converting degrees per second into speed

The title says you want to convert degrees into speed, but your post actually says you need to convert the speed into degrees. Because you already know how fast the objects should move around the orbit, but you don't know how much degrees to put into the RotateAround.
Do I understand it right?
If so, pretty simple math here. I'm not gonna use Sins and Tans, because I'm too dumb for that, so here's how I would do it in a simplified way:

 float desiredSpeed = 10f; //meters per sec
 float distanceToOrbitCenter = Vector3.Distance(transform.position, centerObject.position);
 float orbitLength = 2 * Mathf.PI * distanceToOrbitCenter; // Circle length from school: L = 2 * Pi * R
 float metersInOneDegreeAroundCurrentOrbit = orbitLength / 360f;
 float speedToDegrees = desiredSpeed / metersInOneDegreeAroundCurrentOrbit;
 transform.RotateAround(centerObject.position, Vector3.up, speedToDegrees * Time.deltaTime);