Conditions in the very early universe may have allowed far lighter black holes to form. One weighing the mass of Earth would have an event horizon –– the point of no return for infalling objects –– about as wide as a U.S. dime coin.

How NASA’s Roman Mission Will Hunt for Primordial Black Holes

https://www.nasa.gov/missions/roman-space-telescope/how-nasas-roman-mission-will-hunt-for-primordial-black-holes/