Is there an official English pronunciation of Planetes?
The name "planet" comes from the Ancient Greek term πλανήτης (planḗtēs), meaning "wanderer." In modern Greek, it's more or less pronounced "planítis. "

Note that the Japanese romanization is "Puranetesu," プラネテス. Planets is romanized "Puranettosu, " プラネットス.

By inference, we can assume that the title is pronounced "PLA-NE-TES. "