When is the moon not a moon? If a new proposal for defining planethood is adopted, the moon could be considered a planet in its own right.
The meaning of the word “planet” has been a sore point since 2006, when, after a hurried argument, the International Astronomical Union (IAU) adopted a definition demanding that a planet be nearly round and be massive enough to have accreted or flung away other objects in its orbital neighbourhood.
Famously, this excludes Pluto, recently revealed by the New Horizons mission to be a stunningly complex world.