Orbital Motion of Planets

Phenomenon:

Earth orbits the sun every 365 days. Each planet in our solar system has a different orbital period. Inner planets orbit faster than outer planets.

Student Task:

Construct an explanation for patterns of orbital motion observed in the solar system (i.e., why inner planets orbit faster than outer planets).

Explanation:

A planet orbits the sun at a constant speed due to gravity and inertia. The force of gravity pulls a planet toward the Sun. Inertia keeps a planet moving in a forward direction. When the force of gravity balances a planet’s inertia the result is circular motion.

A planet needs to be moving at just the right speed to stay in orbital motion around the sun. If a planet is moving too fast, it can escape the solar system. If a planet is moving too slow, it can be pulled in and crash into the sun. Objects were set in motion during the formation of our solar system. Those objects moving at just the right speed relative to their distance from the sun became planets.

As a planet’s distance from the sun increases, the strength of the force of gravity decreases. This explains why distance from the sun is proportional to the speed or orbital period for each planet, or why inner planets move faster than outer planets.

Formative Assessment:

Use what students have written in their science notebooks as a formative assessment, including their written explanation for what causes a planet to orbit the sun and why inner planets orbit faster than outer planets.

Do students understand that the force of gravity is an interaction between any two objects with mass and is always attractive? Do students understand that the strength of the force of gravity depends on the mass of the two objects and the distance between them? Do students understand that a balance between gravity and inertia is required to maintain the orbital motion of planets? Do students understand that patterns of motion in the solar system were established early in its history?