<p>There are many normal methods that we use to detect exoplanets, but none of them work well in the case of the 9th planet. Here are some of the main ones.</p>

<ul>
<li><strong><a href="https://en.wikipedia.org/wiki/Methods_of_detecting_exoplanets#Radial_velocity" rel="noreferrer">Radial velocity.</a></strong> The Sun is not moving significantly with respect to Earth, and the hypothetical planet is too far away from the Sun to have much of an impact.</li>
<li><strong><a href="https://en.wikipedia.org/wiki/Methods_of_detecting_exoplanets#Transit_photometry" rel="noreferrer">Transit.</a></strong> This is obviously impossible, as the planet never passes between Earth and the Sun.</li>
<li><strong><a href="https://en.wikipedia.org/wiki/Gravitational_microlensing" rel="noreferrer">Gravitational microlensing.</a></strong> Again, the planet does not pass between us and the Sun.</li>
<li><strong><a href="https://en.wikipedia.org/wiki/Methods_of_detecting_exoplanets#Direct_imaging" rel="noreferrer">Direct imaging.</a></strong> This is difficult to begin with. The planet is also far away and would have <a href="https://en.wikipedia.org/wiki/Planet_Nine#Direct_detection" rel="noreferrer">a low absolute magnitude</a>.</li>
</ul>

<p>As to why indirect evidence for the planet wasn't accumulated earlier - well, <a href="https://en.wikipedia.org/wiki/Kuiper_belt#History" rel="noreferrer">objects in the Kuiper Belt and beyond (including the perturbed Trans-Neptunian Objects (TNOs)) were only first observed in 1992</a>, and we've only built up significant data in recent years.</p>

CC BY-SA 3.0
2016-01-23T01:49:34.043Z
2016-01-24T15:55:18.223Z
2016-01-24T15:55:18.223Z
HDE 226868
<p>This graph from <a href="http://xkcd.com/1633/" rel="noreferrer">XKCD</a> says a lot about why that is the case</p>

<p><a href="https://i.sstatic.net/9uiOO.png" rel="noreferrer" title="Superman lies near the bird/plane boundary over a range of distances, which explains the confusion."><img src="https://i.sstatic.net/9uiOO.png" alt="" title="Superman lies near the bird/plane boundary over a range of distances, which explains the confusion."></a></p>

<p>The bottom line is, the 9th planet is too small to be detected through WISE, and too far/small to have been detected through visible observation. Most likely this hypothetical planet is a long ways away, possibly as far as 1200 AU, and not particularly large, making it difficult to see. WISE was able to rule out the existence of any Saturn sized object up to 10,000 AU. This hypothetical object is about 1/5th to 1/10th the mass of Saturn. It is also far more likely to be more compact, as Saturn is rather low density. Bottom line, if it is visible in WISE, it would be on the very edge.</p>

