<p>Of course it depends on the size of the hole. <a href="https://astronomy.stackexchange.com/a/35064/7982">The only way you are likely to be able to spot a small black hole is by its gravitational effects</a>. If it is a Sun mass hole there would be a lensing effect. The space surrounding the Solar system is empty as far as we know. If it travels at 100 km/s then it is at a distance of 1/3000 lightyear initially.</p>
<p>Would we be able to see deformation of the images of the stars?</p>
<p>Yes.</p>
<p>Microlensing effects can be seen. We could computerize the observations and let them give a warning if it is detected. Would we be able to tell its distance? That is difficult, but assuming it to be one year away from here we can estimate its mass.</p>
<p>If the warning is given then what? Nothing. We would live in fear (that is, the people who know) without being able to do anything about the horrific consequences (the solar system will be disrupted). So it is maybe better not to know. The days that the disrupting starts will tell. If the black hole mass is small (but big enough not to evaporate substantially in a year) it remains to be seen if it can be detected at all. You would only see a black sky with stars. But the impct on Earth will be visible for certain. Black holes are, in that respect, far more sneaky than asteroids, which you can see. I am not sure how big the mass must be for the BH not to vaporize substantially though. Maybe others can tell. It is not so difficult to calculate.</p>

CC BY-SA 4.0
2021-07-20T07:53:12.470Z
A 100 year black hole seems to have peak emission in 0.1 TeV. Wouldn't we detect a strong source of radiation which is getting stronger? Also I would imagine gravity wouldn't be our only problem...
Maja Piechotka
CC BY-SA 4.0
2021-07-20T08:56:46.327Z
@MaciejPiechotka What do you mean by a 100 year black hole? Are you referring to Hawking radiation?
Deschele Schilder
CC BY-SA 4.0
2021-07-20T09:07:12.160Z
@MaciejPiechotka I think such a hole could be dicovered already much sooner than a year ahead.
Deschele Schilder
CC BY-SA 4.0
2021-07-21T18:59:55.130Z
A black hole which evaporates in 100 years. Yes. I'm referring to Hawking radiation - I'm not sure if 'The only way you are likely to be able to spot a small black hole is by its gravitational effects.' is true for such close black hole since small black holes are hot (that is emit black body radiation of hot object).
Maja Piechotka
CC BY-SA 4.0
2021-07-19T22:02:35.217Z
2021-07-20T07:20:24.953Z
2021-07-20T07:20:24.953Z
Deschele Schilder
<p>A black hole approaching our solar system would have been detected hundred of years in advance due to nearby stars shifting position, although it's true nature wouldn't be known until decades ago, when the concept of black holes was first postulated and then confirmed.</p>
<p>Even if the black hole were to enter the solar system, rather then pass nearby it, it wouldn't consume any planets. It's just too tiny, so the chances are ridiculously low. The orbits of planets be definitely be perturbed, which will have other consequences.</p>
<p>Kurzgesagt - In a Nutshell made a video about what would happen if a brown dwarf would fly through the inner solar system and cause Earth to be ejected. While not as massive as a black hole, the effects would be similar: <div class="youtube-embed"><div>
                <iframe width="640px" height="395px" src="https://www.youtube.com/embed/gLZJlf5rHVs?start=0"></iframe>
            </div></div></p>

CC BY-SA 4.0
2021-07-20T10:46:30.493Z
"It's just too tiny", "while not as massive as a black hole" - you seem to have made assumptions about the size of the hypothetical black hole, but the question did not specify a particular size.
JBentley
CC BY-SA 4.0
2021-07-20T17:25:47.777Z
@JBentley Yeah, I never specified the size.
A. Kvåle
CC BY-SA 4.0
2021-07-20T09:08:39.970Z
2021-07-20T09:08:39.970Z
Arthur Westerman
<p>Arrive whithin a year = almost surely already within the heliosphere.</p>
<p>I assume a stellar remnant black hole (bigger ones will be even more noticeable, smaller ones are still unknown).</p>
<p>It will be a rather bright shiny x-ray source in the sky (we do have enough matter around to support a bright accretion disk).</p>
<p>It will distort our outer planet's orbits for probably 10's of years beforehand.</p>
<p>And yes, it will be a disaster.</p>
<p>It took bilions of years to get the solar system orbits in more or less stable and circluar state and we pretty much depend on this state of affairs.</p>

