What if Planet 9 is a Primordial Black Hole? | Hacker News Reader

What if Planet 9 is a Primordial Black Hole?

link.aps.org

129 pointsmxcrossb119 comments

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

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.

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.

PBS SpaceTime

Half of their stuff goes over my head. And I have to watch videos multiple times, but very interesting channel indeed.

This channel makes me feel dumb. It all goes over my head but he is explaining it like he is teaching a group of 10 year olds.

Its good to remind yourself you are not the smartest person in the room.

I'm absolutely abysmal at even basic math (arithmetic, algebra), let alone anything involved in theoretical physics, but I've never had any issue understanding that channel because as far as I can remember, they don't actually bring up any equations or other math. Maybe there are some math-y episodes I haven't seen, dunno, but I was under the impression it was targeted at laymen and designed to be as accessible as possible, like many PBS things. I've learned a ton from it and have never felt they went too advanced for regular people.

Now, minutephysics or 3blue1brown? Love their videos, but most of them go way over my head. I find PBS Space Time exponentially easier to follow than minutephysics. Just saying this in case anyone anyone were to otherwise be discouraged from checking it out.

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

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

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

Anton Petrov https://www.youtube.com/user/whatdamath goes through astrophysics papers quite often. The channel is pretty cool if you like space in general.

thank you for this, i laughed out loud

I love it. I found it actually useful to get a physical scale understanding of the size of one.

It was the very first thing I wondered when I read the title.

My guess is that they wanted it removed since it is more useful communicating outside the field and that printing out that figure would take a lot of ink. Even if the journal doesn’t do much actual printing, I know many researchers that prefer to print out articles before reading them.

could have replaced it with a white filled circle though, or a grayscale version that uses much less ink.

Was it removed because “exact scale” only makes sense if the article is printed on a specific paper size, which isn’t specified?

Objects in a PDF are specified in points, which is a physical measure (1/72 of an inch, or 0.3528 mm).

And printer drivers often have a "fit on page" feature enabled.

... not that A4 vs US letter would be a huge percentual difference, for this image.

I too rushed to check if the figure survived to the referees' comments. Alas, what a pity!

> The associated DM halo however extends to the stripping radius ∼8AU, this would imply a DM halo which extends roughly the distance from Earth to Saturn (both in real life and relative to the image).

Well, how can the article authors know we're reading this on Earth and not some other planet?

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.

"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.

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.

You can't "walk" in this 1.6G. You'd experience it while floating freely in space. And the stress on your body would be tension instead of compression. I guess it would feel like hanging upside down, except that your blood rushes to both your feet and your head.

The difference in gravitational acceleration between two points, not the gravitational acceleration at the points.

I can't imagine we will be building things like that manually.

By the time a swarm of robots will process blueprints and take raw materials and construct the structure semi-autonomously or even completely autonomously.

The structure would most certainly not admit human presence. It only has to redirect radiation to where it is useful (maybe send a starship on a laser beam, for example).

Maybe ships will need laser power . It remains to be seen if near-C travel is at all feasible, due to radiation/erosion from impacting interstellar material. If not, a ship can easily carry its own fuel, it will just be a generation/hibernation ship.

Or, perhaps our extropian descendants won't care about travel time. ;-)

OR, maybe the black hole could power a hyperspace portal or something equally exotic...

Regardless, having a compact object nearby to play with would be fun!

One thing will have to change if we ever can travel at relativistic speeds or have function cryogenics (or anything similar) will be markets and investments, just think of putting even a relatively small amount into a yielding investment even an index fund and going to “sleep” for 200 years not only would you get immediately to benefit from 200 years of advancement but you’ll also get to benefit form 200 years of what effectively is compound interest.

I really wonder what will the societal implications of such a path being possible are going to be.

There are plenty of people that will be willing to push a pause button right now to be able to wake up in a world where you could buy a ticket to mars, add to that the quite likely possibility of becoming a millionaire if not a billionaire if you put enough money into any inflation beating investment and you get quite the incentives package.

Sure you might never wake up because the world is going to end in a nuclear holocaust or your icebox will experience a kernel panic 17 years into your suspended animation but overall this quite likely be a much lower possibility than dying prematurely today.

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.

BTW, regarding "standing" in this gravity field...

If there were a structure to stand on, rather than you being in orbit, the force of gravity would be about 160 billion times the force of gravity on Earth (~1,594,000,000,000 m/s^2).

That's for a 10,000,000 Earth mass black hole at 50 km.

If Planet 9 were indeed a black hole its significance would surely be that prevalence of lower mass black holes is orders of magnitude more common than currently known.

This particular one may not be practical for energy generation, but it would be obviously worth looking for others of more useable mass.

Why not? There's various approaches to extracting energy from a black hole of any size, and being smaller generally makes things easier.

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

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.

