Re: Stable orbits around a black hole?
There are no stable circular orbits sufficiently close to a black hole. For a Schwarzschild (non-spinning) BH the critical radius is 6GM/c^2, where M is the mass of the object.
The objects they consider in the paper have masses in the range 10^6-10^9 solar masses. A 10^6 solar mass black hole has an innermost stable circular orbit radius of about 0.06au (ie about 1/16th of the Earth's orbital radius, and about 1/7 of the orbital radius of Mercury), while a 10^9 one is about 60au, which is outside the orbit of Pluto by a bit. A light year (ly) is about 63,000au.
The planets which might form around these objects have orbital radii of about 10ly: stable orbits very definitely exist at such radii.