OPINION - Miserable June: I don't believe it but this bleak weather has me reaching for my gilet

Approximately 4.5 billion years ago, astrogeologists (that is, people who study planetary geology, not the grifters who ask for your star sign and suggest that while you might feel overwhelmed, now is a good time to address the big picture stuff in your life) hypothesise that a young Earth collided with a Mars-sized planet called Theia.