Valerian and Laureline by... vol 2

The Delphs are an artificial species who depend entirely on an extremely rare substance, that not only allows them to live and reproduce, but also fuels their creative talents - for the Delphs are the most sought-after storytellers in the galaxy. They call upon Galaxity to find a new source of it, as their own is exhausted. The problem is, the only available deposit is on 21st century Earth, and the agents on station... are still going to high school!