Planetary means relating to planets or having to do with the nature of planets. When scientists study planetary motion, they're examining how planets move through space. When astronomers discover a planetary system around a distant star, they've found planets orbiting that star, just like Earth and Mars orbit our Sun.

The word also describes something that affects or involves an entire planet. A planetary emergency is a crisis so large it concerns everyone on the planet, like a comet headed towards the Earth.

Scientists use planetary to describe things similar to planets in size or behavior. A planetary nebula is a cloud of gas surrounding a dying star that looks somewhat circular like a planet (though it has nothing to do with actual planets).

You might hear people talk about thinking on a planetary scale, meaning considering how decisions affect the whole Earth rather than just one country or community. When space probes visit other planets, scientists call them planetary missions. The word reminds us that Earth is just one planet among many, and that understanding planets helps us understand our place in the universe.