Planet names are just another way for you to customize, your planet. By naming them it can make them easier to access when looking through them, and can make your planet stand out, from the others in your solar system. To change your planet name, click on the name of your planet on the overview screen.
Planet names may contain alphanumeric characters and spaces. Using underscores, dashes and other special characters is not allowed because it breaks Espionage Reports.
Update note: These symbols are now allowed to be used in planet names/account names/alliance names:
"- _ . ! ?"