This month, the Dawn probe will give scientists their first clear view of dwarf planet Ceres. Like many objects in our solar system, it has a checkered past. Astronomers deemed the body a planet, then an asteroid, then a dwarf planet, all in the span of 200 years. The designation isn’t up for debate on this mission. But if history is any guide, the term used to describe a rocky planetoid is still anything but solid.

This article was originally published in the March 2015 issue of Popular Science.