morning star vs major planet

morning star and major planet both are nouns.

Word	Noun	Adjective	Verb	Adverb
morning star	Yes	No	No	No
major planet	Yes	No	No	No

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

morning star: a planet (usually Venus) seen just before sunrise in the eastern sky
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 morning star include planet.

morning star (noun)	major planet (noun)
a planet (usually Venus) seen just before sunrise in the eastern sky	(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