
If Planet Nine Is a Tiny Black Hole, This Is How to Find It - lelf
https://www.discovermagazine.com/the-sciences/if-planet-9-is-a-black-hole-this-is-how-to-find-it
======
qubex
This is both a very obvious and a very interesting proposal: locate an
apparent source of gravitation by observing the gravitational effects it has
upon a fleet of small (almost point-like) craft.

However, there’s something that niggles me.

If it’s a primordial black hole that has been captured by the sun, why is it
(as far as I understand) apparently in the same plane (ecliptic) as all the
other major bodies in the solar system? It makes sense for all planets _&
cetera_ to be in the same plane because they all originated in the same
swirling (flat) planetary nebula... but a captured object could potentially
orbit at any inclination (and, considering how massive it is, I doubt it would
be coerced into being coplanar by comparatively minor intersections with light
bodies such as those present in the Oort Cloud, which are arranged roughly
spherically anyway).

~~~
7373737373
If SpaceX can afford to put a car into orbit around the sun as a publicity
stunt during rocket testing, I wonder how much this would cost.

Edit: Probably around $100MM:
[https://www.spacex.com/about/capabilities](https://www.spacex.com/about/capabilities)
for the launch, several $100MMs for the (space based?) accelerator and
spacecraft, and then a few more for operations and research. Yeah, $1 billion
as described in the article seems like a good estimate:
[https://arxiv.org/pdf/1805.01306.pdf](https://arxiv.org/pdf/1805.01306.pdf)

Since knowledge arising from this experiment will probably only be useful as a
common good, there aren't really incentives for this to happen, unless
governments decide to go through with it.

NASA's 2020 budget is only $22.6 billion:
[https://en.wikipedia.org/wiki/Budget_of_NASA](https://en.wikipedia.org/wiki/Budget_of_NASA)

The Apollo program cost $153 billion:
[https://en.wikipedia.org/wiki/Apollo_program#Costs](https://en.wikipedia.org/wiki/Apollo_program#Costs)

Okay, this thing is ten times further away from the sun (400 AU) than Pluto
(40 AU), 80 times the distance of Jupiter from the sun (5 AU), very far.
Still, it's much closer than our next closest star, Proxima Centauri, which is
265612 AU away, or over 660 times the distance of Planet 9.

~~~
ramraj07
If we get actual evidence supporting that there could indeed be a black hole
within our solar system, that seems like a true opportunity to advance our
knowledge of physics. The potential to study a real black hole up close seems
worth massive investments given what kind of breakthroughs could he possible
from it.

~~~
codethief
Yes and no. I mean, how would you study a black hole of radius ~5cm without
getting really close? Consider that:

1\. Time dilation dictates that it would take ages for any instrument to get
near the black hole, conduct measurements and send its results back to Earth.

2\. Time dilation also means you would never see anything fall into the black
hole. So studying the inside of the black hole is out of question, too. But
arguably this is what would be required to make any breakthroughs in our
understanding of black holes.

3\. Tidal forces near the event horizon of such a tiny black hole would have
catastrophic consequences[1] for anything (human or object) that gets too
close to it.

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

~~~
7373737373
Wouldn't this mean that these tidal forces may (have) cause(d) highly
accelerated matter to shoot through our solar system?

~~~
codethief
I assume you're referring to astrophysical jets[1] that are common with SMBHs.
I'm afraid, though, I'm not familiar enough with jet physics to answer your
question. A priori, the tidal forces only cause infalling matter to get
shredded (and heated up). Whether a proper accretion disk together with a
(significant) jet can form, given the size of the black hole, I don't know.

On a related note: It'd be interesting to see in how far such a small BH would
_actually_ be capable of "sweeping [its] local neighbourhood clear of objects"
like planets are (as was claimed in some comment above). Only then would I
expect a significant accretion disk and, possibly, a jet. The reason the
answer to this question is not obvious (at least to me) is once again the tiny
size of the BH: How would anything manage to fall into the black hole, given
that it's so tiny and, thus, very easy to miss?

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

------
knzhou
When the original paper about planet 9 being a black hole came out a year ago,
everybody thought it was a cool-sounding but just completely wild idea -- both
physicists and pop science articles. Not impossible by any means, but not
within the top 250 new physics ideas in plausibility. Then the discussion died
down. I figured it would end up being one of those forgotten but cool gems.

Then the most authoritative string theorist in the world, who hasn't worked on
anything relating to experiments in decades, suddenly writes a very short,
simple, single-author paper to use this idea to promote Yuri Milner and Mark
Zuckerberg's space probes. Now it's the conjunction of _two_ crazy ideas, and
it's really taking off. I just don't know why this is happening, or what to
make of it.

