“On earth” is not just about the planet. People say it when they are very surprised, confused, or upset.
A Common Use
It is often used in questions. For example:
“What on earth is that noise?”
This means: “What is that noise? I really want to know!”
A Simple Example
Ben lost his keys. He looked under the bed and asked,
“Where on earth are my keys?”
He was confused and maybe a little angry.
A Stronger Way to Ask
“On earth” makes your question sound stronger. It shows you are feeling more emotion (strong feeling).