Wasn't Recurecur talking about a 1 mass-of-earth BH, with a dyson-sphere-like structure 50 km away from it, rather than a 50km black hole? I don't understand the physics here myself, just checking if you haven't missed his point.

You can calculate the orbital velocity of an object from your distance from the object. v=sqrt(Gm/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.

10mass 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.

The point of using primordial black hole is that it can be very small (in the range of centimiters for its Schwarzshild radius). You can then calculate safe distance from the black hole where the gravitation and gradient generates acceptable forces for your structure. This is of course much more than the centimiters given but still very small compared to astronomical distances you would need to encircle entire Sun.

Another point of using black hole is that you decide its output (by deciding how much matter falls in) which greatly helps building dyson sphere. You can't modify Sun output.

The schwarzchild radius doesn’t really matter for engineering purposes. Think about the earth; when you are standing on the surface, 1 radius away from the center of mass, you experience 1g of acceleration. Imagine how much more force you experience if you were closer to the center of mass, and replaced that mass with a black hole.

It’s hard enough to build a dyson sphere at 1*earth radius, the amount of forces acting on the structure at 50km radius is insane.

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.

Cool, and absolutely terrifying.

On the flip side, you wouldn't be any less dead if you fell into Jupiter and that's a lot easier to get to

Falling into Jupiter is an eye-opening experience tho, esp. if you fall into the "big eye" (pun intended).

Just the sheer massive size of a turbulent, harsh planet that covers all the horizon... The world becomes flat much faster near such a planet.

We're already on a planet that covers the horizon; some say there's more beyond the horizon. Would Jupiter look subjectively bigger, if you couldn't also see the Earth for comparison?

I guess it wouldn't, but it probably would be a much longer fall until either pressure is crushing you or your heatshield fails.

Crossing into the event horizon of a black hole has a very profound poetry to it in that you will never be able to communicate back your findings. I find the romance of the destruction of knowledge very intriguing - falling into a black hole is similar to shooting a mathematician in the head in that regard.

Great way to go, all things considered.

Yes you'd be dead.

I am prepared to put down MTW and dream

Except that with a black hole this small, you'll be pulled apart by the tidal forces long before you even reach the event horizon. If you want to make it across the event horizon intact, you need a much larger black hole, like the one in the centre of our galaxy.

Which conversely means that supermassive black holes allow you to pass through the event horizon unharmed.

Not sure if you meant it that way but information actually can't be created or destroyed in quantum mechanics. This is known as the no-hiding theorem [1] which has been experimentally proven and which leads to the black hole information paradox. [2]

1. https://en.wikipedia.org/wiki/No-hiding_theorem

2. ttps://en.wikipedia.org/wiki/Black_hole_information_paradox

So, is it kind of like a cryptographic hash function?

Somewhat. Like a hash function, it projects some large state space (the hash input/you) onto a different state space (hash digest/2d-surface of a BH). But a hash digest actually has lost information, i.e. it's guaranteed by pigeonhole principle that some two inputs have the same digest, which means you lose the information about which one it was. Whereas a black hole scrambles you, but none of the "bits" are actually erased, just smeared out and projected onto the surface.

In what sense? It poses no threat to us.

This is not scary. Yes the event horizon will expand the more matter falls into its gravity well, but otherwise a black hole acts like a normal planet orbiting the Sun.

Is there a theoretical limit on the minimum mass of a black hole?

It's counter-intuitive to me that something could get dense enough to form a black hole and not have more mass than the Sun, or at least enough mass to steal Pluto.

Yeah, the event horizon the size of a Planck unit.

Again, unless you get super close, it acts identical to a regular planet of the same mass.

Do we actually have a theory that predicts a theoretical minimum size of a Planck length?

Black holes are an artifact of gravity and the theory of relativity. The planck unit comes up as a result of quantum mechanics. Without a working theory of quantum gravity, we do not have a description of what happens in scenerios where both theories are relevent.

There's an approximation that's expected to hold, semiclassical gravity, where both theories are relevant. This is the background against which calculations of Hawking radiation and black hole evaporation are made.

I don't think so. With the various colliders there was talk of black holes but they would be small and super short lived. The question may be more interesting if we talk about stable black holes at least by human time scales.

Small black holes evaporate fairly quickly from Hawking radiation. The rate increases dramatically the smaller the mass. Since the only time primordial black holes would have formed was in the Big Bang there is a lower limit based on the black hole surviving 13.8 billion years. iirc the number is on the order of 10^12 kilograms.

Well since the mass of the Earth is ~ 5x10e24 seems we're still in the realm of possibility

Yes, Tolman–Oppenheimer–Volkoff limit would estimate a star around 1.5-2.5 sun masses could result in a stellar black hole.

We don't know much about stars collapsing, and the mass wasted in the process to have a more accurate number at this point.

That's only true for black holes created from the collapse of stars. Primordial black holes would have been created in the early universe and be much much smaller.

