4.0
2020-07-17T16:12:22.010Z
2020-07-17T16:12:22.010Z
Natsfan
<p>Since 2019 it is proposed that the hypothetical Planet IX may in fact be a primordial black hole. Black holes are being looked after differently than planets. You can look for a gravitational lens or for an accretion disc. However, the black hole theory doesn't seem to be taken serious by many.</p>
<p>Is there any ongoing search for such black hole rather than a planet, or are there any plans to look after a primordial black hole if the planet theory would get disproved?</p>

CC BY-SA 4.0
2020-07-15T05:53:48.280Z
How do you search for it, and if so :how is that search different from a search for a planet?
planetmaker
CC BY-SA 4.0
2020-07-15T06:50:17.400Z
@planetmaker like this! https://astronomy.stackexchange.com/a/36882/7982
uhoh
CC BY-SA 4.0
2020-07-15T07:20:03.203Z
Related https://astronomy.stackexchange.com/questions/35063/can-the-wise-telescope-detect-black-holes https://astronomy.stackexchange.com/questions/33535/how-would-a-small-nearby-black-hole-be-detected-and-confirmed-as-such https://astronomy.stackexchange.com/questions/27983/could-planet-nine-be-a-tiny-black-hole-or-an-exotic-compact-object-such-that-it  https://astronomy.stackexchange.com/questions/27925/could-a-black-hole-pass-quiescently-through-the-oort-cloud
ProfRob
CC BY-SA 4.0
2020-07-15T07:21:16.490Z
I may have to revisit https://astronomy.stackexchange.com/questions/27925/could-a-black-hole-pass-quiescently-through-the-oort-cloud since my answer suggested such an object would be easily detectable by eROSITA for example.
ProfRob
CC BY-SA 4.0
2020-07-15T07:31:32.003Z
@RobJeffries So ROSAT All Sky Survey would already have detected it if it truly were a primordial black hole, if I understand you correctly?
Ioannes
CC BY-SA 4.0
2020-07-15T08:34:03.467Z
@Greenhorn there are quite a few unidentified X-ray sources in the ROSAT all-sky survey.  A single survey datapoint would not be enough to identify it as a solar-system BH candidate. You would need to see it move w.r.t. background stars at the appropriate rate. eROSITA is doing several scans of the sky over several years. However see the comment immediately above yours.The work referred to by uhoh suggests that my (back-of-the-envelope) estimate of the X-ray flux may be optimistic, but I am not yet sure why.
ProfRob
CC BY-SA 4.0
2020-07-15T12:15:58.350Z
@RobJeffries Looking at that previous answer, I’d suggest that the assumption that the object would have an accretion disk could be the issue. I wouldn’t expect a star passing through the Oort Cloud to pick up an accretion disk, and as you know a black hole is no different gravitationally at large R.  The capture cross section for material to get into a bound orbit is so small that I’d expect it to kick a lot of things around, but not to pick up a disk.
Eric Jensen
CC BY-SA 4.0
2020-07-15T12:39:55.413Z
@ELNJ Material cannot be directly accreted without losing angular momentum. It is difficult to see how that happens without a disk. The capture cross-section is not small. It is the Bondi-Hoyle radius. That is not the root of any problem.
ProfRob
CC BY-SA 4.0
2020-07-15T12:54:04.533Z
Doh. Note my answer in https://astronomy.stackexchange.com/questions/27925/could-a-black-hole-pass-quiescently-through-the-oort-cloud refers to a **1 solar mass** black hole and the luminosity scales as $M^2$. Thus "quiescent" accretion onto a 10 Earth-mass black hole will be 12 orders of magnitude lower. @ELNJ
ProfRob
CC BY-SA 4.0
2020-07-15T12:57:22.407Z
Ah, makes sense.  And thanks for the pointer to read up on Bondi-Hoyle accretion - interesting! I come from the stellar/planetary side of things and was thinking of accretion of discrete Oort Cloud bodies, not ISM gas, which I now see is the density you end up using in the final bit of your answer.
Eric Jensen
CC BY-SA 4.0
2020-07-15T14:03:39.757Z
The paper linked to by uhoh considers the accretion of Oort cloud objects. They are tidally disrupted before they get close to the black hole and *then* form a (very short-lived) accretion disc around it. The idea there is that you see "flares" of UV emission from such events and that these might be detectable. @ELNJ  And I misspoke when I said 12 orders of magnitude lower. That should be 8-9 orders of magnitude lower.
ProfRob
<black-hole><9th-planet><trans-neptunian-object><hypothetical>
Is there or will there be any search after a primordial black hole 500 au from the Sun?
Ioannes
# Is there or will there be any search after a primordial black hole 500 au from the Sun?

