
What If Planet 9 Is a Primordial Black Hole? - anpat
https://arxiv.org/abs/1909.11090v1
======
henearkr
Some potentially interesting uses for it:

\- definitive sink to send all of our nuclear waste (it would be the /dev/null
of the Solar System)

\- gravitational energy generator (limitless, until we have no more mass to
throw in)

~~~
dvdkhlng
I think the most obvious way to utilize a close-to-earth black hole is for
space-flight. By utilizing the Oberth effect [1] it is possible to gain extra
kinetic energy when firing engines very deep inside the black hole's gravity-
well (while performing a sling-shot). The Penrose Process [2] may also allow
for some kinetic energy boost.

Unfortunately both effects do not seem to offer the kind of multiple-orders-
of-magnitude gain required to make interstellar travel practical.

[1]
[https://en.wikipedia.org/wiki/Oberth_effect](https://en.wikipedia.org/wiki/Oberth_effect)

[2]
[https://en.wikipedia.org/wiki/Penrose_process](https://en.wikipedia.org/wiki/Penrose_process)

~~~
henearkr
Isn't there some way to stockpile the kinetic energy in several passes,
instead of only one slingshot, then finally use all of that for a final throw?

~~~
guelo
The limit is the escape velocity of the orbit.

~~~
hinkley
Not if you have time. And three bodies. If you've achieved escape velocity
from the 2nd body (eg, Earth) but not the first body (eg, the Sun) you can
keep swinging around the 1st body stealing speed off of the 2nd one each time.

Takes some careful orbits and a long time, but NASA does it all the time.

~~~
dvdkhlng
It would be very cool to have a binary black hole system somewhere in the
solar-system. I.e. two black holes in a tight orbit around each other at very
high orbital velocities. This could allow for very powerful slingshots to
bring probes to interstellar speeds.

~~~
hinkley
If you have the energy to get to black holes rotating around each other, you
probably can build ships that don't need to slingshot. 'Cept perhaps for
braking maneuvers.

------
dreamcompiler
Interesting hypothesis. Primordial black holes -- if they exist -- have a
tendency to be very small, and very small black holes evaporate extremely
quickly. The smallest ones couldn't have survived to the present day. An
Earth-mass black hole is big enough to have survived until now, but I was
under the impression PBHs that big were posited to be very rare.

~~~
subroutine
They are looking at several objects between approx. 1-20 Earth-masses
(presumably they were bigger during primordial creation; this is their size,
now, after 13 billion years of evaporation).

~~~
DuskStar
The bigger the black hole, the slower it evaporates. By the time you get up
into planetary sizes they basically don't change mass on the time scale of
stars. (From this wonderful calculator [0], a black hole with a mass of
6.0x10^24 kg - about that of the earth - would have a lifetime of ~5.75x10^50
years)

So if there's a 1-20 Earth-mass black hole out there, it hasn't evaporated
from anything appreciably larger to get to that point.

0: [http://xaonon.dyndns.org/hawking/](http://xaonon.dyndns.org/hawking/)

~~~
raducu
There go my hopes humanity could use this to convert mass to energy.

How long would we have to wait for it to evaporate before we could use it for
energy production?

------
ginko
Very interesting. I guess having a stable black hole like that in our solar
system would be an incredible opportunity to send a probe and check our
current understanding of physics.

If anything it'd be a lot more valuable then just finding another rock in
space.

~~~
est
The odds of finding blackholes everywhere in the universe suddenly increased?

~~~
Simon_says
Unless there's some unknown effect of having a black hole that makes life more
likely to appear when PBH orbit the star, in which case it might decrease
estimates of the frequency of life in the universe.

It's theorized that the existence of Jupiter played a role in clearing out the
inner solar system of kinetic kill vehicles, helping create a stable
environment for life to evolve on Earth.

------
rapsey
I apologize in advance for the dumb question. How is an earth mass black hole
formed? I thought they form from collapsing large stars.

~~~
opmac
Primordial black holes were formed during the big bang. That's the only way we
think such objects could form.

~~~
dr_dshiv
What mechanism could create them? By the time there was matter, I thought the
only pressure forces were ambient light and baryonic acoustic waves, neither
of which should be strong enough...

