How small can a black hole be? For several decades, astronomers have worked to answer this question by tallying the black holes in our corner of the universe.

They’ve found plenty of big and medium-size ones over the years—including a supermassive monster at the heart of our galaxy. But until recently, they’ve seen no signs of small ones, and that’s presented a long-standing mystery in astrophysics.

Now, astronomers have discovered a black hole with just three times the mass of the sun, making it one of the smallest found to date—and it happens to be the closest known black hole, at just 1,500 light-years from Earth.      

The discovery “implies that there are many more [small black holes] that