don't panic, but there might be some black holes right nearby

A new discovery drastically reduces the distance between us and the nearest known black holes. Here's why you don't need to be alarmed, yet.

Up until very recently, the closest known black hole to Earth was a rather comfortable 1,560 light years away in the equatorial constellation Ophiuchus. It wasn’t too massive either, just under ten solar masses and less than 30 kilometers across, more or less as big as the Indian city of Mumbai. On top of that, it wasn’t even active, calmly swinging the Sun-like star Gaia BH1 around itself without so much as treating itself to a couple billion tons of plasma every once in a while. Unfortunately, it might not be the closest known black hole to us anymore after scientists noticed some rather odd movements of stars in the Hyades star cluster in the northern constellation Taurus.

Just like we used the suspiciously rapid orbits of stars around invisible masses which seemed much greater than they should be given the volume of space studied, a team of European researchers calculated that the motion of stars in the Hyades seemed to indicate not just one, but as many as three stellar mass black holes. And they weren’t even trying to do this intentionally. They were just trying to explain why some clusters of stars seemed to stay together and the detections of gravitational waves indicating a merger between black holes coming from their general directions. Hyades was just the closest cluster they could study in detail, a mere 150 light years away.

So, how big are these new black holes? We don’t know. Are they still there? Again, we don’t know. It’s possible that one was ejected after a merger of the birth of new stars, and is up to 45 light years from the cluster right now, most likely away from us. What’s adding to the uncertainty here is that the cluster isn’t rotating around a clear center of mass but seems to have binary systems consisting of one typical star orbiting a black hole companion, much like Gaia BH1. Which systems are the binaries? There’s not yet enough data to pinpoint them, but exclude them from the models and the paths of the rest of the stars’ trajectories don’t make as much sense anymore.

That brings us to the big question of whether we should be worried with black holes a mere 150 light years away. After all, stars drift all the time. It’s possible that one could float by and consume our little planet, or rip it out of the Sun’s orbit and send it out of the solar system to join the massive ranks of “rogue worlds” seemingly littering space. And while yes, that’s true, the black holes in question are a) not confirmed, and b) 1.4 quadrillion kilometers away, so that’s certainly not something we should be concerned about in our lifetimes. In fact, we are more likely to be harmed by a rogue red dwarf in our immediate stellar neighborhood, but even that’s millions of years in our future.

Over the long term, however, it’s certainly possible that a rouge black hole floating by will consume or warp the Earth with its gravitational tides, whether it was ejected and just ended up close enough through the push and tug of stars over the eons. This is a potentially apocalyptic event for future humans if any still exist many millions of years from now, lack the ability to flee into space, and have no other worlds where humanity has already settled. But given that it would be extremely challenging for complex life to exist on our world after the next 600 million years, we probably shouldn’t lose any sleep about the far future either since we won’t be here to see it.

See: S Torniamenti et al. (2023) Stellar-mass black holes in the Hyades star cluster? MNRAS, Vol. 524, Issue 2, DOI: 10.1093/mnras/stad1925