planet [ME] Early Greek astronomers observed certain heavenly bodies moving around the night sky in contrast to the stars, which stayed permanently in a fixed position in relation to one another. This is why they are called planets, from Greek planētēs ‘wanderer’. The Sun and the Moon were once thought of as planets too. Plankton [L19th], the term for small and microscopic organisms floating in the sea, comes via German from the related Greek word planktos, ‘wandering or drifting’. See also aeroplane.