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Comment by smoyer | Hacker News Reader

Wouldn't the capture of a PBH by Sol be more like forming a binary system with a large distance between the two bodies? Intuitively it feels like capturing something with that much mass would result in our sun "wobbling" with respect to its expected path through the Milky Way.

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The Sun also wobbles because of the planets. Primordial black holes are (depending on theory of creation and path in later life) smaller than stellar black holes, maybe far smaller, in the order of lunar mass or even particle magnitude mass. So the wobble from a PBH will be overshadowed by that of Jupiter and the other planets. The sun doing a wobble of more than 30AU (what you would expect for a 1 solar mass PBH somewhere past neptune) with respect to the galaxy would have been detected ages ago, even the parallax measurements of yore are precise enough for that. From that fact, the magnitude of disturbance in other planets' and TNOs' orbits and the absence of gravitational lensing observations, our PBH should be in the order of planetary masses.

> particle magnitude mass

IIRC, that small would evaporate almost instantly[0]. I think you’re thinking of particle radius, which would be mountain mass[1].

[0] this formula says less than a plank time and is therefore wrong: https://www.wolframalpha.com/input/?i=5120π%28G%29%5E2%28mas...

[1] https://www.wolframalpha.com/input/?i=0.1nm*c%5E2%2F2G

Yes, if it would have particle mass at the time of the big bang it would have evaporated long ago. But all small black holes do evaporate (the smaller, the faster), so if PBHs were e.g. car-mass at the big bang, they could just be the right size for particle-masses right now (and gone in a few years).

1.7e11 kg is a pretty big car: https://www.wolframalpha.com/input/?i=%28%28%28h-bar%29%28c%...

And again, the predicted lifespan of a proton-mass black hole is significantly less than a plank time, which in turn is therefore a sign that — one way another — the maths doesn’t actually apply to that situation in the first place.

The minimum mass for a black hole is the Planck mass, approximately 21 micrograms. Such a black hole is a Planck length in radius, has a Hawking temperature of the Planck temperature, and will decay in Planck time, releasing Planck energy of 1.9 GJ (about the same as a tank of gasoline).

I think it depends on the mass of the object so captured? AIUI the mass of planet 9/PBH is estimated at 5-10 earth masses, which at that distance would not wobble sol (330,000 earth masses) very much.

Not all black holes have to be big; a 10 earth mass black hole would be quite tiny.

The paper actually depicts a 5 earth mass black hole at 1:1 scale, if you are curious.

Black holes in general are much smaller than people think. The event horizon of a 10 solar mass black hole is only something like 12 km across. That’s basically the head of a pin at space scale.

Supermassive black holes are on a whole different level however.

The most mind-boggling bit is the inverse relation between mass and density. Quoting[1]:

> A black hole of 387 million solar masses would have the average density of water and would be comparable to a giant water balloon extending from the sun almost to Jupiter.

[1]: https://arxiv.org/abs/1312.0340

A sphere of water extending to the orbit of Jupiter is a LOT of water.

Average density. What would the density be within the orbit of Mercury?

By nature of black holes: we don't know. For us it is pretty much a mass removed from this universe. For all we know, it has the average density everywhere. But the currently prevalent theory posits the density of the volume of the black hole is actually zero, with all the mass in a single point (singularity).

Interesting! Thank you for explaining.

If the event horizon of a 5-earth BH hole is several inches across as the paper states then doubling the mass/volume of said sphere would less than double the radius methinks ... maybe the figure is 12cm?

It's not any different (at large distance) than any other object of the same mass.

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