~~~
qubex
Of course if there was enough dust in interstellar space to reflect a laser
beam (as happens when we see the traces of glowing beams shone through carbon
dioxide gas) we could just prove the heavens with laser beams until suddenly
we saw a beam bend. Unfortunately that isn’t the case so we have to fling
stuff at it and see if they veer off track.

~~~
robocat
You can’t easily linearly scan space for a moving target, among other show-
stopper problems.

~~~
qubex
No, but since using satellites on trajectories you hope will be perturbed is
essentially ‘scanning’ anyway, it would be preferable to do it with a light-
beam than with physical objects.

------
tejtm
A well characterized black hole in our solar system would be our (one way)
ticket out of here. The velocity we could gain would be limited to the vessel
integrity. And thanks to math, being able to choose approach trajectories
means we could go anywhere.

Well worth looking real hard just in case it does exist.

~~~
T-A
Assuming that you're thinking of the Oberth effect, it's no miracle worker. In
the (non-relativistic) impulsive burn approximation you gain a speed factor
sqrt(1 + 2*v_escape/delta_v) [1].

With v_escape = c and delta_v = 10 km/s that's a factor 245, for a final speed
of 2450 km/s. That will get you to Alpha Centauri in 535 years.

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

~~~
tejtm
Alas I am not well enough educated on the subject to have ever come across
that particular term. (I do wish KSP was available on *nix) And I do see your
point as it pertains to solid matter.

How ever as a layperson I am at least vaguely aware that there are hyper
velocity stars, some observed w/delta-v close to 10% of C if memory serves ...

Which would indicate to me that having a gravity well we can delve arbitrarly
deep into has profound implications compared to gravity wells where it you go
too deep you run into clouds or dirt.

For example the vessel could be paired, co-orbiting with a sacrificial mass
that is dumped into the hole while the vessel leaves at at factor greater than
a few hundred.

~~~
T-A
The fastest hypervelocity star I can find with a quick bit of googling is
S5-HVS1, at 1700 km/s [1], slower than the hypothetical black-hole-accelerated
ship example.

You can't delve arbitrarily deep into a black hole's gravity well if you want
to get something back; you need to stop at the horizon, where the escape
velocity equals light speed.

Dumping sacrificial mass into the hole is built into the Oberth effect
scenario: the ship's extra speed (relative to the hole) is matched by a lower
speed for the ejected reaction mass, which therefore falls toward the hole.

I suppose you could imagine something like a circular accelerator built around
the hole and powered by dropping masses into it.

[1] [https://newatlas.com/space/fastest-star-hypervelocity-
ejecte...](https://newatlas.com/space/fastest-star-hypervelocity-ejected-
milky-way/)

------
tjpnz
The article talks about a scattergun approach and that detection would require
getting within a few dozen AU of the object. That would require a huge number
of probes and a means of coordinating beams spread across multiple continents.
Wouldn't it be easier to determine the objects rough position ahead of time?

------
austincheney
Could a black hole orbit our tiny yellow dwarf sun? Even if it were tiny for a
black hole it would be hard to imagine that it would have less mass than our
sun. If it had more mass than our sun wouldn’t our system be a binary system,
and if so it would suggest a black hole is not really a satellite of the sun.

How small can a black hole be and still qualify as a black hole? I guess a
better question is what is the least massive black hole observed and what is
the smallest current theories allow?

I have no idea what I’m talking about so all of that is probably wrong. I am
just not able to wrap my head around it.

~~~
jniedrauer
> How small can a black hole be and still qualify as a black hole?

There's no theoretical lower bound on the mass of a black hole. Once matter
has ceased to repel other matter, it will continue to fall inward. A violent
impact could generate the forces necessary on a small scale to create micro
black holes. Black holes do seem to slowly lose energy though, so small ones
would not live very long[0]:

[0]:
[https://en.wikipedia.org/wiki/Micro_black_hole#Hawking_radia...](https://en.wikipedia.org/wiki/Micro_black_hole#Hawking_radiation)

~~~
codethief
> so small ones would not live very long

Actually, this is completely unknown. While Hawking radiation is a well-
established effect in the semiclassical limit (read: when gravity is weak), we
don't really know what happens when the Schwarzschild radius reaches scales
where tidal effects become noticeable already near the event horizon and,
eventually, quantum gravitational effects (supposedly) take over.

In fact, this whole question lies at the heart of the black hole information
paradox. Do black holes evaporate completely? Or will evaporation stop at some
point, leaving behind a black hole remnant?

------
sbierwagen
From the paper:

>However, the .01 c mission sketched in [10] has a spacecraft mass of only 6.6
mg, divided between the sail and the payload. Multiplying this by 100 while
keeping the sail mass fixed leaves a payload mass of barely .65 grams.

Oof. That's not a lot.

------
mmmBacon
Wouldn’t a black hole that small evaporate due to Hawking radiation?

