But wouldn’t the ability to maintain an orbit be a function mass as well as speed?
Nope. It doesn't matter if it's a feather or a brick. If you apply an equal force to objects of different mass, the less massive object will accelerate more - however - the amount of force exerted by gravity depends on the mass of the objects. A larger object will produce more gravitational force, but it requires more force to accelerate. A smaller object will produce less gravitational force, but requires less force to accelerate.
The mass of the object gets canceled out in the equation, leaving you only with the mass of the other object (i.e. the planet being orbited/fallen towards) and the distance between them.
Nope. It doesn't matter if it's a feather or a brick. If you apply an equal force to objects of different mass, the less massive object will accelerate more - however - the amount of force exerted by gravity depends on the mass of the objects. A larger object will produce more gravitational force, but it requires more force to accelerate. A smaller object will produce less gravitational force, but requires less force to accelerate.
The mass of the object gets canceled out in the equation, leaving you only with the mass of the other object (i.e. the planet being orbited/fallen towards) and the distance between them.