BIG THINK
- From the perspective of any inhabited planet, the greater Universe is full of hazards: exploding stars, comets and asteroids, gamma-ray bursts, and black holes among them.
- But black holes pose a particular danger because of their invisible and indestructible nature; there is no “Armageddon”-like solution to being swallowed by a black hole.
- Although the odds of Earth getting swallowed by a black hole, or any Solar System planet, for that matter, are low, it’s definitely a real possibility.
Of all the ways that planet Earth could meet its eventual demise, death by black hole is among the most spectacular. While gamma-ray bursts, nearby supernovae, or giant collisions with asteroids or comets could easily pose a threat to all life on our planet, a black hole offers an even more grim fate: the possibility of destroying the Earth itself entirely, perhaps even swallowing it whole. While life on Earth is expected to come to an end within ~2 billion years as the Sun continues to swell, expand, and heat up, we expect the Earth itself to stick around for another 5-7 billion years, until the Sun becomes a red giant, at which point it will engulf Mercury, Venus, and possibly the Earth as well.
But there’s always the possibility that a black hole will randomly, as the stars and stellar remnants dance throughout the Milky Way, pass into our Solar System, devouring our planet in the process. Which leads to this week’s question from Andrea Hall, who wants to know:
“Can Earth or any of our other planets eventually get swallowed into a black hole? Or is it way too far out there to affect us?”
This is a challenging question, because while the black holes that we know of are too far away to swallow us anytime in the foreseeable future, we know that there are plenty of invisible lurkers out there, and those are perhaps the most dangerous ones of all.
So far, there are only four main ways we know of to directly detect black holes. One is through their emissions of light, particularly X-ray light.
You might immediately object, and say, “Wait a minute, I thought the defining feature of black holes was that they’re black, as in, no light can escape from them.” And that’s true: from within their event horizons. There’s an imaginary surface you can draw around any black hole — a sphere for a non-rotating black hole and a flattened, oblate spheroid for a spinning black hole — that separates its outsides from its insides. If anything crosses over to the inside of the event horizon, it cannot escape; it must inevitably hit the central singularity, where it only adds to the black hole’s mass and energy.
But the event horizons of black holes are very small. While stars like our Sun are more than ~1 million kilometers across, and giant stars like Betelgeuse can be larger than Jupiter’s orbit around the Sun (over ~1 billion km across), black holes are the densest objects in the known Universe. A black hole of the mass of the Sun would have an event horizon of just ~3 kilometers in radius; the supermassive black hole Sagittarius A* at our galaxy’s center — the largest in the Milky Way — is about ~20 million km across. Whenever a clump of matter intersects that black hole, whether a planet, star, cloud of gas, or anything else, only a fraction of the mass gets devoured; the rest gets torn apart and accelerated, where it emits radiation we can observe.
We see this around active black holes at the centers of galaxies all the time: the active ones emit spectacular streams of radiation…