CC BY-SA 4.0
2021-07-20T12:41:30.247Z
and not just an X-ray source. The event horizon emits all over the spectrum, wouldn't be surprised if all the space dust and other objects being sucked towards it will cause it to emit light in the visual spectrum as well.
jwenting
CC BY-SA 4.0
2021-07-20T15:23:41.020Z
The event horizon emits Hawking radiation, for a stellar mass black hole (or bigger) it is WAY colder than the cosmic microwave background. X-rays, gamma-rays (and everything softer as well) come from the accretion disk and from the polar jets. And yes, it will be probably visible in UV-Vis as well.
fraxinus
CC BY-SA 4.0
2021-07-20T17:58:14.920Z
Can you establish that it would be a bright X-ray source? I think this is the key point.
ProfRob
CC BY-SA 4.0
2021-07-20T18:32:15.667Z
The famous Cygnus X-1 is estimated at four orders of magnitude higher luminosity in x-ray than our Sun in its whole spectrum. Its accretion disk is powered by its companion's stellar wind. Well, our environment is rather weaker in stellar winds, but goog luck missing a black hole near the Oort cloud.
fraxinus
CC BY-SA 4.0
2021-07-21T10:37:09.507Z
By my calculations; if using the smallest estimate for the Oort cloud's outer bound (0.08 light years), and using the highest estimate for the black hole's velocity (1000km/s), then it would enter the Oort cloud when it is about 30 years away from us. So, then the question is, if it's 0.08 ly away from us, to what degree would we see its accretion disk? Could we see it with the naked eye?
A. Kvåle
CC BY-SA 4.0
2021-07-21T11:24:51.103Z
Pretty much yes.
fraxinus
CC BY-SA 4.0
2021-07-21T11:36:06.557Z
Would it look like another star on the sky, or would it look more unique? At what point would we be able to actually see the accretion disk?
A. Kvåle
CC BY-SA 4.0
2021-07-21T13:19:16.390Z
Yes, it will look like a star. Brighter and brighter. No, I think we will never be able to resolve the accretion disk with naked eye - the Earth will be blown into pieces before that (tidal forces) and the atmosphere will be long gone even before that (x-rays). And I think there is no safe distance for a human to see the stellar black hole accretion disk.
fraxinus
CC BY-SA 4.0
2021-07-21T15:03:53.270Z
Not sure where I got this idea (I thought someone commented it on this post), but is it true that the smaller the black hole, the brighter the accretion disk? According to Wikipedia, the higher the mass of the central body, the higher the frequency of the EM radiation. Now, is the luminosity similarly correlated? The higher the mass, the higher the luminosity? If so, it would probably make my first idea incorrect.
A. Kvåle
CC BY-SA 4.0
2021-07-21T19:15:28.547Z
The smaller the black hole, the brighter is the black hole itself (Hawking radiation). But we are not aware of any black hole that has even theoretically detectable Hawking radiation. The properties of the accretion disk depend on a lot of factors that I am not competent to comment, but the main factor in their brightness/luminosity is the matter available for accretion. Bigger (heavier) black holes are brighter as a matter of scale (they attract matter from bigger volume of space).
fraxinus
CC BY-SA 4.0
2021-07-26T06:56:52.593Z
@A.Kvåle Take a look at the [Hawking radiation calculator](https://www.vttoth.com/CMS/physics-notes/311-hawking-radiation-calculator).
PM 2Ring
CC BY-SA 4.0
2021-07-26T06:59:05.587Z
@PM2Ring ...only to see how much unimportant is the Hawking radiation in the context of the question.
fraxinus
CC BY-SA 4.0
2021-07-26T22:50:36.027Z
Do smaller black holes release more Hawking radiation because they have less gravity?
A. Kvåle
CC BY-SA 4.0
2021-07-27T07:47:00.250Z
@A.Kvåle in these terms, smaller black holes release more Hawking radiation because of stronger tidal effect near the event horizon.
fraxinus
CC BY-SA 4.0
2021-07-20T09:45:11.253Z
2021-07-20T09:45:11.253Z
fraxinus
<p>Basically, is our observational technology and coverage sufficient enough to pretty much always spot incoming objects like a black hole when they're a year from arriving? If so, within what time frame will it not be necessarily spotted by us?</p>
<p>I know <em>necessarily</em> is a bit of a strong word as of course, it is not 100 % certain that we would spot it in any case, but at certain points it becomes very unlikely that we don't spot it. Where does this point lie?</p>
<p><strong>EDIT:</strong></p>
<p>From what I've gathered from your responses, there's three ways to notice an incoming black hole:</p>
<ol>
<li>Lensing affect; the gravitational effects of the black hole distorts light, which we can see on our space imaging.</li>
<li>Electromagnetic radiation caused by accretion disks, everything from gamma rays to x-rays, including visible light.</li>
<li>Gravitational effects on the solar system. If the orbits of our solar system change noticeably, then this could lead to the discovery of an incoming black hole.</li>
</ol>
<p>Now, to what degree each of the above points would play a role in the detection of the black hole depends on the characteristics of the black hole. This means that there's probably many different time frames in which the BH is likely to be detected, depending on the BH characteristics. If this is the case, then I am looking for an upper and lower bound.</p>

