Distance Learning Module: What is a Planet?

You may think of outer space as a vast, empty void—but space is full of objects! Both within and beyond the bounds of our solar system, there are countless stars, nebulae, asteroids, comets—and, of course, planets. But what IS a planet, anyway? Why was Pluto “demoted” from a true planet to a dwarf planet, and what makes a planet different from other objects in space?

In this module, we’ll look at the official definition of what makes a planet, and put the current criteria into historical context by reviewing some of the ways that astronomical understanding has evolved throughout history. This content is most appropriate for middle- and high-schoolers, based on reading level.