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Comment by WaxProlix | Hacker News Reader

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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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.

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

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

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

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.

That makes sense, thanks!

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

yes. you can get a lot closer to it.

And you can pick how strong you want the gravity to be up to the event horizon

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.

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.

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.

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))

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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