The motion of planets and moons in stable orbits around their respective stars is governed by the fundamental principles of gravity and inertia. These principles, as described by Isaac Newton's laws of motion and his law of universal gravitation, help us understand why celestial bodies remain in orbit instead of drifting away or falling into their parent stars. Here's a simplified explanation:
Gravity: Every object in the universe with mass attracts other objects with a gravitational force. This force depends on the masses of the objects involved and the distance between them. In the case of planets and moons, they are attracted to their parent star (e.g., the Sun) or planet (e.g., Earth) by the force of gravity.
Centripetal Force: When an object moves in a circular path, it experiences an inward force called the centripetal force. In the context of celestial bodies in orbit, the gravitational force between them and the central body acts as the centripetal force. It keeps the planet or moon moving in a curved orbit instead of flying off in a straight line.
Orbital Velocity: For an object to remain in a stable orbit around another object, it must have a specific velocity known as the orbital velocity. The orbital velocity depends on the mass of the central body and the distance between the objects. When an object, such as a planet or moon, attains the appropriate orbital velocity, the gravitational force and the centripetal force balance each other, resulting in a stable orbit.
Conservation of Angular Momentum: Angular momentum is a property of rotating or orbiting objects. According to the conservation of angular momentum, an object in motion will maintain a constant angular momentum unless acted upon by an external torque. In the absence of significant external forces, planets and moons will continue to move in their orbits with a constant angular momentum, ensuring their stability.
These principles collectively explain why planets and moons stay in their orbits around stars and planets. The gravitational attraction between celestial bodies provides the necessary centripetal force to keep them in stable motion. As long as the forces and conditions remain balanced, the orbits can persist for long periods.
It's worth noting that while orbits can be stable over immense timescales, there are external factors that can perturb orbits over time. For example, gravitational interactions with other objects, tidal forces, or the effects of general relativity can introduce small changes in orbits. However, these perturbations are typically gradual and do not lead to planets or moons suddenly drifting away or falling into their parent bodies.