There isn't a single, universal Celtic word for "planet". The Celtic languages are a family of languages, not a single language, and each branch has its own vocabulary.

Here are some examples from different Celtic languages:

* Irish: "planaid"

* Scottish Gaelic: "planaid"

* Manx: "planaid"

* Welsh: "planed"

* Breton: "planedenn"

* Cornish: "planedh"

These are all modern loanwords from Latin "planeta", ultimately from the Greek "planētēs" meaning "wanderer".

It's worth noting that ancient Celts, like other ancient cultures, likely had their own ways of understanding and naming celestial bodies. However, those names are largely lost to time, as their knowledge was passed down orally.