Yes, PLANET is a Scrabble word!

Letter values in Scrabble US: P 3, L 1, A 1, N 1, E 1, T 1.

Is it a valid word?

PLANET is a valid word in the official Scrabble dictionary (US), the UK Scrabble Dictionary and Words with Friends (ENABLE).

(astronomy) any of the nine large celestial bodies in the solar system that revolve around the sun and shine by reflected light; Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto in order of their proximity to the sun; viewed from the constellation Hercules, all the planets rotate around the sun in a counterclockwise direction
a person who follows or serves another
any celestial body (other than comets or satellites) that revolves around a star

Also: major planet.