~~~
ben_w
You don’t need to matter to create a black hole, a Kugelblitz black hole is
made entirely of light.

~~~
dr_dshiv
Fascinating! But, if that's the case, the densities of light after the big
bang would have created endless numbers of these?

~~~
Simon_says
That's the idea.

~~~
dr_dshiv
My understanding is that things were incredibly homogeneous until the time in
which baryonic acoustic waves started creating compressed and expanded areas.
If there were black holes in that homogeneous soup, when things were so dense,
it seems like _everything_ would turn into a black hole or get sucked it.

Assuming light going into a black hole adds to its mass?

But assuming some light and matter didn't get sucked in and that's what
remains, then I can understand why black holes are proposed as a possible
answer to the mystery of dark matter.

~~~
ben_w
Wolfram Alpha says that the Schwarzschild radius for the mass of the visible
universe is bigger than the visible universe, so all I can say is something is
missing from my understanding (but I already knew _that_ ).

[http://www.wolframalpha.com/input/?i=schwartzchild%20radius%...](http://www.wolframalpha.com/input/?i=schwartzchild%20radius%20for%20mass%20of%20universe)

~~~
hnuser123456
What you might be missing is that the radius of a black hole is proportional
to its mass, so in fact, two half-visible-universe-mass-BHs would take much
less volume than a vis-universe-mass one, moreso for four quarters, etc.

Additionally, it's been proposed this universe is a 4d-spatial black hole. In
this case, you can re-consider aggregating the entire 3d-spatial universe as a
4d black hole, and the Schwarzschild radius calculation works again.

[https://www.nature.com/news/did-a-hyper-black-hole-spawn-
the...](https://www.nature.com/news/did-a-hyper-black-hole-spawn-the-
universe-1.13743)

~~~
ben_w
That’s isn't something I missed, as it happens.

