Final answer:
The net force on planet B would be zero due to the gravitational forces from planets A and C canceling each other out, as explained by Newton's universal law of gravitation and the symmetry of the system.
Step-by-step explanation:
The net force on planet B when the three planets are along the same horizontal line would be zero. This is because the gravitational force that planet A exerts on planet B is equal in magnitude but opposite in direction to the gravitational force that planet C exerts on planet B. According to Newton's universal law of gravitation, the force between two masses is directly proportional to the product of their masses and inversely proportional to the square of the distance between their centers. When planets A, B, and C are in a straight line, with equal distances between them and planet B is in the middle, the forces exerted by planets A and C on B cancel each other out.
Gravitational interactions between multiple bodies, such as planets, are complex and can cause perturbations in their orbits. However, in this specific scenario with equal masses and distances, symmetry simplifies the outcome. The mutual gravitational effects of the planets A and C on planet B would effectively nullify each other.