CC BY-SA 3.0
2016-01-23T04:08:47.483Z
Who is 'me' here? Earth is probably not that far from me!
MycrofD
CC BY-SA 3.0
2016-01-23T04:49:20.450Z
Me is Randal, and he is measuring from the center of the planet.
hildred
CC BY-SA 3.0
2016-01-23T22:43:20.740Z
Shouldn’t the region “planets we would see during the day” include the moon?
Peter LeFanu Lumsdaine
CC BY-SA 3.0
2016-01-23T23:07:02.657Z
Yea, the chart isn't perfect. Mars shows up as further than Jupiter, for instance. That should be the case, and I don't think all of the planets are quite in the right area...
PearsonArtPhoto
CC BY-SA 3.0
2016-01-24T03:09:09.913Z
@Peter: I think the point is that the "visible at night" / "visible in daytime" line seems to be drawn at around 0.1 solar diameters, i.e. the size of a small red dwarf star. Basically, anything bigger than that would have to be a proper star, capable of fusing hydrogen and emitting its own light, which would make it more visible. But yes, to properly match the description, the line really should turn down on the left enough to include the moon (and, for that matter, the earth, and some birds and airplanes) in the range of daytime-visible objects.
Ilmari Karonen
CC BY-SA 3.0
2016-01-25T03:44:17.720Z
@MycrofD Randall Munroe is measuring distance between his location on the surface of the Earth and the center of any depicted object.  I'm a little surprised he didn't include the sun on this chart.
WBT
CC BY-SA 3.0
2016-01-25T12:35:54.880Z
The problem is 'me' and earth both should be at origin..that is what i think.. where am i wrong?
MycrofD
CC BY-SA 3.0
2016-01-25T14:14:52.913Z
@MycrofD: As above, Randall is measuring from the center of planets.
Charles
CC BY-SA 3.0
2017-02-26T05:46:25.997Z
So would the sun's center or edge be at the top of the main diagonal line that Earth is on (1 AU)? While I find the graphic cool and applaud the creativity, it's a bit unintuitive and hard to follow for me. I guess people's minds work differently than others and am glad it clears things up for others.
iMerchant
CC BY-SA 3.0
2016-01-23T01:49:17.197Z
2017-06-25T16:23:46.697Z
2017-06-25T16:23:46.697Z
PearsonArtPhoto
<p>Possible reasons that the planet has not been picked up previously:</p>

<ol>
<li>It's not there.</li>
<li>Photographic proper motion surveys cover the whoke sky. To avoid these planet 9 would have to be fainter than about 18th or 19th magnitude. This puts a lower bound in the size/albedo/distance combination for any planet 9. The proposed planet could easily be fainter than 20th magnitude.</li>
<li>Targeted searches for <a href="https://en.wikipedia.org/wiki/Trans-Neptunian_object">trans-Neptunian objects</a> (TNOs) use big telescopes with wide fields of view. Nevertheless they do not cover anywhere near the whole sky and are generally limited to perhaps magnitude 22-23.</li>
<li>Deep searches can go much fainter, but are "pencil beam" surveys and so have little chance of blindly picking up a particular object, or are single epoch and so cannot detect motion due to parallax and orbital motion.</li>
<li>Infrared surveys (<a href="https://en.wikipedia.org/wiki/Wide-field_Infrared_Survey_Explorer">WISE</a> and <a href="https://en.wikipedia.org/wiki/2MASS">2MASS</a>) covered the whole sky. They were sensitive enough to pick up gas giants at the proposed distance of planet 9, because giants are kept hot by gravitational contraction. However, if planet 9 were rocky/icy and had no internal source of heat then it could have been too cold to detect.</li>
</ol>

CC BY-SA 3.0
2016-01-23T08:52:09.767Z
2016-01-23T13:20:10.407Z
2016-01-23T13:20:10.407Z
ProfRob
<p>Adding to the other excellent answers here, if there were an unobserved &quot;Planet 9&quot; the possibility of it being a difficult to observe primordial black hole has been raised.</p>
<p>According to Phys.org's <a href="https://phys.org/news/2020-07-scientists-planet-primordial-black-hole.html" rel="noreferrer">Scientists propose plan to determine if Planet Nine is a primordial black hole</a> if Planet 9 were a primordial black hole it could be detected by the <a href="https://en.wikipedia.org/wiki/Vera_C._Rubin_Observatory" rel="noreferrer">Vera C. Rubin Observatory</a> by observations of occasional accretion flares from cometary debris.</p>
<p>But its <em>a big if.</em></p>
<blockquote>
<p>Dr. Avi Loeb, Frank B. Baird Jr. Professor of Science at Harvard, and Amir Siraj, a Harvard undergraduate student, have developed the new method to search for black holes in the outer solar system based on flares that result from the disruption of intercepted comets. The study suggests that the LSST has the capability to find black holes by observing for accretion flares resulting from the impact of small Oort cloud objects.</p>
<p>&quot;In the vicinity of a black hole, small bodies that approach it will melt as a result of heating from the background accretion of gas from the interstellar medium onto the black hole,&quot; said Siraj. &quot;Once they melt, the small bodies are subject to tidal disruption by the black hole, followed by accretion from the tidally disrupted body onto the black hole.&quot; Loeb added, &quot;Because black holes are intrinsically dark, the radiation that matter emits on its way to the mouth of the black hole is our only way to illuminate this dark environment.&quot;</p>
</blockquote>
<p>This is discussed further in their arXiv preprint <a href="https://arxiv.org/abs/2005.12280" rel="noreferrer">Searching for Black Holes in the Outer Solar System with LSST</a> accepted for publication in Astrophysical Journal Letters.</p>
<p>Note that LSST now refers to the Legacy Survey of Space and Time, and the Vera C. Rubin Observatory used to be called the Large Synoptic Survey Telescope. <a href="https://astronomy.stackexchange.com/q/34647/7982">What is the LSST now?</a></p>

