[elorant]

And the closest Neutron star is 200 ly away moving our way, thankfully in a safe distance in some 300.000 years. Had this thing flung in a short distance it could disturb planetary trajectories.

Supernovas can have the same effect as magnetars, as also black holes. There could even be a tiny one lurking in the outskirts of our solar system (the so-called planet 9). What I wanted to point out is that the Universe is a very hostile place. There could have been myriads of civilization that were eradicated because of a cosmic event every now and then.

[est31]

How is a black hole of the mass of a planet more dangerous than a planet of the mass of a planet?

[elorant]

A black hole can emit gamma rays.

[jjcon]

Black holes do not emit gamma rays... black holes emit nothing (aside from potentially hawking radiation). Matter around supermassive black holes can give off gamma rays but planet massed black holes are just as likely to cause gamma as planets themselves are.

[arrow7000]

Regular planets don't have the incredible concentration of gravitational pull that an event horizon does, so no, planet mass black holes are not only as dangerous as planets are.

[est31]

The formula for the gravitational pull is the same for black holes and regular objects. That means, you can orbit a black hole of the mass of the earth from 10 thousand km away just as easily as you can orbit earth itself 10 thousand km away from earth's center. Low earth orbit is in fact below that at only 7 thousand km.

The difference really starts to matter as you get inside a planet's radius. As you wander closer to earths center, the gravitational pull continues trending downward and gets replaced by pressure, aka having to support increasingly large portions of the planet. Eventually you'll be killed by that pressure, as no material is known to survive the pressure. Humanity hasn't made it further than a dozen km into the rocky part of earth's surface, the fraction of a fraction of earth's radius. Mostly, earth's insides are inaccessible to us.

For black holes, this is different. The gravitational pull just keeps getting stronger and stronger the closer you get to the event horizon. Eventually you'll be killed by tidal forces. But at the height of a circular orbit at the same height as earth's surface, those are the same as on earth, aka not really something to worry about for humans.

To give it in explicit numbers, a 2 meter tall object of 80 kg mass standing on the surface of earth gets about 0.5 milinewtons of tidal forces (while having to deal with 785 newtons due to normal gravity): https://www.wolframalpha.com/input?i=80kg+*+G+*+mass+of+eart...

Thus, you can get much closer to a black hole than you can get to earth's center.

[arrow7000]

Yes, and when you get closer to the black hole than you can get to a planet's centre of mass, that's when you get the massive gravitational forces that can result in the extremes of temperature that result in gamma rays

[batch12]

Mass or size? If mass is the same, the effect is the same.

[arrow7000]

With a black hole the mass is concentrated in a much smaller location than it is in a planet. Which means you can get a hell of a lot closer to the centre of mass of the black hole, which is where you start getting the extreme gravitational forces that you would never get on a planet