Given some of the answers on the Physics StackExchange, I think my error is
using a static approximation like the Schwarzschild solution for a dynamic
situation — given my grasp of the Einstein field equations is “ooh pretty
symbols” this isn’t a huge surprise, though my lack of detailed understanding
is a personal frustration.

~~~
raattgift
I'm not sure what your question is, exactly, but a couple small addition to
your observation about black holes forming from collapsing radiation.

You're quite right that formation by collapse excludes Schwarzschild.

What you're heading towards is the Vaidya metric -- I don't know of any easy
overview of it, but one can think of it in terms of Schwarzschild.

Schwarzschild is a static solution: an eternal, time-independent, everywhere-
vacuum, pointlike mass. There is no radiation in Schwarzschild.

Vaidya has the same spherical symmetry, but the central mass is time-
dependent. The central mass can radiate away or absorb incoming radiation, but
with the condition that the radiation is spherically symmetrical.

Radiation in Vaidya is technical: it is a "null dust" \-- it follows null
geodesics and does not self-interact, so it shares properties with light
((classical) light rays have no charge, and don't clump; breaking down light
rays into particle-like elements leaves each element having no rest mass in
its own Local Inertial Frame).

As long as it is sufficiently close to a null dust, Vaidya can model
practically any collapse of radiation to a black hole. Unfortunately the
spherical symmetry is a hard constraint for the _exact_ solution, and is
easily broken by matter self-interactions. However one can certainly play
around with numerical approximations to the Vaidya solution, and if one does
that enough for a particular family of perturbations of the exact solution, an
intuition is likely to develop.

However, a kugelblitz from a collapsing null dust is within the gift of the
exact Vaidya solution.

You can take a "swiss cheese" approach to a very early expanding universe
filled with radiation (of the technical type above) peppered with Vaidya
regions which evolve. Vaidya is time-dependent and can deal with a tapering
off of incoming radiation as long as it's always spherically symmetrical, with
the result that eventually you have a "cheese" that is an expanding radiation-
filled spacetime and "holes" which are vacuoles which asymptote towards
Schwarzschild, and around each "hole" a thin-shell Israel junction. The
asymptotic behaviour is because the dust crossing the junction into the Vaidya
vacuole is (a) cosmologically redshifted within the "cheese" and (b) diluted
by the metric expansion of the "cheese". The radiation already inside the
junction at early times collapses onto the central mass. The two combine to
effectively shut off the incoming radiation, leaving behind a very close
approximation of the Schwarzschild vacuole.

(Aside: for massive dusts, one would use a Lemaître-Tolman-Bondi metric
instead of Vaidya, and one still runs into problems when breaking the
conditions of spherical symmetry and no-self-interactions in the dust. LTB has
a couple neat properties which are suitable for physical cosmology "swiss
cheese" models if one assumes that the radiation exiting the galaxy-cluster
"holes" is negligible -- starlight arriving in our galaxy cluster from distant
galaxies likely adds basically nothing to the mass of our cluster as a whole,
and our galaxy-cluster isn't losing much weight through its starshine out to
infinity; ditto for neutrinos and heavier particles).

In summary, primordial black holes are pretty easy if the very early universe
is filled with a homogeneous, isotropic dust -- radiation as a null dust, or
some massive dust, or even some combination. In the initial dust one expects
Jeans instability, and a power-law distribution for the total masses of the
resulting vacuoles. Plenty of small primordial black holes, fewer big ones.
What would cut off really small black holes originating along these lines?
Unknown. If nothing is found, this type of model loses its attractiveness.

There are several other models for primordial black holes, but they look a lot
less like the kugelblitzes you talked about a few comments above.

One other note, although I don't have space to develop it here: we can form
black holes from gravitational radiation alone, even in a spacetime with zero
matter. Gravitational radiation is not the same as matter radiation in the
technical sense above: apart from the mathematical details of which tensors
encode it (Riemann vs stress-energy) more physically gravitational radiation
strongly self-interacts, so we generally can't treat it as a null dust.

> personal frustration

GR doesn't really come easily to anyone, even (and sometimes especially) with
people who are mathematically gifted. Even Einstein and Hilbert struggled with
it, and in the last century only a small number of exact solutions -- none of
them better than a fair-enough-to-be-useful approximation to astrophysical
observations -- have been found. There have been many many many false starts.
Consequently one has to develop an understanding of where exact theory starts
to diverge from exact observation (and what one can do about it with arcane
tricks), and I don't think that's really possible without understanding the
exact theory first.

Lastly, I don't know what you're thinking about here:

> the Schwarzschild radius for the mass of the visible universe is bigger than
> the visible universe

The visible universe is not even slightly approximated by the interior part of
Schwarzschild metric. In particular, galaxy clusters are flying apart rather
than collapsing to a point. Additionally, there are no apparent tidal stresses
on galaxy clusters, even the ones at highest redshift: the Weyl tensor, which
essentially encodes tidal stresses, is nothing like a black hole solution (not
even under time-reversal wherein we get a "white hole", because we would then
see spaghettification in reverse: galaxies evidencing ellipsoidal early
galaxy-clusters with later galaxy-clusters becoming markedly rounder).

Event horizons can appear all over the place, including in perfectly flat
spacetime (for Rindler observers, for example). There are lots of very-not-
like-black-hole-spacetime settings in which there are global event horizons.
The salience is in what trajectories radiation and other types of matter take,
rather than that the matter-in-the-bulk can be partitioned by horizons that
produce Lorentz-contraction observables.

Going back to our swiss-cheese model above: in the far far far future the
cheese part is essentially empty of radiation because it has all diluted away
with expansion, while the holes are also empty of radiation because it has all
fallen onto the central mass. Both empty, highly curved spacetimes, but with
very different trajectories for the null dust: flying to infinity versus
flying into a point.

~~~
ben_w
Thanks! That’s an absolutely fantastic description which clears up several of
my misconceptions.

------
aruggirello
A PBH in our Solar System would quickly become a very popular target for
science, allowing us to test both relativity theory and quantum physics at an
unprecedented level. And who knows, we might even be able to use it as a
slingshot for interstellar travel, or as a gravitational lens - that would be
great!

Edit: so, how do we locate it exactly?

~~~
danbruc
It took New Horizons about ten years to reach Pluto and this black hole would
be on the order of ten times further away from the sun than Pluto. We would of
cause still point all kinds of instruments at it from Earth but we would not
be able to fly circles around it and throw stuff into it any time soon.

~~~
sliken
Actually I believe new horizons had a pretty primitive propulsion system and
achieved most of it's delta V from a sling shot around jupiter.