CC BY-SA 4.0
2020-07-11T04:38:21.447Z
2020-07-15T07:08:57.227Z
2020-07-15T07:08:57.227Z
uhoh
<p>Brown and Batygin, the authors of the paper on the possible planet, have a <a href="http://www.findplanetnine.com/p/blog-page.html" rel="noreferrer">webpage</a> addressing this.</p>

<p>A few reasons not already covered:</p>

<ul>
<li>It moves quite slowly - the authors estimate 0.2-0.6 arc seconds per hour - so standard surveys may not notice the movement and fail to recognize it as a solar system object.</li>
</ul>

<blockquote>
  <p>Eris, which is the most distant confirmed object still known in the
  solar system, moves at a speed of 1.5 arcseconds per hour, which is so
  slow that it was missed the first time around. Most surveys of the
  outer solar system would not be able to find Planet Nine, even if it
  were quite bright, as they would just think it is a stationary star.</p>
</blockquote>

<ul>
<li><p>If the planet is near aphelion, it might be an order of magnitude further away than any major or minor planet we've found so far (excluding exoplanets, which are found by methods that don't apply in this case). The authors suggest an aphelion between 500 and 1200 AU. For comparison, Pluto is at 30-50 AU, while Eris at around 100 AU wasn't discovered until 2005. The potential 9th planet would be far larger than Eris, but is also likely to be much further away, and thereby fainter.</p></li>
<li><p>The WISE survey eliminated <a href="http://www.space.com/24989-planet-x-search-nasa-telescope.html" rel="noreferrer">Saturn-sized planets within 10,000 AU</a>, and Jupiter-sized planets within 26,000 AU. But the potential 9th planet is far smaller than those. WISE has also done a more sensitive search, which would pick up Neptune-sized objects, but that search has so far covered only a limited part of the sky.</p></li>
<li><p>The planet will be far harder to spot if it has the Milky Way in the background - there are too many stars potentially drowning out a faint object.</p></li>
</ul>

<p>Here's the authors' summary:</p>

<p><a href="https://i.sstatic.net/UDCTJ.png" rel="noreferrer"><img src="https://i.sstatic.net/UDCTJ.png" alt="Estimated 9th planet orbit"></a></p>

<p><em>Estimated orbit for the putative 9th planet. The horizontal axis is the right ascension. The colored segments are regions where it should have been found by existing surveys.
<br>Illustration by Brown and Batygin, assuming <a href="https://en.wikipedia.org/wiki/Fair_use" rel="noreferrer">fair use</a> applies.</em></p>

<blockquote>
  <p>The biggest unexplored territory is where, statistically, it is most
  likely to be: near aphelion. Sadly, aphelion is also very close to the
  Milky Way galaxy. Ugh.</p>
  
  <p>So where is it? Probably distant. 500 AU+. Probably fainter than 22nd 
  magnitude. Very possibly in the middle of the Milky Way galaxy.</p>
  
