Primordial black holes seem likely since many models predict them. They’re not a fringe idea.

They are also a dark matter candidate, though this is more controversial. The ones we are seeing here would be huge ones but their masses could range the spectrum. Smaller ones would have evaporated already but there could be tons of asteroid, moon, and planetary mass ones around.

At least some dark matter may be black holes the size of a hydrogen atom with the mass of an asteroid, and similar objects. These would be incredibly hard to detect. The only way would be their gravitational effects on other bodies or weak anomalous radiation bursts when they rarely encounter matter.

They’re also awesome and weird. One could, for example, shoot right through the Earth. If it was small nothing might happen. Larger ones might cause seismic events or perhaps Tunguska type events due to induced fusion in the atmosphere. What was Tunguska anyway?

The most exciting thing is that if small mass PBHs exist and are common enough, we could find one someday in our solar system, maybe captured as a moon or in an asteroid belt. That would be close enough to send a probe to go look at it and do experiments with it. Being able to directly examine a black hole could be the thing that lets us “finish” physics. It would let us see conditions far beyond anything any imaginable terrestrial accelerator could ever produce.

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WaxProlix 3 days ago | parent | next [-]

I encountered a theory that 'planet x' might be such a PBH, explaining its ability to gravitationally impact post Neptunian bodies and its elusiveness. Would be incredibly cool to have something so exotic (or commonplace?) so close to home.

Cool idea on Tunguska - would such an explanation make predictions that we could verify? Radioactivity or changes to carbon in stones or the rings of local trees... An interesting thought.

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api 3 days ago | parent | next [-]

If planet X exists and is a planetary mass PBH it could unlock the universe in many ways. We could use it as a gravitational slingshot to fire probes at significant fractions of the speed of light out for flyby surveys of other solar systems.

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dwaltrip 3 days ago | parent | next [-]

It would make a better slingshot than a planet of the same mass?

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adgjlsfhk1 3 days ago | parent [-]

yes. you can get a lot closer to it.

	▲	positron26 2 days ago \| parent [-]
		And you can pick how strong you want the gravity to be up to the event horizon

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perihelions 2 days ago | parent | prev | next [-]

What's a plausible mechanism for that? There's no net change in speed in a two-body interaction. The conventional slingshot mechanism is a three-body interaction that involves a massive planet's rotation around the sun, but that's a very low speed for Planet 9—much slower than i.e. Jupiter.

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api 2 days ago | parent [-]

Oberth effect is one:

https://en.m.wikipedia.org/wiki/Oberth_effect

Fire a super high thrust engine during flyby.

If the PBH were in orbit around the Sun I don’t see why a conventional gravity assist would not work, but an Oberth effect maneuver would be more powerful.

	▲	perihelions 2 days ago \| parent [-]
		When you have a conventional gravity assist, the speed (magnitude) is unchanged in the coordinate frame of the planet that's providing the assist. All that happens is the velocity vector is rotated in that frame. Thus the usefulness hinges on arriving with a large relative velocity, to start with—a large vector to rotate, allows for a large velocity change. There's no heliocentric velocity in a slow-moving outer planet.

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BriggyDwiggs42 3 days ago | parent | prev [-]

Probably a dumb question but at those energies would we be risking de-orbiting the black hole with such a maneuver?

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api 3 days ago | parent | next [-]

It’s hard to visualize how weird and extreme black holes are.

A black hole with the mass of the Moon would be smaller than a BB but would have the mass and inertia of the Moon. It would be basically immovable as far as we are concerned. Chuck stuff at it all day and its trajectory change would be so small we probably wouldn’t be able to measure it.

	▲	BriggyDwiggs42 3 days ago \| parent [-]
		That makes sense, thanks!

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dtech 3 days ago | parent | prev | next [-]

No, for the same reason slingshotting on a planetary body now has no significant effect on it. The mass difference is too enormous.

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adgjlsfhk1 3 days ago | parent | prev [-]

no. if it has the mass of a planet, it has the inertia of a planet

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perihelions 2 days ago | parent | prev | next [-]

There's several large HN threads about that hypothetical,

https://hn.algolia.com/?query=planet%20black%20hole&type=sto... ("What If Planet 9 Is a Primordial Black Hole?" (+ title variations))

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zero_bias 2 days ago | parent | prev [-]

After the Chelyabinsk meteorite, we know that the Tunguska event has a mundane explanation: certain types of meteorites are prone to breaking up in the upper layers of the atmosphere, and Tunguska simply exploded the same way the Chelyabinsk meteorite did

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antognini 2 days ago | parent | prev | next [-]

Planetary and moon mass black holes are ruled out by gravitational microlensing surveys. Microlensing puts an upper bound on the mass of primordial black holes at ~1/5 the mass of Ceres.

	▲	zero_bias 2 days ago \| parent [-]
		These surveys assume that primordial black holes are distributed uniformly across galaxies, but this may not be the case if they form small dark globular clusters in the outskirts of galaxies. By small, it is meant that with a total mass of 100–1000 M, they would produce no significant lensing, and by dark, that such clusters would consist entirely of black holes.

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thehappypm 2 days ago | parent | prev [-]

It would be an incredible thing if you could build a device that emitted tiny black holes over and over and over again, just strip out horizontal lines of matter