BUR #4378 has been approved by @evazion.
create alias earth -> earth_(planet)
create implication earth_(planet) -> planet
If the earth is being depicted from outside, as the tag says, then it fits under the definition of planet.
This is different from the other planets' tags, which for example could be used for images set in the surface of said planets.
The alias is because I see the tag routinely mistagged for character sitting on the ground or scenery pictures.