CC BY-SA 4.0
2021-07-19T20:32:52.913Z
Might depend on how fast it was going.  Typical velocities of nearby stars could be about 100km/s but black holes might be going faster... maybe 1000km/s, if they were accelerated by the supernova explosion that formed them.
James K
CC BY-SA 4.0
2021-07-19T20:34:06.230Z
@JamesK If a black hole was travelling at that speed, within what time frame would we probably spot it?
A. Kvåle
CC BY-SA 4.0
2021-07-19T23:38:26.573Z
We would also be able to detect its gravitational effect on other Solar System bodies; their orbits would be perturbed by the black hole’s presence.
Pierre Paquette
CC BY-SA 4.0
2021-07-20T07:15:10.033Z
When you say the black hole would "arrive" is this the point at which the extra gravitational pull is noticable or the point at which we're within the black hole's event horizon crosses into the solar system?
Lio Elbammalf
CC BY-SA 4.0
2021-07-20T17:21:23.063Z
@LioElbammalf Depends on what you mean by noticeable. I guess if the effect was noticeable then the black hole would probably be found quite quickly, because people all over the world would want to find out what was causing the mysterious gravitational effects. Both curiosity and fear would probably make a lot of resources be directed at finding the source of the disturbance, which makes me believe that the black hole would be spotted quite quickly.
A. Kvåle
CC BY-SA 4.0
2021-07-21T09:21:51.853Z
@A.Kvåle I'm just trying to get an idea of where this blackhole will need to be in a year's time to have "arrived" in our solar system.
Lio Elbammalf
CC BY-SA 4.0
2021-07-21T10:07:39.947Z
@LioElbammalf It was a bit of an arbitrary time frame, I just kind of used it as an example. What my question really is, is this; "if a black hole was headed towards us, within what time frame of its arrival would we almost certainly spot it?"
A. Kvåle
CC BY-SA 4.0
2021-07-26T07:02:13.423Z
Is this a normal stellar BH (so with a mass around 3 $M_\odot$ or so)? Or is it a (highly hypothetical) [primordial BH](https://en.wikipedia.org/wiki/Primordial_black_hole)?
PM 2Ring
<observational-astronomy><black-hole><deep-sky-observing>
If a black hole was headed for our solar system and would arrive within a year, would we necessarily know of it?
A. Kvåle
# If a black hole was headed for our solar system and would arrive within a year, would we necessarily know of it?

(Asked by: username_0 on 2021-07-19)

Basically, is our observational technology and coverage sufficient enough to pretty much always spot incoming objects like a black hole when they're a year from arriving? If so, within what time frame will it not be necessarily spotted by us?

I know *necessarily* is a bit of a strong word as of course, it is not 100 % certain that we would spot it in any case, but at certain points it becomes very unlikely that we don't spot it. Where does this point lie?

**EDIT:**

From what I've gathered from your responses, there's three ways to notice an incoming black hole:

1. Lensing affect; the gravitational effects of the black hole distorts light, which we can see on our space imaging.
2. Electromagnetic radiation caused by accretion disks, everything from gamma rays to x-rays, including visible light.
3. Gravitational effects on the solar system. If the orbits of our solar system change noticeably, then this could lead to the discovery of an incoming black hole.

