I think it's good to use obvious symbols. Stuff that we use for protection, like umbrella which protects us from the sun (just to let you know, umbrellas, originally, were used to protect people from the sun, not the rain ;) ).
Otherwise you can try with shields, fences, walls, some kind of a guardian. Anything which could show protection of Earth against Sun.
In the attachment you can find my take on the topic :) I used umbrella :)