More recent solar sails and plasma based propulsion systems have achieves
significant improvements since then. If it was a priority I don't see why
would couldn't go 10x as far within in the next decade.

------
ryan_j_naughton
It blows my mind to think about sub-stellar mass black holes can exist. As I
was ignorant of their theoretical possibility, I thought the lower bound limit
to a blackholes mass was the Chandrasekhar limit [1] (or even greater as that
simply is the boundary between white dwarves and further collapse to either a
neutron star or black hole).

To learn that the conditions of the early universe could have create sub-
stellar mass black holes means there could be tons of small black holes out
there lurking in interstellar space.

What would happen if one of them got close enough to our Sun to begin
accreting gas from the Sun?

\- Could it eat the entire Sun and cause our solar system to go dark?

\- As it gained sufficient mass from the Sun, it would switch from orbiting
the Sun to them being a binary system. Would that destabilize the orbits of
planets?

\- Or disturb a ton of Oort cloud bodies and potentially cause tons of comets
and raise the risk of impact events?

\- At what point would the Sun's fusion stop?

\- Obviously this depends on the binary dance of the 2 bodies, but rapidly the
sun would be pulled apart.

\- Would there be X-rays and particle jets like a micro-version of a quasar?

I really hope someone is modeling this!

[1]
[https://en.wikipedia.org/wiki/Chandrasekhar_limit](https://en.wikipedia.org/wiki/Chandrasekhar_limit)

~~~
justfor1comment
Your premise is extremely unlikely to begin with. Let's assume somehow it
comes to pass, still the time scales of these phenomenon are on the order of
millions of years. If today a blackhole were to start eating the Sun, it would
take a couple of million years to finish that meal. Given a max human lifespan
of 120 years. You and your next 10^4 generations have nothing to worry about.

~~~
ryan_j_naughton
For it to eat the entire sun, you are correct.

But for it to destabilize the orbit of planets through the orbit of a binary
system, it is quite plausible once the blac hole has gained sufficient mass
for it to count as a binary system and not simply a planetary mass black hole
orbiting a stellar mass sun[1].

[1] [https://arstechnica.com/science/2013/01/binary-star-
systems-...](https://arstechnica.com/science/2013/01/binary-star-systems-make-
for-unstable-planets/)

------
tlb
How can you tell for sure whether something is a black hole or a regular rock
of the same mass, when you're too far away to resolve the size of the object
in a telescope?

Do you have to send a probe close and throw something in it?

~~~
sliken
Black holes are of course much smaller than anything else of the same mass. So
the gravitational gradient is much higher, which causes some detectable side
effects.

One possibility is gravitational lensing. The abstract also mentions detecting
"annihilation signals from the dark matter microhalo around the PBH". Not that
I understand that completely, and I don't believe (corrections welcome) that
dark matter annihilation by black holes has been confirmed experimentally.

I believe high energy particles are released by black holes as they consume
matter, so that might be another way to detect a small black hole instead of a
similar mass rock.

~~~
yread
Would evaporation give out any signal? Anti-particles could be a signal, no?

~~~
sliken
Hawking radiation is a signal, one so strong it's on the order of a large
nuclear bomb when a black hole finally winks out of existence.

However if it's big enough to be messing with orbits around neptune it's
likely pretty large. If it's large then the hawkings radiation isn't going to
be visible.

So if it's moon mass it's going to be 0.1mm and at 2.7 kelvin or so. Earth
mass is 9mm, but even colder. Generally for the orbital changes they are
seeing in a wide variety of objects it seems like the mass is even larger
still.

Even seeing pluto is hard, even a jupyter mass black hole is only 3 meters or
so. So no I don't think we could detect it via hawking radiation.

We might however see high energy particles resulting from the somewhat messy
feeding that black holes are known for. If we sent a probe it could plausibly
get close enough to directly observe hawkings radiation. Pretty amazing
thought, maybe we will get that lucky.

