Pluto hasn't been a planet for almost 20 years. In the early 2000s, scientists discovered several objects of a similar size to Pluto. So, during the summer of 2006, members of the International ...

A simple comparison of Pluto’s size with other planets, explaining why its small scale influenced its reclassification.

Pluto was once considered one of the solar system’s planets but was later downgraded to the status of ‘dwarf’ planet after several more similarly sized objects were discovered throughout our solar ...

A new study provides the best estimate to date of the diameter and heft of Pluto’s moon Charon and suggests that the satellite likely has no atmosphere. Charon is the largest known moon in relation to ...

NASA chief Jared Issacman appears to be among those who support Pluto being reclassified as a planet. Could President Trump ...

As elementary schoolers, we learned the order of the planets. Maybe your teacher even had a snappy mnemonic like “My Very Educated Mother Just Served Us Nine Pizzas” or “My Very Educated Mother Just ...

Editor’s note: A version of this story originally appeared in Astronomy’s 2007 special issue 50 Greatest Mysteries of the Universe. For a more recent discussion of Pluto’s planethood, see “An ...