Now, to what degree each of the above points would play a role in the detection of the black hole depends on the characteristics of the black hole. This means that there's probably many different time frames in which the BH is likely to be detected, depending on the BH characteristics. If this is the case, then I am looking for an upper and lower bound.

**Tags:** observational-astronomy, black-hole, deep-sky-observing

> **Comments:**
> **username_1**: Might depend on how fast it was going.  Typical velocities of nearby stars could be about 100km/s but black holes might be going faster... maybe 1000km/s, if they were accelerated by the supernova explosion that formed them.  
> **username_0**: @JamesK If a black hole was travelling at that speed, within what time frame would we probably spot it?  
> **username_2**: We would also be able to detect its gravitational effect on other Solar System bodies; their orbits would be perturbed by the black hole’s presence.  
> **username_3**: When you say the black hole would "arrive" is this the point at which the extra gravitational pull is noticable or the point at which we're within the black hole's event horizon crosses into the solar system?  
> **username_0**: @LioElbammalf Depends on what you mean by noticeable. I guess if the effect was noticeable then the black hole would probably be found quite quickly, because people all over the world would want to find out what was causing the mysterious gravitational effects. Both curiosity and fear would probably make a lot of resources be directed at finding the source of the disturbance, which makes me believe that the black hole would be spotted quite quickly.  
> **username_3**: @A.Kvåle I'm just trying to get an idea of where this blackhole will need to be in a year's time to have "arrived" in our solar system.  
> **username_0**: @LioElbammalf It was a bit of an arbitrary time frame, I just kind of used it as an example. What my question really is, is this; "if a black hole was headed towards us, within what time frame of its arrival would we almost certainly spot it?"  
> **username_4**: Is this a normal stellar BH (so with a mass around 3 $M_\odot$ or so)? Or is it a (highly hypothetical) [primordial BH](https://en.wikipedia.org/wiki/Primordial_black_hole)?  

---

## 3 answers

### Answer by username_5 on 2021-07-19. Score: 4

Of course it depends on the size of the hole. [The only way you are likely to be able to spot a small black hole is by its gravitational effects](https://astronomy.stackexchange.com/a/35064/7982). If it is a Sun mass hole there would be a lensing effect. The space surrounding the Solar system is empty as far as we know. If it travels at 100 km/s then it is at a distance of 1/3000 lightyear initially.

Would we be able to see deformation of the images of the stars?

Yes.

Microlensing effects can be seen. We could computerize the observations and let them give a warning if it is detected. Would we be able to tell its distance? That is difficult, but assuming it to be one year away from here we can estimate its mass.

If the warning is given then what? Nothing. We would live in fear (that is, the people who know) without being able to do anything about the horrific consequences (the solar system will be disrupted). So it is maybe better not to know. The days that the disrupting starts will tell. If the black hole mass is small (but big enough not to evaporate substantially in a year) it remains to be seen if it can be detected at all. You would only see a black sky with stars. But the impct on Earth will be visible for certain. Black holes are, in that respect, far more sneaky than asteroids, which you can see. I am not sure how big the mass must be for the BH not to vaporize substantially though. Maybe others can tell. It is not so difficult to calculate.

> **Comments:**
> **username_6**: A 100 year black hole seems to have peak emission in 0.1 TeV. Wouldn't we detect a strong source of radiation which is getting stronger? Also I would imagine gravity wouldn't be our only problem...  
> **username_5**: @MaciejPiechotka What do you mean by a 100 year black hole? Are you referring to Hawking radiation?  
> **username_5**: @MaciejPiechotka I think such a hole could be dicovered already much sooner than a year ahead.  
> **username_6**: A black hole which evaporates in 100 years. Yes. I'm referring to Hawking radiation - I'm not sure if 'The only way you are likely to be able to spot a small black hole is by its gravitational effects.' is true for such close black hole since small black holes are hot (that is emit black body radiation of hot object).  

---

### Answer by username_7 on 2021-07-20. Score: 1

A black hole approaching our solar system would have been detected hundred of years in advance due to nearby stars shifting position, although it's true nature wouldn't be known until decades ago, when the concept of black holes was first postulated and then confirmed.

