When something happens once in a blue moon, it very rarely happens.
"Since my daughter was born, I only go out with my friends once in a blue moon."
"My brother is so good at chess I only beat him once in a blue moon."
When you say that someone is living on another planet, you are saying either they do not notice what is happening around them or they are not realistic.
"He's living on another planet if he thinks he's going to get £5000 for his old car."
"She wasn't listening to me at all. Sometimes I think she's living on another planet."
We come back down to earth when we have to start doing something boring/not fun after doing something fun.
"I came back down to earth on my first Monday back in work after my holiday in Spain."
"She came back down to earth when she got back after the weekend to find she had so many work emails to answer."
Everything under the sun is everything that exists or is possible:
"We stayed up all night talking. We must have talked about everything under the sun."
"She's very intelligent and knowledgeable. She can talk about everything under the sun."
When you are over the moon, you are very happy about something. This expression is used but these days it is considered a cliché (a phrase or an idea that has been used so often that it no longer has much meaning and is not interesting).
"I was over the moon to find out my wife's pregnant."
"He was over the moon with his IELTS score."
To help you remember these idioms, try this quick quiz:
