Which one is correct in the planet or on the planet?

Usually "on the planet" is what we mean.

oh... thank you

You're welcome.

To determine whether it is correct to use "in the planet" or "on the planet," we need to consider the context in which these phrases are used.

"In the planet" is not commonly used in English since "in" suggests being inside something, whereas a planet is typically understood as a solid object. However, there might be some exceptional cases such as referring to being inside a cave or an underground structure on a planet.

"On the planet" is the more commonly used phrase and is generally used to indicate being situated or located on the surface of a planet. It describes the idea of a person, object, or phenomena existing or happening on the surface of a particular planet.

For example:
- "Humans live on the planet Earth."
- "The rover landed on the surface of Mars."

So, "on the planet" is generally the correct phrase to use when referring to existing or happening on the surface of a planet, while "in the planet" is rarely used and may have specific, less common circumstances in which it could be appropriate.