Black holes in their stellar or supermassive form are extreme objects that should inspire awe in all of us. But should we fear them? Is our planet at risk of being devoured by a black hole? As far as we know, there is nothing to worry about. Still, maybe some science will help us soothe this worry.
Black holes are not some tremendous, universal plugholes. Yes, there is a pretty big supermassive black hole at the center of our galaxy, but it is not sucking in everything in the Milky Way. There are actually plenty of stars orbiting it, and they have survived for billions of years. Sagittarius A* (pronounced A-star), our friendly neighborhood supermassive black hole, is not only not snacking on any of its nearby stars, it’s not snacking at all. Only when a supermassive black hole begins feeding is there trouble – and you don't even have to be near it. A feeding supermassive black hole can affect its entire galaxy and beyond.
If stars or gas get really close to a supermassive black hole, the intense gravity will smash and mush them into hot plasma, which will be slowly sucked in by the gargantuan object. The processes that go on around a black hole can produce jets of high-energy particles that expand way beyond the galaxy, heating up gas to tens of millions of degrees. They can also generate galaxy-wide winds that warm up the interstellar material, stopping stars from forming and inundating planets with cosmic rays. So a planet is more likely to be fried than mashed.
The issue with black holes is that they are proverbially black and we can’t see them. The best way for us to spot them is to wait for one of them to start feeding and light up. Based on these observations, we know that Sagittarius A* is not the only black hole in the galaxy. According to simulations and models, it appears that it might not even be the only supermassive black hole in the Milky Way. And when it comes to smaller black holes, the estimated number is much, much bigger. There could be millions of stellar-sized black holes all around us, and the center is probably crammed with them.
Based on these estimates, we can calculate that there should be a black hole every 125 light-years on average. This means there could be a quiet object just chilling relatively close by and we would be none the wiser. So, should we be worried?
I guess it depends. It is quite unlikely that one is so close to us as to pose a threat. But If you like to know where all potential celestial dangers are, then you should be quite worried. A black hole 10 times the mass of the Sun would be less than 30 kilometers (19 miles) across. Looking for such an object even a handful of light years away would be like looking for a black cat in a black room at night during a blackout. And the cat might not be there.
If there was a nearby black hole, we might be able to see gravitational effects. We might see positions and movements of stars shifting in a way that can only be caused by a black hole. And in the future, we might detect them with more sensitive gravitational wave observatories, or it's even possible we could see an increase in comets if one passes near the Solar System, like Scholz’s star.
If we detect a black hole and it is on a collision course with the Solar System, well there’s little we can do. We need to hope that it is not moving too fast for us to come up with some contingency plan. It doesn’t even have to hit us – a gravitational kick from a passing black hole would wreak havoc in the Solar System.
You can sleep soundly knowing that black holes are not out to get us. And even if they are, there’s little we can do, so there’s no point losing sleep over it.