Ted Kazinski was concerned that high energy particle physics would create a blackhole large enough to become self sustaining. If I remember correctly, high energy physics creates tiny black holes during particle collisions, but they "evaporate" faster than they can absorb mass.

So to answer your question with my limited understanding, yes, there's a lower limit to stable black holes. Beyond the critical mass, they simply evaporate in a matter of seconds.

> Beyond the critical mass, they simply evaporate in a matter of seconds.

For a sense of scale, a black hole with a mass of 2.2 * 10^5 kg will evaporate in roughly 1 second. A 1 kg black hole would evaporate in roughly 100 attoseconds.

"Evaporate", meaning "vaporize everything for miles around", right?

And for the 10^5 kg one, "vaporize the entire planet" if one is nearby?

Schwarzschild radius and the Compton wavelength both combine to say that in theory, the minimum mass required for a quantum black hole is the Planck mass(ish). They might not be possible or they might require more energy than what we can produce today.

https://link.springer.com/article/10.1140/epjc/s10052-017-54...

This article looks interesting and will probably go into more detail if you are wanting to know more. I will read it tomorrow. Great question!

It's a little complicated... Hawking radiation establishes the minimum size of stable black hole in our universe. If the temperature of the event horizon is higher than the temperature of the CMB, than a black hole will lose mass by Hawking radiation, which raises is temperature further and so on. Using a knowledge of expansion of the universe (and thus the cooling of the CMB) plus the basic math of hawking radiation, you can pretty easilly calculate the minimum size of a black hole that you should see.

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.

wait, 2020 is not over yet...

I assume M-plus means Mass of earth?

Yes. Similarly, M with a dot in the circle is the sun.

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

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.

its hung about out there for a few millennia, I doubt it's going to be a problem for us in a the entire lifespan of the human race...

A fun way to not be terrified -

"What would happen if the Moon were replaced with an equivalently-massed black hole?"

https://what-if.xkcd.com/129/

Also need one for lava, but looking up quicksand is one of the few that might make it worse for you.

No more terrifying than any gravity well.

Yes the cool thing about black holes is that the terrible things inside them can never get out. It's like a prison-planet with the most terrible criminals in there, but they can never escape. Laws of Physics prohibit that

Less scary than most, since the paper predicts it would be somewhere in the range of 5cm diameter. Very difficult to hit, even if you were trying.

Acrophobia, anyone?

Cool, and absolutely terrifying.

Sibling comments are questioning this.

But ...

what if the black hole's orbit around the sun isn't something approximately circular?

What if the orbit resembles a long-period comet? Highly elliptical? What if it puts in an appearance near the sun every 100,000 years? How would we know? What in the historical record could we study?

They have a pretty good idea of what the orbit would look like to explain the eccentric orbits of TNOs. It's not circular, but it's not that eccentric: https://en.wikipedia.org/wiki/Planet_Nine#/media/File:Planet...

somehow it never occurred to me that black hole could actually orbit what does it mean for the event horizon to move one side is fairly predictable as space gets swallowed as the black hole advances but the space at the rear side would just sort of pop out of the event horizon or be stretched from the previous position

what would happen to the space behind a black hole moving at relativistic speed?

Bet there some paper I cannot understand about it somewhere

Much less terrifying than a 5-Earth-mass planet. Such a planet would be quite easy for a spacecraft to crash into. But if all that mass is the size of a tennis ball, it would be very easy to miss. Unless your aim was perfect you'd just slingshot around it.

it's not more terrifying than a planet or a sun, it's just more dense and invisible to the eye :)

I don't see how this is more terrifying than, say, have a sun in the neighbourhood.

Seconded.

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?

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.

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.

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.

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-...

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

> 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

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. :)

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.

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...

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

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?

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.

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.

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.

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.

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

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

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

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

I really doubt Pluto is a black hole.

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

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

Asking for Nathan Poe.

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?

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.

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

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

This paper postulates a black hole with 5-15 times Earth's mass, which is considerably less than the sun.

I see. That's lighter that Jupiter. Do PBHs have singularity like regular black holes?

If PBHs are real, then they should be identical to "regular" black holes, just smaller.

Wikipedia says "Physicists are undecided whether the prediction of singularities means that they actually exist, or that current knowledge is insufficient to describe what happens at such extreme densities."

incredibly hard to conceive 15 Earths shrunk down to the size of a grapefruit! fascinating

> While an M⊕ object is too light to be an astrophysical black hole formed by stellar collapse, PBHs arise from over densities in the early universe [15, 16] and as a result can be substantially lighter than M .

Normally black holes do have at least the mass of ~1.4 times our sun because they are formed by stellar collapse. That is why here they are considering Primordial Black Holes which a) formed differently and b) radiated some of their mass away

I guess I mixed PBH with a regular black hole.

Reply on news.ycombinator.com