The universe has quasi-moons, mini-moon, and moonlets, but no official definition of what counts as a moon.

Astronomers are proposing a new, more quantitative definition of what makes a planet. The new definition looks more directly at the object's mass — but it would still leave Pluto out of the running.

If you’re someone who is still upset about Pluto’s planetary demotion from more than a decade ago, then an upcoming debate about the definition of the word “planet” may be of interest to you. One of ...