
What if Planet 9 is a Primordial Black Hole? - mxcrossb
https://link.aps.org/doi/10.1103/PhysRevLett.125.051103
======
ISL
One should not miss Figure 1 of the arXiv preprint, which is, alas, absent
from the final PRL.

[https://arxiv.org/pdf/1909.11090.pdf](https://arxiv.org/pdf/1909.11090.pdf)

It is among the rarest journal-article figures in physics. If I'd been asked
to referee the paper, I'd have campaigned hard to keep it in.

~~~
themodelplumber
Fascinating. And kind of funny. I wonder, are there any YouTubers who walk
through the math in papers like this? Asking because it seems really
interesting and sometimes it's fun to learn bits here and there from applied
math rather than always working up from the fundamentals.

~~~
wrnr
The channel I find most accessible for laymen, that goes into the maths, is
DrAPhysics[1]. It's done by some random English radio jockey with a background
in Physics. Nothing on the cutting edge, but I like his simple presentation of
manipulating symbols on paper by hand. If you want visuals there is this[2].

[1]
[https://www.youtube.com/watch?v=foRPKAKZWx8](https://www.youtube.com/watch?v=foRPKAKZWx8)

[2]
[https://www.youtube.com/watch?v=UfThVvBWZxM](https://www.youtube.com/watch?v=UfThVvBWZxM)

~~~
DarmokJalad1701
This kind of makes me want to do Youtube videos about stuff that I know.
Procrastination is a b*tch though ...

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lmilcin
If this was true, this would be extremely fortunate for our civilization if we
can figure out not to kill ourselves in next couple hundred years.

Black holes offer about the most effective process to extract energy from
matter, orders of magnitude more efficient than fusion and it is not picky
about what kind of matter you put in.

You can put a Dyson sphere/swarm around small black hole and extract something
on the order of up to 40% of mass-energy equivalent of the falling matter.
With BH of the mass of Earth the sphere would be pretty small and easy to
make.

~~~
wcoenen
"Small" Dyson sphere is relative. The tidal forces at 50km from an earth-mass
black hole would still be pretty significant, so it probably couldn't be
smaller than that.

E.g. to pick something on the scale of a human, the difference in
gravitational acceleration (being GM/(r^2)) between two points at 50000 and
50002 meters from such a black hole would be about 1.6 G.

~~~
pas
What does that do? I mean you can walk in 1G just fine, so probably we could
go there, take a walk/sit there in 1.6G, and leave, right?

That means that if we put a 100km wide structure around it, we can service it
easily(?) even without robots.

~~~
Recurecur
It's not that, it's the difference in the gravity field between two points. On
Earth, the gravity field is very close to uniform.

Near a small black hole, the field changes rapidly and is very strong.

It's a "pull" effect, meaning that if you were oriented with your feet towards
the object as described by the GP, and 2m tall, your feet would experience 1.6
G more "gravity" than your head. It's a tidal force, literally working to pull
you apart (same effect causes tides on Earth).

If you were to fall into the black hole, at some point you'd be "stringified"
as that force stretched you into spaghetti shape.

~~~
Recurecur
So, I just calculated the orbital speed at 50 km. It came out to 282,378 km/s.
The speed of light is 299,792.5 km/s, so we're way into relativistic
territory.

Good luck having enough energy to build a structure around it! ha

~~~
lmilcin
Black hole with radius of 50km (escape velocity of speed of light at radius X
is definition of Schwarzshild radius for nonrotating black hole) would have
mass of 17 suns.

We are talking about small primordial black holes, in the vicinity of 10-50
times mass of Earth.