(Asked by: username_0 on 2020-07-15)

Since 2019 it is proposed that the hypothetical Planet IX may in fact be a primordial black hole. Black holes are being looked after differently than planets. You can look for a gravitational lens or for an accretion disc. However, the black hole theory doesn't seem to be taken serious by many.

Is there any ongoing search for such black hole rather than a planet, or are there any plans to look after a primordial black hole if the planet theory would get disproved?

**Tags:** black-hole, 9th-planet, trans-neptunian-object, hypothetical
> **Comments:**
> **username_1**: How do you search for it, and if so :how is that search different from a search for a planet?  
> **username_2**: @planetmaker like this! https://astronomy.stackexchange.com/a/36882/7982  
> **username_3**: Related https://astronomy.stackexchange.com/questions/35063/can-the-wise-telescope-detect-black-holes https://astronomy.stackexchange.com/questions/33535/how-would-a-small-nearby-black-hole-be-detected-and-confirmed-as-such https://astronomy.stackexchange.com/questions/27983/could-planet-nine-be-a-tiny-black-hole-or-an-exotic-compact-object-such-that-it  https://astronomy.stackexchange.com/questions/27925/could-a-black-hole-pass-quiescently-through-the-oort-cloud  
> **username_3**: I may have to revisit https://astronomy.stackexchange.com/questions/27925/could-a-black-hole-pass-quiescently-through-the-oort-cloud since my answer suggested such an object would be easily detectable by eROSITA for example.  
> **username_0**: @RobJeffries So ROSAT All Sky Survey would already have detected it if it truly were a primordial black hole, if I understand you correctly?  
> **username_3**: @Greenhorn there are quite a few unidentified X-ray sources in the ROSAT all-sky survey.  A single survey datapoint would not be enough to identify it as a solar-system BH candidate. You would need to see it move w.r.t. background stars at the appropriate rate. eROSITA is doing several scans of the sky over several years. However see the comment immediately above yours.The work referred to by uhoh suggests that my (back-of-the-envelope) estimate of the X-ray flux may be optimistic, but I am not yet sure why.  
> **username_4**: @RobJeffries Looking at that previous answer, I’d suggest that the assumption that the object would have an accretion disk could be the issue. I wouldn’t expect a star passing through the Oort Cloud to pick up an accretion disk, and as you know a black hole is no different gravitationally at large R.  The capture cross section for material to get into a bound orbit is so small that I’d expect it to kick a lot of things around, but not to pick up a disk.  
> **username_3**: @ELNJ Material cannot be directly accreted without losing angular momentum. It is difficult to see how that happens without a disk. The capture cross-section is not small. It is the Bondi-Hoyle radius. That is not the root of any problem.  
> **username_3**: Doh. Note my answer in https://astronomy.stackexchange.com/questions/27925/could-a-black-hole-pass-quiescently-through-the-oort-cloud refers to a **1 solar mass** black hole and the luminosity scales as $M^2$. Thus "quiescent" accretion onto a 10 Earth-mass black hole will be 12 orders of magnitude lower. @ELNJ  
> **username_4**: Ah, makes sense.  And thanks for the pointer to read up on Bondi-Hoyle accretion - interesting! I come from the stellar/planetary side of things and was thinking of accretion of discrete Oort Cloud bodies, not ISM gas, which I now see is the density you end up using in the final bit of your answer.  
> **username_3**: The paper linked to by uhoh considers the accretion of Oort cloud objects. They are tidally disrupted before they get close to the black hole and *then* form a (very short-lived) accretion disc around it. The idea there is that you see "flares" of UV emission from such events and that these might be detectable. @ELNJ  And I misspoke when I said 12 orders of magnitude lower. That should be 8-9 orders of magnitude lower.  

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## 1 answers

### Answer by username_5 on 2020-07-17. Score: 1 | Accepted answer

Maybe, Harvard scientists have proposed a way to determine, once and for all, whether Planet Nine actually could be a black hole. Specifically, the new method would scour the outer solar system for evidence of telltale flares that are emitted when a black hole devours a comet or other distant object. Such flares, they say, should be detectable by the upcoming Vera C. Rubin Observatory in Chile, which is expected to begin a 10-year survey of the southern sky within the next few years.

I suppose if funding is available for this it will be done when the telescope comes on-line. This may solve the problem of if it's a black hole but a large planet could still be possible but unknown.

Ref: <https://astronomy.com/news/2020/07/is-planet-nine-a-black-hole-or-a-planet-harvard-scientists-suggest-a-way-to-find-out>

> **Comments:**
> **username_0**: Very interesting article! Thanks very much.  
> **username_5**: thank you!  i hope you enjoy it!  
> **username_2**: additional reading available here: https://astronomy.stackexchange.com/a/36882/7982  

---

