Such asteroids would rapidly have adopted orbits that cut across those of the inner planets, including Earth.

ECONOMIST: Asteroid impacts

And the hole's outer edge is roughly where rocky planets, like the inner planets of this solar system (including the earth), are expected to be.

ECONOMIST: Dusty genesis

Small planets, like the inner four in our own solar system, are rocky - larger ones, like the outer four, aren't much more than balls of gas.

BBC: Viewpoint: The obsession with a 'twin Earth'

Without it such a galaxy's outer parts would spin more slowly than the inner ones (just as the outer planets of the solar system move around the sun more slowly than the earth does).

ECONOMIST: The galactic gourmand