  <p>Now go find planet nine.</p>
</blockquote>

<p>More details on the authors' webpage: <a href="http://www.findplanetnine.com/p/blog-page.html" rel="noreferrer">http://www.findplanetnine.com/p/blog-page.html</a></p>

<hr>

<p>Finally, the gravitational dominance of the Sun reaches halfway to the nearest star. There's still plenty of unexplored territory for planets smaller than Saturn to hide in.</p>

<p><a href="https://i.sstatic.net/fRmFX.jpg" rel="noreferrer"><img src="https://i.sstatic.net/fRmFX.jpg" alt="Solar system distances out to the Oort cloud and beyond"></a>
<br><em>Note the log axis. We have a good map for the inner 50 AU, and are starting to find objects around 100 AU, but solar system objects might exist all the way to the outer edges of the Oort cloud.
<br>Illustration from <a href="https://en.wikipedia.org/wiki/Oort_cloud" rel="noreferrer">wikipedia</a>.</em></p>

CC BY-SA 3.0
2016-01-23T23:13:41.757Z
The current dominance of the Sun might extend half-way to the nearest star now, but statistically we come close to other stars frequently, maybe a light year or closer, which could easily mean that any far out object would no longer be orbiting, depending on how close of a pass.
PearsonArtPhoto
CC BY-SA 3.0
2016-01-23T23:42:13.283Z
Good point. I'm assuming that if the Oort cloud can be stable, it should be possible for larger bodies to remain stable as well. But I'm a hobbyist, not a professional astronomer, so I'll leave it for someone more knowledgeable.
j-g-faustus
CC BY-SA 3.0
2016-01-23T23:45:16.313Z
Wow, nice job finding that page! +1 for that alone. But seriously, excellent answer. Nice job picking up on the parallax angle (pun absolutely intended).
HDE 226868
CC BY-SA 3.0
2016-01-24T17:59:32.790Z
@PearsonArtPhoto - more precisely, the "gravitational dominance" of the Sun (assuming that we're talking about 2 stars) would extend to the center of mass between them. Add more bodies and it gets much more complicated (as well as begging a definition of "gravitational dominance").
Comintern
CC BY-SA 3.0
2016-01-23T22:52:38.660Z
2016-01-23T22:52:38.660Z
j-g-faustus
<p>In the comments to <a href="https://astronomy.stackexchange.com/q/13313/182">this question</a>, there was considerable doubt placed as to the subject of if the so called "9th planet" really exists. That wasn't really the intent of the previous question, so I ask this. Why hasn't this "9th planet" been detected before now, if it even exists?</p>

CC BY-SA 3.0
2016-01-24T19:53:45.607Z
[Closely related question](http://physics.stackexchange.com/q/230973/40456) on [physics.se].
David Richerby
CC BY-SA 3.0
2016-01-25T04:37:08.740Z
naively, because it is very small and far away.
njzk2
CC BY-SA 3.0
2016-01-25T09:15:42.507Z
Last year, i started cataloging all the pebbles in my garden.  I've found a nice shiny one by the apple tree roots.  My question is - why wasn't this pebble found before now?
Max Williams
CC BY-SA 3.0
2017-02-25T20:24:36.383Z
Strongly related: http://astronomy.stackexchange.com/questions/8605/if-alpha-centauri-as-solar-system-exactly-mirrored-our-own-what-would-we-be-ab
ProfRob
<solar-system><planet><observational-astronomy><9th-planet>
Why hasn't the "9th Planet" been detected already?
PearsonArtPhoto
# Why hasn't the "9th Planet" been detected already?

(Asked by: username_0 on 2016-01-23)

In the comments to [this question](https://astronomy.stackexchange.com/q/13313/182), there was considerable doubt placed as to the subject of if the so called "9th planet" really exists. That wasn't really the intent of the previous question, so I ask this. Why hasn't this "9th planet" been detected before now, if it even exists?