Even if the black hole were to enter the solar system, rather then pass nearby it, it wouldn't consume any planets. It's just too tiny, so the chances are ridiculously low. The orbits of planets be definitely be perturbed, which will have other consequences.

Kurzgesagt - In a Nutshell made a video about what would happen if a brown dwarf would fly through the inner solar system and cause Earth to be ejected. While not as massive as a black hole, the effects would be similar:

> **Comments:**
> **username_8**: "It's just too tiny", "while not as massive as a black hole" - you seem to have made assumptions about the size of the hypothetical black hole, but the question did not specify a particular size.  
> **username_0**: @username_8 Yeah, I never specified the size.  

---

### Answer by username_9 on 2021-07-20. Score: 1

Arrive whithin a year = almost surely already within the heliosphere.

I assume a stellar remnant black hole (bigger ones will be even more noticeable, smaller ones are still unknown).

It will be a rather bright shiny x-ray source in the sky (we do have enough matter around to support a bright accretion disk).

It will distort our outer planet's orbits for probably 10's of years beforehand.

And yes, it will be a disaster.

It took bilions of years to get the solar system orbits in more or less stable and circluar state and we pretty much depend on this state of affairs.

> **Comments:**
> **username_10**: and not just an X-ray source. The event horizon emits all over the spectrum, wouldn't be surprised if all the space dust and other objects being sucked towards it will cause it to emit light in the visual spectrum as well.  
> **username_9**: The event horizon emits Hawking radiation, for a stellar mass black hole (or bigger) it is WAY colder than the cosmic microwave background. X-rays, gamma-rays (and everything softer as well) come from the accretion disk and from the polar jets. And yes, it will be probably visible in UV-Vis as well.  
> **username_11**: Can you establish that it would be a bright X-ray source? I think this is the key point.  
> **username_9**: The famous Cygnus X-1 is estimated at four orders of magnitude higher luminosity in x-ray than our Sun in its whole spectrum. Its accretion disk is powered by its companion's stellar wind. Well, our environment is rather weaker in stellar winds, but goog luck missing a black hole near the Oort cloud.  
> **username_0**: By my calculations; if using the smallest estimate for the Oort cloud's outer bound (0.08 light years), and using the highest estimate for the black hole's velocity (1000km/s), then it would enter the Oort cloud when it is about 30 years away from us. So, then the question is, if it's 0.08 ly away from us, to what degree would we see its accretion disk? Could we see it with the naked eye?  
> **username_9**: Pretty much yes.  
> **username_0**: Would it look like another star on the sky, or would it look more unique? At what point would we be able to actually see the accretion disk?  
> **username_9**: Yes, it will look like a star. Brighter and brighter. No, I think we will never be able to resolve the accretion disk with naked eye - the Earth will be blown into pieces before that (tidal forces) and the atmosphere will be long gone even before that (x-rays). And I think there is no safe distance for a human to see the stellar black hole accretion disk.  
> **username_0**: Not sure where I got this idea (I thought someone commented it on this post), but is it true that the smaller the black hole, the brighter the accretion disk? According to Wikipedia, the higher the mass of the central body, the higher the frequency of the EM radiation. Now, is the luminosity similarly correlated? The higher the mass, the higher the luminosity? If so, it would probably make my first idea incorrect.  
> **username_9**: The smaller the black hole, the brighter is the black hole itself (Hawking radiation). But we are not aware of any black hole that has even theoretically detectable Hawking radiation. The properties of the accretion disk depend on a lot of factors that I am not competent to comment, but the main factor in their brightness/luminosity is the matter available for accretion. Bigger (heavier) black holes are brighter as a matter of scale (they attract matter from bigger volume of space).  
> **username_4**: @A.Kvåle Take a look at the [Hawking radiation calculator](https://www.vttoth.com/CMS/physics-notes/311-hawking-radiation-calculator).  
> **username_9**: @PM2Ring ...only to see how much unimportant is the Hawking radiation in the context of the question.  
> **username_0**: Do smaller black holes release more Hawking radiation because they have less gravity?  
> **username_9**: @A.Kvåle in these terms, smaller black holes release more Hawking radiation because of stronger tidal effect near the event horizon.  

---

