outer planet vs major planet

outer planet and major planet both are nouns.

Word	Noun	Adjective	Verb	Adverb
outer planet	Yes	No	No	No
major planet	Yes	No	No	No

As nouns, major planet is a hypernym of outer planet; that is, major planet is a word with a broader meaning than outer planet:

outer planet: (astronomy) a major planet whose orbit is outside the asteroid belt (Jupiter, Saturn, Uranus, Neptune, Pluto)
major planet: (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

Other hypernyms of outer planet include planet.

outer planet (noun)	major planet (noun)
(astronomy) a major planet whose orbit is outside the asteroid belt (Jupiter, Saturn, Uranus, Neptune, Pluto)	(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