If an astronomical object were no longer within the gravitational field of any other object, it would not necessarily be motionless. The absence of gravitational forces from other objects would mean that there are no external forces acting upon it, which could result in its motion being unaffected by other gravitational influences. However, the object could still have its own intrinsic motion.
Several factors could contribute to the motion of an astronomical object even in the absence of gravitational forces from other objects. Here are a few examples:
Initial momentum: If the object had a momentum before it moved beyond the gravitational field, it would continue to move with that momentum until acted upon by another force.
Internal forces: Internal forces within the object, such as pressure or collisions, could cause it to experience motion or changes in its shape.
Tidal forces: If the object is part of a binary system or is close to a massive object, it might experience tidal forces that can induce motion or deformations.
External non-gravitational forces: Other non-gravitational forces, such as radiation pressure from nearby stars or magnetic fields, could influence the object's motion.
Relativistic effects: If the object is moving at relativistic speeds, relativistic effects, such as time dilation or length contraction, may come into play.
In summary, while the absence of gravitational forces from other objects would eliminate their direct influence, there could still be other factors at play that could cause the object to exhibit motion or changes in its state.