**Tags:** solar-system, planet, observational-astronomy, 9th-planet

> **Comments:**
> **username_1**: [Closely related question](http://physics.stackexchange.com/q/230973/40456) on [physics.se].  
> **username_2**: naively, because it is very small and far away.  
> **username_3**: Last year, i started cataloging all the pebbles in my garden.  I've found a nice shiny one by the apple tree roots.  My question is - why wasn't this pebble found before now?  
> **username_4**: Strongly related: http://astronomy.stackexchange.com/questions/8605/if-alpha-centauri-as-solar-system-exactly-mirrored-our-own-what-would-we-be-ab  

---

## 5 answers

### Answer by username_14 on 2016-01-23. Score: 60 | Accepted answer

Brown and Batygin, the authors of the paper on the possible planet, have a [webpage](http://www.findplanetnine.com/p/blog-page.html) addressing this.

A few reasons not already covered:

* It moves quite slowly - the authors estimate 0.2-0.6 arc seconds per hour - so standard surveys may not notice the movement and fail to recognize it as a solar system object.

> Eris, which is the most distant confirmed object still known in the
> solar system, moves at a speed of 1.5 arcseconds per hour, which is so
> slow that it was missed the first time around. Most surveys of the
> outer solar system would not be able to find Planet Nine, even if it
> were quite bright, as they would just think it is a stationary star.

* If the planet is near aphelion, it might be an order of magnitude further away than any major or minor planet we've found so far (excluding exoplanets, which are found by methods that don't apply in this case). The authors suggest an aphelion between 500 and 1200 AU. For comparison, Pluto is at 30-50 AU, while Eris at around 100 AU wasn't discovered until 2005. The potential 9th planet would be far larger than Eris, but is also likely to be much further away, and thereby fainter.
* The WISE survey eliminated [Saturn-sized planets within 10,000 AU](http://www.space.com/24989-planet-x-search-nasa-telescope.html), and Jupiter-sized planets within 26,000 AU. But the potential 9th planet is far smaller than those. WISE has also done a more sensitive search, which would pick up Neptune-sized objects, but that search has so far covered only a limited part of the sky.
* The planet will be far harder to spot if it has the Milky Way in the background - there are too many stars potentially drowning out a faint object.

Here's the authors' summary:

[![Estimated 9th planet orbit](https://i.sstatic.net/UDCTJ.png)](https://i.sstatic.net/UDCTJ.png)

*Estimated orbit for the putative 9th planet. The horizontal axis is the right ascension. The colored segments are regions where it should have been found by existing surveys.
  
Illustration by Brown and Batygin, assuming [fair use](https://en.wikipedia.org/wiki/Fair_use) applies.*

> The biggest unexplored territory is where, statistically, it is most
> likely to be: near aphelion. Sadly, aphelion is also very close to the
> Milky Way galaxy. Ugh.
>
> So where is it? Probably distant. 500 AU+. Probably fainter than 22nd
> magnitude. Very possibly in the middle of the Milky Way galaxy.
>
> Now go find planet nine.

More details on the authors' webpage: <http://www.findplanetnine.com/p/blog-page.html>

---

Finally, the gravitational dominance of the Sun reaches halfway to the nearest star. There's still plenty of unexplored territory for planets smaller than Saturn to hide in.

[![Solar system distances out to the Oort cloud and beyond](https://i.sstatic.net/fRmFX.jpg)](https://i.sstatic.net/fRmFX.jpg)
  
*Note the log axis. We have a good map for the inner 50 AU, and are starting to find objects around 100 AU, but solar system objects might exist all the way to the outer edges of the Oort cloud.
  