~~~
Smithalicious
>one so strong it's on the order of a large nuclear bomb

Surely a "large nuclear bomb" isn't very strong when you're talking about
things on black hole scale?

~~~
sliken
Sure, but that's when it's near zero size/mass.

Ah, looks like I was "a bit" low. Wiki claims 5×10^6 megatons of TNT when it
finally evaporates. Apparently about the energy the shoemaker level struck
Jupiter.

------
Apocryphon
I am reminded of a tale from Yukinobu Hoshino's science fiction anthology-
series 2001 NIGHTS, which had a then-tenth planet named Lucifer, with a
retrograde orbit and composed of antimatter- a former sun that collapsed not
into a black hole, but a gas giant made of antimatter, left over from the Big
Bang.

[https://arche-arc.blogspot.com/2018/11/mythcomics-lucifer-
ri...](https://arche-arc.blogspot.com/2018/11/mythcomics-lucifer-
rising-2001-nights.html)

Were such theories about a hypothetical planet beyond Pluto already existent
when this series was created, back in the mid-'80s?

~~~
clort
More science fiction: Larry Niven wrote about Jack Brennan, a human turned Pak
Protector who set up a small environment around a micro black hole to provide
gravity in the outer solar system. I don't think it was earth-mass size
though, more likely a singularity

~~~
ben_w
All black holes have singularities, what do you mean? (An Earth mass black
hole is pretty tiny — Schwarzschild radius order of a centimetre).

~~~
logfromblammo
A sub-Moon-mass singularity of 1e22 kg provides an acceleration of 9.8 m/s^2
at a radius of 261 km, while having a Schwarzchild radius of 1.5e-5 m.

The combined system of that singularity plus a hard spherical shell at radius
261 km (anchored by some sci-fi means that does not contribute significant
mass) has a density of 134 g/mL, 6 times as dense as pure osmium, but being
mostly empty space inside. And having a surface area of 856000 km^2, 0.6% the
land area of Earth. Such a body could retain an atmosphere and sustain
ecological cycles.

~~~
perl4ever
I had the question "how much mass does it take for a black hole to have 1G at
the same radius as its event horizon" and the answer appears to be about 10^12
times that of the Sun.

------
hsnewman
Wouldn't we see some radiation from dust falling in these primordial black
holes?

------
batarjal
There a lot of talk about the idea of an Earth mass Primordial Black Hole
punching through Earth and the effects of such an event. However, Planet nine
is predicted to be about five Earth masses, not one.

I don't have a physics degree, but how close would Earth have to get to such
an object before the Earth is within that object's Roche limit?

~~~
semi-extrinsic
The Roche limit depends on the density difference between the two bodies in
question. For a black hole (of any size), the density is basically infinite
compared to regular planets, so the Roche limit is basically zero. As in, less
than one meter.

The radius of the event horizon for a black hole even the size of Jupiter is
only 2.5 meters.

~~~
CodesInChaos
The gravity field of a spherical object only depends on its mass and not its
density/radius. So for the Roche limit only the density/radius of the object
at risk of breaking apart (called minor object on wikipedia) (earth) should
matter, not the one of the major object (black hole).

Wikipedia gives the formula d=R_m*(2m_M/m_m)^{1/3} which matches that
intuition.

~~~
semi-extrinsic
Ah, you're right.

So plugging in numbers you get 2.15 times the radius of Earth, which becomes
13 750 km.

For comparison, Earth-Moon distance is approx. 400 000 km.

~~~
perl4ever
Yes, but you don't have to be within the Roche limit to have substantial tidal
effects, which could destroy habitability at a much greater distance. Just
look at Io.

------
jcims
Wouldn't we expect such a black hole to slingshot dust and other material to
some non-negligible percentages of the speed of light? Wouldn't some of that
eventually hit the earth's atmosphere? What would that look like? Tunguska?

~~~
aruggirello
Not really, it would just form a (tiny) accretion disk. Dust and other
material would fall into it, emit gamma rays and be gone forever. It's a black
hole.

~~~
Simon_says
Or both. Accretion disks can shoot a tiny bit of matter out the poles while
most falls in. IIRC, the dynamics that cause this effect are not well
understood.

------
parliament32
The coolest part is the 1:1 scale figure on page 5 of the PDF. I don't think
I've ever seen a 1:1 figure in an astronomy paper before.

------
wruza
PBH is a greatest component of alchemy, a philosopher's stone. If you mix one
with a nearby neutron star in good proportions, you can get _lots_ of gold.

[https://phys.org/news/2017-08-theory-heavy-elements-
primordi...](https://phys.org/news/2017-08-theory-heavy-elements-primordial-
black.amp)

------
moconnor
My favourite part of this paper is the 1:1 scale image of the posited black
hole in the appendix...