~~~
Dylan16807
No. At even 1% of Earth's mass, a black hole's already down to 2 Kelvin of
output, across a surface as big as a grain of very fine sand. It would take
more than 10^44 years to evaporate on its own, but since the cosmic microwave
background is 2.7K it would actually grow over time.

For some other data points, a thousand ton black hole would explode in 84
seconds. A million ton black hole would last 2600 years as an unimaginably hot
point five hundred times smaller than a proton. A billion ton black hole would
last 2.6 trillion years as an unimaginably hot point twice as big as a proton.

------
lenkite
Discovering a mini black-hole at the boundaries of our solar system would also
mean discovering a lovely power source just waiting to be tapped, right ?

~~~
the8472
Black holes themselves do not generate power. If they have an accretion disk
disk - i.e. they're feeding on matter - then that can be quite good at
converting mass into radiation which you can then harvest. But you could also
do that with the radiation of a more easily accessible mass-energy converter:
the sun

~~~
gorgoiler
You are both talking about the _Penrose Process_ , or perhaps only one of you
is?

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

------
Insanity
It's such an unimaginable thing. 5cm across, yet "super" massive (not SMBH).

------
baja_blast
Planet Nine is a Mass Effect Relay

------
sandworm101
>>> about 5 centimeters across. Consequently, it is almost impossible to spot
with a telescope.

If it is 5cm across, then it should be rather bright (Hawking radiation).
While still tiny, it wouldn't be black.

~~~
speakeron
This calculator[1] indicates that the temperature for a 2.5 cm radius black
hole should be 0.007 K. So below the temperature of the cosmic background (2.7
K).

[1] [https://www.vttoth.com/CMS/physics-notes/311-hawking-
radiati...](https://www.vttoth.com/CMS/physics-notes/311-hawking-radiation-
calculator)

~~~
alextheparrot
Does this mean that the cosmic background could be adding energy to the
blackhole (Maybe helping with stability?)? I'm not sure how temperature and
heat transfer would work in this context.

Edit: For those interested, there is an entire article on "Black Hole
Thermodynamics" on Wikipedia
([https://en.wikipedia.org/wiki/Black_hole_thermodynamics](https://en.wikipedia.org/wiki/Black_hole_thermodynamics))

~~~
qubex
You are correct: a black hole whose Hawking temperature is less than that of
the cosmic background radiation is “colder” than the space it is immersed in
and consequently absorbs energy (and mass-energy equivalence) from the heat
bath it is immersed in. The easiest way to envision it is that the black hole
swallows up photons of the microwave background radiation and is emitting less
energy as Hawking radiation, leading to a net imbalance in the black hole’s
favour.

Of course eventually the expanding universe will drop the temperature of the
cosmic background radiation further and the process will reverse.

------
jonshariat
So could something like Starlink be used to detect things like this?

~~~
qubex
No. Who implied that?!

Starlink will be a network of very near earth communication satellites. This
black hole, if it exists, is far far further out than Pluto.

The probes wouldn’t be even remotely reminiscent of Starlink satellites
either.

~~~
jonshariat
It was a question. I am not an expert and so I asked.

Also I said "like this".

If you have a mesh of satellites around the earth and used that data along
with other astronomy tools, I'm sure there are other things we can learn.

------
nafey
Does anyone else feels like that projects like these should be avoided unless
we are certain that our corner of the galaxy is not part of a larger dark
forest? The consequences of such actions can be catastrophic. If we do send
out probes they should be designed to disintegrate after some fixed period of
time.

~~~
NikolaeVarius
I too choose to not use anything that radiates electromagnetic waves on the
off chance aliens detect them.

~~~
nafey
Electromagnetic waves will die off and become indistinguishable from
background radiation after a certain distance. Not so much for physical
objects.

~~~
maxander
The famous Douglas Adams quote is relevant: "Space is big. You just won't
believe how vastly, hugely, mind-bogglingly big it is. I mean, you may think
it's a long way down the road to the chemist's, but that's just peanuts to
space."

The odds of an alien civilization picking up one of our probes, even if we
send out thousands of them, is preposterously small. Imagine trying to such an
object if it had been left floating at an unknown point in the ocean; now
consider how much bigger interstellar space is than Earth's oceans.

~~~
gorgoiler
“It’s like trying to find your car keys in a field using a pair of
binoculars.”

I cannot remember where this quote came from, but I think it was something to
do with a missing Russian cargo vessel in the Atlantic.

