Quote Watsisname (

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Planet 9's satellite system could be huge (in terms of size, not mass). Its Hill Sphere is a whopping 6AU in radius -- greater than the distance from the Sun to Jupiter! We might expect it to have captured moons within about 1/3 to 1/2 of that range.
Okay, based on these values, here are a few moons of Planet 9

(Planet 9 is here)