There isn't a single perfect antonym for "earth" because it has several different meanings:

1. The planet we live on: In this sense, there is no true antonym. You could use phrases like "other planets" or "outer space."

2. Soil or ground: Antonyms could be "air," "water," "sky," or "above."

3. The material of the Earth's surface: Here, antonyms might be "metal," "wood," "plastic," or "concrete."

4. A person's home or birthplace: In this context, the antonym could be "foreign land," "away," or "abroad."

It's important to consider the specific context in which "earth" is used to determine the most fitting antonym.