As you’d expect, this means a small planet. But it also refers to the bits and bobs (that’s the technical term) which can come together to make up a planet. Apparently there’s a theory that that’s how the earth was formed. Called, rather unimaginatively, the planetesimal hypothesis, it states that planets form out of cosmic dust grains that collide then stick together to form larger bodies.

Etymology AND science? I know, spoil you.