If it's rotating, you can extract its rotational energy using the https://en.wikipedia.org/wiki/Penrose_process https://en.wikipedia.org/wiki/Penrose_process A

If it's rotating, you can extract its rotational energy using the https://en.wikipedia.org/wiki/Penrose_process https://en.wikipedia.org/wiki/Penrose_process Additionally, all black holes constantly emit Hawking radiation https://en.wikipedia.org/wiki/Hawking_radiation https://en.wikipedia.org/wiki/Hawking_radiation Unfortunately for the planet 9 black hole, it only loses mass on balance it's below a certain size, about 0.007 earth mass. Primordial black holes of that size, if they exist, would be quite useful. However, they would be monumentally hard to find: the black hole above would be 62 micrometres in radius, and have an event horizon temperature of 2.93K. Assuming no changes to the cosmic microwave background, this black hole is unstable: it will eventually radiate away all its mass and disappear. You'll be waiting a while for that to happen, at 20 picowatts, it'll take 1.9e35 gigayears to die.