At 10 Earths, the radius is 8cm (!!!) and at 50 Earths it is still measly
44cm.

~~~
jychang
You can calculate the orbital velocity of an object from your distance from
the object. v=sqrt(G _m /r) where v is the velocity, G is the gravitational
constant, m is the mass of the black hole, and r is the distance from the
black hole. Any slower and it’ll fall in; faster and it’ll drift away.

For an object 10x the size of the earth, a satellite would need to travel at
282300m/s to orbit the object at 50km. He’s off by a factor of 1000, due to
units change from m to km. This is about a thousandth of the speed of light,
but about 4x the speed of the fastest manmade object.

If you weren’t at orbital velocity and were stopped instead, you’d be pulled
towards the black hole at 162600g of acceleration.

10_mass of the earth is a lot of mass. You get pulled towards the earth at the
rate of 1g when you’re (earth’s radius) distance away from the center of
earth, you’re dealing with an insane amount of force when you’re only 50km
away.

------
wrnr
A person should be allowed to dream sometimes, how freaking cool would this
be. A block hole right in our neighbourhood, even closer than Alpha Centauri.
Just imagine what kind of things can be learned from studying it.

~~~
127
Cool, and absolutely terrifying.

~~~
deepspace
Agree with cool, but not with terrifying. According to the paper, the mass
would be ~5-15 M⊕ at a distance of 300-1000AU, so not a threatening object in
any way.

~~~
dyslexit
And at 5M⊕ would have a diameter of about 9cm (or at 10M⊕ about the size of a
bowling ball).

~~~
echelon
How would we ever find that? Gravitational lensing of the background stars?

Do we stand a chance of uncovering it at all?

Edit: from the abstract,

> The observational constraints on a PBH in the outer Solar System
> significantly differ from the case of a new ninth planet. This scenario
> could be confirmed through annihilation signals from the dark matter
> microhalo around the PBH

Wild.

------
jug
The interesting bit about this to me, if this is happening in our immediate
neighborhood, is if it’s an indication that it’s actually a commonplace
phenomenon and further - an explanation of our missing matter to bind
galaxies.

A primordial black hole is not visible but it would interact gravitationally,
potentially with multiple of them clustering up in groups. Dark matter work
this way seemingly including clustering. Some galaxies seem to be made up by a
ton of the stuff, others by barely anything. Could that be due to random
chance; i.e. primordial black holes distributed unevenly since the cosmic
inflation era, much like the non-uniform distribution of matter at large?

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Causality1
_annihilation signals from the dark matter microhalo around the PBH._

What? Since when do we know enough about dark matter to say something like
that?

Even if we did the diameter of a black hole with the mass of a dozen earths is
smaller than a beach ball and the density of dark matter in the outer solar
system is one AMU for every three cubic centimeters. I'm skeptical we have
equipment sensitive enough to detect such a small number of interactions at
the distance of 500+ AU.

~~~
dumbfoundded
That part seemed more speculative.

I imagine most likely this is being used as an alternative hypothesis if we
search for the undiscovered planet and fail to find it. Simultaneously if we
become more certain about the observed gravitational effects, the probability
of a PBH would increase.

~~~
Causality1
There's speculation and then there's fantasy. Hypothetically you could build a
detector sensitive enough and a set of algorithms robust enough to observe the
vibrations of a single crystal and decode from them every sound being made on
the entire surface of the earth. Just because something is not physically
impossible does not make it realistic or keep it from being foolish.

------
jbotz
This paper has been on Arxiv since nearly a year ago; surprisingly it
apparently didn't make HN at the time, even though it did get a bit of
publicity then, including the MIT technology review[0]. Despite the sense of
humor evident in the included to-scale "picture" of a black hole, the paper is
quite serious... it may be speculative, but it's science. Also, the authors
are not the first to speculate that the OGLE microlensing events could be best
explained by a population of primordal black holes[1]. However, Planet 9 could
still just easily just be an ordinary planet.. the final point of the paper is
that we should look for both, the two requiring different search strategies.

Anyway, no matter how plausible or implausible, this post made my day. A mini
black hole, close enough that we could send a probe there? How cool is that?!

[0] [https://www.technologyreview.com/2019/09/30/410/is-
planet-9-...](https://www.technologyreview.com/2019/09/30/410/is-
planet-9-actually-a-primordial-black-hole/)

[1] [https://arxiv.org/abs/1901.07120](https://arxiv.org/abs/1901.07120)

~~~
T-A
> This paper has been on Arxiv since nearly a year ago; surprisingly it
> apparently didn't make HN at the time

[https://news.ycombinator.com/item?id=21078068](https://news.ycombinator.com/item?id=21078068)

------
rishav_sharan
We know that most heavy elements in the solar system come from a supernova
explosion. I have always thought that the 9th "planet" was the remnant of this
explosion and that this dead was the ancient binary companion of sun.

