Comment by ryan_j_naughton | Hacker News Reader

Comment by ryan_j_naughton | Hacker News Reader

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ryan_j_naughton

It blows my mind to think about sub-stellar mass black holes can exist. As I was ignorant of their theoretical possibility, I thought the lower bound limit to a blackholes mass was the Chandrasekhar limit [1] (or even greater as that simply is the boundary between white dwarves and further collapse to either a neutron star or black hole).

To learn that the conditions of the early universe could have create sub-stellar mass black holes means there could be tons of small black holes out there lurking in interstellar space.

What would happen if one of them got close enough to our Sun to begin accreting gas from the Sun?

- Could it eat the entire Sun and cause our solar system to go dark?

- As it gained sufficient mass from the Sun, it would switch from orbiting the Sun to them being a binary system. Would that destabilize the orbits of planets?

- Or disturb a ton of Oort cloud bodies and potentially cause tons of comets and raise the risk of impact events?

- At what point would the Sun's fusion stop?

- Obviously this depends on the binary dance of the 2 bodies, but rapidly the sun would be pulled apart.

- Would there be X-rays and particle jets like a micro-version of a quasar?

I really hope someone is modeling this!

[1] https://en.wikipedia.org/wiki/Chandrasekhar_limit

My understanding is that mass in a black hole doesn't operate any differently at a distance. So if there was an earth-mass black hole in a tight orbit around the sun, it shouldn't have a significant impact on the orbits of objects in the solar system, even after it consumed the entirety of Sol's mass. Maybe Mercury would get messed up, depending on how tight the orbit is.

HOWEVER. Getting a black hole into a tight orbit - I don't know how that might happen. A black hole from outside the solar system would be coming in on a parabolic path past the sun. It would shoot right back out of the solar system.

If the BH managed to absorb enough mass from the sun, I would imagine that would throw off it's trajectory enough it could become captured into an orbit. On each subsequent fly by / through the sun, it would capture additional mass. This would reduce the period of the orbit.

In a situation like this, you have an increasingly massive object passing through the solar system in an irregular pattern. That can't be good.

Your premise is extremely unlikely to begin with. Let's assume somehow it comes to pass, still the time scales of these phenomenon are on the order of millions of years. If today a blackhole were to start eating the Sun, it would take a couple of million years to finish that meal. Given a max human lifespan of 120 years. You and your next 10^4 generations have nothing to worry about.

For it to eat the entire sun, you are correct.

But for it to destabilize the orbit of planets through the orbit of a binary system, it is quite plausible once the blac hole has gained sufficient mass for it to count as a binary system and not simply a planetary mass black hole orbiting a stellar mass sun[1].

[1] https://arstechnica.com/science/2013/01/binary-star-systems-...

I seem like that adds up. If somehow black hole consumed enough material from the sun so that it's orbit stayed inside the sun it would start growing exponentially.

First the outer layers of the sun are not very dense, but as a larger fraction of the matter for the black hole comes from the sun the slower (relative to the sun) the black hole would get. The slower the black hole is that closer it would come to the center of the sun. The closer to the center of the sun the denser the sun is.

The pressure of the sun is at least 10,000 times greater than the center of the earth which is 3,500 kilobar. Wouldn't the amazing gravity gradient and the 3,500 kilobar pressure result in a very well fed black hole that would double in mass within say a few days? Sure a accretion disk would form and start pushing back the matter at the north and south poles to reduce the feeding rate.

Sure black holes generally increase is size slowly, but they aren't usually inside a gas cloud of 1.4grams/cm^3 at a pressure of 3,500 kilobar and having an entire suns worth of mass to provide resistance to the accretion disk allowing for matter to fall in quicker.

I've heard numbers like you mentioned for atom sized size black holes that fell within the earth, but the main problem is that the likelyhood of swallowing an atom is so small that it grows incredibly slowly.

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