Which of Newton's laws explains why massive planets are able to move their parent stars more than less massive planets, thus making massive planets easier to find using the Doppler technique?

Newton's first law of motion

Explanation:

Newton's First Law of motion also known as law of inertia (inertia is the measure of how massive a body is), states that a body in motion keeps the same motion unless acted upon by an outside force. Likewise, if it is not moving, it remains that way unless a force acts on it. The force of gravity causes the moving planets to travel in roughly circular orbits around the sun or massive planets.

Gravity is the attraction between two objects that have mass. The amount of gravity is directly proportional to the amount of mass of the objects.

Therefore, massive planets will have more force of gravity that makes it possible for them to move their parent stars more than less massive planets, thus making massive planets easier to find using the Doppler technique.