But yeah. Just fanciful thoughts here and no evidence, or even, good arguments
to back it up. :)

------
wyldfire
So would this PBH orbit the Sun?

> This scenario could be confirmed through annihilation signals from the dark
> matter microhalo around the PBH.

How long does this experiment take to construct, and to execute?
Days/weeks/decades?

BTW has anyone thought of getting in touch with the Lectroids? They're bound
to know more about Planet 9 than we do.

~~~
czbond
Planet X and the Annunaki. I really enjoy some of those 'out there' outer
space/alien documentary shows because even if untrue, they expand my mind with
ideas. Sort of the mind expansion of getting "high", without the substance
part.

They have an episode on Planet X and Annunaki and predict a black hole is
Planet X.

[https://www.amazon.com/The-
Annunaki/dp/B07JW478HG/ref=sr_1_1...](https://www.amazon.com/The-
Annunaki/dp/B07JW478HG/ref=sr_1_12?dchild=1&keywords=Anunnaki&qid=1596081149&sr=8-12)

------
thunderrabbit
Assuming its location can be determined, what experiments would you like to
try in its vicinity?

------
geoffmunn
I've been wondering - if the black hole was very small (I've read elsewhere
that it would be grapefruit sized), would it be possible to build some kind of
box around it so you couldn't accidentally go into the event horizon?

~~~
Sharlin
There's no material that you could make the box or shell out of that could
withstand the gravitational forces near the black hole. Even the gravity
_differential_ (tidal forces) is strong enough to rip anything apart (the so-
called spaghettification). And if you made the shell large enough (planet-
diameter), it would collapse into a spherical body just from its self-gravity,
never mind the additional gravity of the PBH.

~~~
geoffmunn
But given that it's so small, what would the gravitational forces be if you
made a shell with a 100km radius? Or pick a number - surely at some point it
might be practical to build something.

~~~
xingyzt
Size of the object doesn’t matter. Gravitational force depends on its mass and
how far away you are from the black hole. Standing on a shell ~6300 km above a
black hole with the mass of the Earth is equivalent to standing on the Earth
itself.

~~~
jborichevskiy
I have a hard time really accepting this concept given how unintuitive it is
to me but I found your comment helpful in that regard.

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xingyzt
If true, would this be the smoothest planet in the solar system?

~~~
Sharlin
I guess that _technically_ the IAU definition of planethood would not exclude
a primordial black hole, but I doubt that's the intended interpretation.

~~~
m_mueller
someone, somewhere who sat in the planet definition committee: "seriously?
again? what else didn't we think about...."

------
dmead
shouldn't this be something we can look for without resorting to additions to
the standard model?

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autokad
I really doubt Pluto is a black hole.

~~~
p1mrx
Yeah, anchoring a Mass Relay to a black hole would be too hazardous.

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smitty1e
Are we still allowed full verbal use of the spectrum, or should we say
"singularity" here?

Asking for Nathan Poe.

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sidcool
Since it's a black hole, it's gravitational effect would be at least as much
as the Sun. Is there evidence to support this tug of war?

~~~
317070
Black holes do not have more gravity per mass than any other object.

Put the other way, if the sun would suddenly change into a black hole without
losing a lot of mass, the earth would still follow the exact same orbit.

~~~
sidcool
That's true. My point was that if there were a solar mass object beyond Pluto,
it would have significant effect on orbits of all planets. But apparently PBHs
can be lighter

~~~
hliyan
I would suppose so. I would expect a lot more perturbation in the kuiper belt
if a solar mass object was lurking out there.

