Hello good (and not-so-good, we don’t judge) people!
It’s Tuesday and it’s time for our appointment with the history of the English vocabulary.
Today’s word is “planet”.
Everybody in the modern world knows of planets, whether it be from astronomy classes in school or science fiction, and we all know what a planet is, right?
Well, it turns out that amongst astronomers, the actual definition of “planet” is still a bit controversial. The currently accepted definition is the following: “an astronomical body massive enough to be rounded by its own gravity (which excludes asteroids), not massive enough to initiate thermonuclear fusion (which excludes stars), and which has cleared its orbital neighbourhood of planetesimals (i.e. small rocky bodies)”. The addition of this last clause to the definition famously resulted in the “demotion” of former planet Pluto to “dwarf planet”, or “Kuiper belt object”, a decision that was made firstly because of its very small size (it’s smaller than Russia!), and secondly because the cascading discovery of similar objects would have made the Solar System have thousands of planets. You don’t want to memorise thousands of names at school, now, do you?
To the ancient Greek, though, the definition of planet was very simple: it was a star which moved instead of staying still. This is why they gave them the name “planetes”, which in Ancient Greek means “wanderer”, and from which our modern word “planet” comes.