For some reason, I couldn't login (error 403) so I created a new account. Thanks Mosfet and JackDole for your help, I finally succeeded.
@Mosfet : Yeah, that's exactly what I was doing, and it didn't work, that's why I didn't understand. It works now, but I tried so many things that I finally don't even know if I was doing something wrong (in typography for example) or if it was my computer and/or the program that had some bug... Well, it's fixed now, so thanks for your help. ^^
@JackDole : Thanks for the example, that's exactly what my script looks like now ! I even made precise adjustments. Don't worry, I carefully read the "create a star/planet" manual section before I post here, many times too, but the coordinates system isn't explained anywhere. Maybe too obvious ?