What would happen if a supermassive black hole came close to the Earth? (Advanced)

Is it theoretical for a supermassive black hole to have an apocolyptic effect on the planet earth? if so (and even if not so) what would happen if such a black hole drifted close enough to annihilate our planet? thanks so much.

I did some thinking about this, and went through some *very* rough calculations. And the short answer is yes, it *could* happen. But it's very unlikely. And we would have quite a bit of warning before anything really bad happens.

Now here's the long answer:

As our website mentions (but doesn't explain in much detail) supermassive black holes are found at the centers of galaxies. And they have masses of perhaps several million times the mass of the Sun. You *could* have a situation in which two galaxies are merging, and they merge in such a way that the supermassive black holes (let's use SBH from now on) at the centers of the two galaxies pass very close to each other, and one SBH gives the other enough of a gravitational "kick" that it gets ejected from the two galaxies. The SBH would probably drag some stars along with it, but it would more or less be moving through the universe on its own. (You would have to be very lucky for this to work so that the two SBH's don't simply merge with one another, but I don't see why it *couldn't* happen.)

(The following is based on some quick calculations that I did, assuming the mass of the SBH is exactly 1 million times the mass of the Sun.)

Now suppose that this SBH happened to come into our galaxy, and in fact came close to our solar system. (Again, this is so improbable as to be hardly worth considering. But it's still fun to think about.) Once it started moving through the Galaxy, we would start to notice the orbits of stars that it encountered being disrupted. And in fact, our solar system's orbit around the center of the Galaxy would be disrupted if it came close. How close? I'm thinking that we would start to notice something was fishy if it came within about 1000 light years of our solar system. But the disruption of our solar system's orbit around the galactic center probably wouldn't have any catastrophic implications for life on Earth. In fact, even if we were on a "collision course" with the SBH, we would probably have a few hundred thousand years between the time we cross that ~1000 ly threshold and the time the *real* disaster begins.

What is that real disaster? Well, once the SBH got within a few hundred AU (1 AU = distance between the Earth and the Sun) of us, it would start to seriously disrupt the orbits of the planets in our solar system, including the Earth. So we would very quickly either boil or freeze to death, as we'd either get too close to or too far away from the Sun. There are any number of different things that could happen to the Earth after that point. We could fall into the Sun. We could be thrown out of the solar system, but into some elliptical orbit around the SBH. We could be thrown *into* the SBH.

Let's consider that last possibility. Once the SBH came within about 1 AU of the Earth, the gravitational tidal forces would rip the Earth itself apart. It would probably take at least a few years after the Earth's orbit around the Sun is disrupted (and everyone either freezes or boils) until the Earth gets close enough to the SBH to break up.

Then the pile of rubble that was once the Earth would fall into the SBH. Physics doesn't address what exactly happens after that. :-)


This page was last updated on June 27, 2015.

About the Author

Christopher Springob

Christopher Springob

Chris studies the large scale structure of the universe using the peculiar velocities of galaxies.  He got his PhD from Cornell in 2005, and is now a Research Assistant Professor at the University of Western Australia.

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