Illustration from [wikipedia](https://en.wikipedia.org/wiki/Oort_cloud).*

> **Comments:**
> **username_0**: The current dominance of the Sun might extend half-way to the nearest star now, but statistically we come close to other stars frequently, maybe a light year or closer, which could easily mean that any far out object would no longer be orbiting, depending on how close of a pass.  
> **username_14**: Good point. I'm assuming that if the Oort cloud can be stable, it should be possible for larger bodies to remain stable as well. But I'm a hobbyist, not a professional astronomer, so I'll leave it for someone more knowledgeable.  
> **username_5**: Wow, nice job finding that page! +1 for that alone. But seriously, excellent answer. Nice job picking up on the parallax angle (pun absolutely intended).  
> **username_15**: @username_0 - more precisely, the "gravitational dominance" of the Sun (assuming that we're talking about 2 stars) would extend to the center of mass between them. Add more bodies and it gets much more complicated (as well as begging a definition of "gravitational dominance").  

---

### Answer by username_0 on 2016-01-23. Score: 38

This graph from [XKCD](http://xkcd.com/1633/) says a lot about why that is the case

[![](https://i.sstatic.net/9uiOO.png "Superman lies near the bird/plane boundary over a range of distances, which explains the confusion.")](https://i.sstatic.net/9uiOO.png "Superman lies near the bird/plane boundary over a range of distances, which explains the confusion.")

The bottom line is, the 9th planet is too small to be detected through WISE, and too far/small to have been detected through visible observation. Most likely this hypothetical planet is a long ways away, possibly as far as 1200 AU, and not particularly large, making it difficult to see. WISE was able to rule out the existence of any Saturn sized object up to 10,000 AU. This hypothetical object is about 1/5th to 1/10th the mass of Saturn. It is also far more likely to be more compact, as Saturn is rather low density. Bottom line, if it is visible in WISE, it would be on the very edge.

> **Comments:**
> **username_6**: Who is 'me' here? Earth is probably not that far from me!  
> **username_7**: Me is Randal, and he is measuring from the center of the planet.  
> **username_8**: Shouldn’t the region “planets we would see during the day” include the moon?  
> **username_0**: Yea, the chart isn't perfect. Mars shows up as further than Jupiter, for instance. That should be the case, and I don't think all of the planets are quite in the right area...  
> **username_9**: @username_8: I think the point is that the "visible at night" / "visible in daytime" line seems to be drawn at around 0.1 solar diameters, i.e. the size of a small red dwarf star. Basically, anything bigger than that would have to be a proper star, capable of fusing hydrogen and emitting its own light, which would make it more visible. But yes, to properly match the description, the line really should turn down on the left enough to include the moon (and, for that matter, the earth, and some birds and airplanes) in the range of daytime-visible objects.  
> **username_10**: @username_6 Randall Munroe is measuring distance between his location on the surface of the Earth and the center of any depicted object.  I'm a little surprised he didn't include the sun on this chart.  
> **username_6**: The problem is 'me' and earth both should be at origin..that is what i think.. where am i wrong?  
> **username_11**: @username_6: As above, Randall is measuring from the center of planets.  
> **username_12**: So would the sun's center or edge be at the top of the main diagonal line that Earth is on (1 AU)? While I find the graphic cool and applaud the creativity, it's a bit unintuitive and hard to follow for me. I guess people's minds work differently than others and am glad it clears things up for others.  

---

### Answer by username_5 on 2016-01-23. Score: 24

There are many normal methods that we use to detect exoplanets, but none of them work well in the case of the 9th planet. Here are some of the main ones.

* **[Radial velocity.](https://en.wikipedia.org/wiki/Methods_of_detecting_exoplanets#Radial_velocity)** The Sun is not moving significantly with respect to Earth, and the hypothetical planet is too far away from the Sun to have much of an impact.
* **[Transit.](https://en.wikipedia.org/wiki/Methods_of_detecting_exoplanets#Transit_photometry)** This is obviously impossible, as the planet never passes between Earth and the Sun.
* **[Gravitational microlensing.](https://en.wikipedia.org/wiki/Gravitational_microlensing)** Again, the planet does not pass between us and the Sun.
* **[Direct imaging.](https://en.wikipedia.org/wiki/Methods_of_detecting_exoplanets#Direct_imaging)** This is difficult to begin with. The planet is also far away and would have [a low absolute magnitude](https://en.wikipedia.org/wiki/Planet_Nine#Direct_detection).

