astul : planet
The Illunse word for planet (wandering star) is astul. Astul is a unusual masculine first name. Astul is a rare last name.
This word is a mixture of the Old English word tungol (luminary, star, planet, constellation) and the Latin word astrum (star, heavenly body, planet / sun / moon). I decided not to use the lesser used Latin word planeta, even though it's the more accurate word.
astula : planets
The Illunse word for planets (nominative plural) is astula. In Latin astula means splinter/chip; shavings.
The Latin plural of astrum is astri. The Old English plural of tungol in is tungol (same as the singular).
I didn't find a word for planet in either Quenya or Sindarin. But there are stars Tolkien named in Quenya that are tentatively identified as our planets.