Earth & SkyGreek → English
Planetoid
/ˈplænɪtɔɪd/noun
A small rocky body orbiting the Sun, especially one larger than a typical asteroid.
The Greek roots
πλανήτης
planḗtēs
wanderer
-οειδής
-oeidḗs
shaped like
Literally: “wanderer-shaped thing”
The story of the word
The Greeks called the planets πλανῆται ἀστέρες, wandering stars, because they were the points of light that drifted against the fixed background. When Ceres and Pallas turned up at the start of the nineteenth century, astronomers argued over what to call them; Herschel's asteroid, star-like, was resented by those who pointed out the objects were not stars at all, and planetoid was the rival term. Asteroid won, and planetoid survives for the larger ones.
- How it travelled
- Ancient Greek → scientific coinage → English
- First recorded
- early 19th century
Same family
planetasteroidspheroidplanetarium
In a sentence
Ceres is big enough that most textbooks now call it a dwarf planet rather than a planetoid.