Wiktionary
n. 1 (context astronomy English) any of the 8 celestial bodies which orbit the Sun: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune (a celestial body that fits the 2006 IAU definition of planet) 2 (context astronomy English) any planet that is significantly larger and more massive than Earth, containing large amounts of hydrogen and helium, such as Jupiter and Neptunehttp://www.seasky.org/astronomy/astronomy_glossary.html
Usage examples of "major planet".
The 4,500-year-old depiction, however, also insists that there was - or has been - another major planet between Mars and Jupiter.
So an entire nation of several tens of millions of human beings resides on a moon that would fit inside the merest whorl in a major planet's atmosphere.
So an entire nation of several tens of millions of human beings resides on a moon that would fit inside the merest whorl in a major planet’.
The two moons of the major planet had disappeared behind a heavy deck of clouds.
An almost random example: an author is presented who argues that a major planet lies beyond Pluto.