~~~
eganist
Page 5 of the pdf
([https://arxiv.org/pdf/1909.11090v1.pdf](https://arxiv.org/pdf/1909.11090v1.pdf))
for anyone wondering.

~~~
tqkxzugoaupvwqr
Where is the scale bar? If the page is not printed in the right dimensions or
displayed 1:1 on a screen, the figure is wrong.

~~~
MPSimmons
The white horizontal edge bottom of the page is 8.5", and the white vertical
edge of the page is 11".

~~~
dkersten
Pff not when I print it on A4.

~~~
spacehome
pdfs literally include their actual size in metadata in the document itself.
Most printers will complain at least a little bit if you try to print it on
the wrong size paper.

This paper is :

215.9 × 279.4 mm (Letter, portrait)

------
sidcool
That would be a bit scary. Even a little wiggle in the orbit of such a body
could spell devastation.

~~~
c3534l
Not really. A black hole isn't any more dangerous than any other object with
the same mass. A black hole with the mass of Earth, for instance, would behave
exactly as Earth does, except when an object collides with it, it continues on
down into the center where it would hit Earth's crust if it were a planet.

~~~
sliken
If a black hole has the mass of the earth it's Schwarzschild radius would be
about 9mm. If it was disturbed from an orbit well past pluto it would have
quite a velocity when it hit earth.

The good news is it would pass effortlessly through the earth and not go to
the earth's core and stay there. After all where is the resistance going to
come from?

Now if the blackhole was brought to rest at the earth surface it wouldn't
reach escape velocity (11.2 km/sec), but anything coming from way past pluto
would likely have a much larger velocity.

The bad news is that having an earth skewer the earth at high speed is going
to cause some pretty crazy gravitational interactions.

Also some of the matter falling into the black hole from earth would be
consumed, but some of it would radiated out, no idea if that would be worse
than the physical disruptions or not.

~~~
KiwiJohnno
>crazy gravitational interactions

Thats putting it very mildly. Its actually quite an interesting thought
exercise - As this black hole approached earth its gravity would counteract
earth's gravity, meaning everything near where it "touched down" would
experience near zero-g as it approached. The opposite side of the earth would
experience greatly increased gravitational pull...

Then while the black hole was inside the earth, everything would be
experiencing close to 2G, then zero G for a bit for the poor souls on the side
of the earth where it exited.

Because of the fantastic amount of momentum it would have, and it would
certainly be travelling at well over earth escape velocity I would expect it
would travel straight through earth and keep going, not even coming close to
stopping inside earth.

The affects on earth would be very... bad.

I'd imagine it would trigger devastating global earthquakes, as well as epic
rock slides from the changing gravitational forces. Not to mention the
tsunamis from the oceans sloshing around like water in a giant bath, as well
as all the rockslides both above and below water. It would be a mess.

Assuming this object would be travelling at 100km/sec, and the gravity would
maybe begin to badly affect us when it was 50,000 km out from the surface of
the earth (?) then it would take about 16 hours to travel from this distance,
impact the earth and then reach this distance out the other side.

I'm not sure how much energy would be released by the matter being consumed by
the black hole - It might not be very much, after all its total gravitational
pull wouldn't be very high, but would be travelling at probably at least 50+
km/second - certainly more than 11km/sec (earth escape velocity) The matter in
front of the black hole might simply be swallowed up without much fuss, or if
there is any "resistance" to this matter falling down past the event horizon
I'd imagine it would be like a continuous series of nuclear bombs going off as
matter was flash heated to the point of spontaneous nuclear fusion before it
passed the event horizon.

The one good point to note in all of this is it would be extraordinarily
difficult to disturb this object from its object so it fell into the inner
solar system. After all, it has the same mass as earth, and we don't have to
be worried about random stuff disturbing earth's orbit.

~~~
debatem1
This would be an extraordinary sci-fi story and I would pay good money to read
it.

~~~
kyralis
Not _quite_ the same thing, but you might be interested in _The Forge of God_
by Greg Bear.

~~~
KiwiJohnno
I second that. Great book. The sequel, anvil of stars is pretty good too, but
its a different setting

