What force keeps planets in orbit around the Sun? Gravity and motion

This question underscores the fundamental concept of orbital motion. Gravity is theonly force acting on the planets as they orbit the Sun. However, gravity alone does not maintain planets' orbits. If gravity acted alone, it would pull the planets straight into the Sun. What averts this collision is the planets' motion. Each planet has a significant sideways velocity that keeps it from crashing. Consequently, the planets neither plunge into the Sun nor drift into open space. Instead, they follow a curved path, maintaining a stable orbit (Fig. 1).

Fig. 1. Computer-generated image of the solar system. Source: tenor.

To understand how motion offsets gravity, we isolate these components and analyze them separately. Without the Sun, a planet would experience no force and travel at a constant speed in a straight line, according to Newton's first law. This uniform motion would cause it to move progressively farther from the location where the Sun once was (Fig. 2, left).

Fig. 2. The roles of velocity and gravity in orbital motion. Left: In the absence of gravity, an object moves at constant velocity along a straight line due to inertia. Middle: In the absence of velocity, gravity pulls the object radially toward the central body. Right: When sideways velocity and gravity act together, gravity continuously bends the object's straight path into an orbit.

In contrast, without motion, gravity would pull the planet inward along a radial line until it crashes into the Sun (Fig. 2, middle). When both motion and gravity are at play, motion works to increase the planet's distance from the Sun, while gravity works to decrease it. If these two tendencies are perfectly balanced, they offset each other, keeping the planet at a stable distance from the Sun. As a result, gravity continuously curves the planet's straight path into an orbit (Fig. 2, right).

It is important to note that this sideways motion is not associated with force; it is a consequence of inertia. Due to the geometry of space, when a force vector is perpendicular to a velocity vector, the force changes only the direction, not the speed. In the vacuum of space, there is no atmosphere to slow the planets down. Thus, they continue to rotate at the speed they gained during the formation of the solar system, as neither the Sun's gravity nor their velocity changes.