Please bear with me, as I'm not in the field of physics, this question may seem a bit simple.
This question is concerning stable circular orbits around celestial bodies.
I know the equation relevant to my question is given by the equality of gravitational to centripetal force:
where $m$ is the mass of the orbiting body, $M$ the mass of the celestial body, $G$ the gravitational constant, $r$ the distance between the centers of mass of both bodies and $v$ the tangential velocity.
Assuming a celestial body without an atmosphere;
When expecting the above formula, it seems that one, should be able to orbit at any
given radius, however is this true in reality?
Assuming a celestial body with an atmosphere;
Will drag be the main reason why one cannot orbit at heights inside the atmosphere?