As to why indirect evidence for the planet wasn't accumulated earlier - well, [objects in the Kuiper Belt and beyond (including the perturbed Trans-Neptunian Objects (TNOs)) were only first observed in 1992](https://en.wikipedia.org/wiki/Kuiper_belt#History), and we've only built up significant data in recent years.


---

### Answer by username_4 on 2016-01-23. Score: 17

Possible reasons that the planet has not been picked up previously:

1. It's not there.
2. Photographic proper motion surveys cover the whoke sky. To avoid these planet 9 would have to be fainter than about 18th or 19th magnitude. This puts a lower bound in the size/albedo/distance combination for any planet 9. The proposed planet could easily be fainter than 20th magnitude.
3. Targeted searches for [trans-Neptunian objects](https://en.wikipedia.org/wiki/Trans-Neptunian_object) (TNOs) use big telescopes with wide fields of view. Nevertheless they do not cover anywhere near the whole sky and are generally limited to perhaps magnitude 22-23.
4. Deep searches can go much fainter, but are "pencil beam" surveys and so have little chance of blindly picking up a particular object, or are single epoch and so cannot detect motion due to parallax and orbital motion.
5. Infrared surveys ([WISE](https://en.wikipedia.org/wiki/Wide-field_Infrared_Survey_Explorer) and [2MASS](https://en.wikipedia.org/wiki/2MASS)) covered the whole sky. They were sensitive enough to pick up gas giants at the proposed distance of planet 9, because giants are kept hot by gravitational contraction. However, if planet 9 were rocky/icy and had no internal source of heat then it could have been too cold to detect.


---

### Answer by username_13 on 2020-07-11. Score: 8

Adding to the other excellent answers here, if there were an unobserved "Planet 9" the possibility of it being a difficult to observe primordial black hole has been raised.

According to Phys.org's [Scientists propose plan to determine if Planet Nine is a primordial black hole](https://phys.org/news/2020-07-scientists-planet-primordial-black-hole.html) if Planet 9 were a primordial black hole it could be detected by the [Vera C. Rubin Observatory](https://en.wikipedia.org/wiki/Vera_C._Rubin_Observatory) by observations of occasional accretion flares from cometary debris.

But its *a big if.*

> Dr. Avi Loeb, Frank B. Baird Jr. Professor of Science at Harvard, and Amir Siraj, a Harvard undergraduate student, have developed the new method to search for black holes in the outer solar system based on flares that result from the disruption of intercepted comets. The study suggests that the LSST has the capability to find black holes by observing for accretion flares resulting from the impact of small Oort cloud objects.
>
> "In the vicinity of a black hole, small bodies that approach it will melt as a result of heating from the background accretion of gas from the interstellar medium onto the black hole," said Siraj. "Once they melt, the small bodies are subject to tidal disruption by the black hole, followed by accretion from the tidally disrupted body onto the black hole." Loeb added, "Because black holes are intrinsically dark, the radiation that matter emits on its way to the mouth of the black hole is our only way to illuminate this dark environment."

This is discussed further in their arXiv preprint [Searching for Black Holes in the Outer Solar System with LSST](https://arxiv.org/abs/2005.12280) accepted for publication in Astrophysical Journal Letters.

Note that LSST now refers to the Legacy Survey of Space and Time, and the Vera C. Rubin Observatory used to be called the Large Synoptic Survey Telescope. [What is the LSST now?](https://astronomy.stackexchange.com/q/34647/7982)


---

