plan‧et [countable]

• 1 a very large round object in space that moves around the sun or another star:
• Mercury is the smallest of all the planets.
• Is there life on other planets?
• the future of Planet Earth
• ! Do not say 'in a planet'. Say on a planet.
• 2

   somebody is (living) on another planet

  , what planet is somebody on? spoken used humorously to say that someone's ideas are not at all practical or sensible:
• He thinks motherhood is glamorous - what planet is he on?
• 3

   the planet

  the world - used especially when talking about the environment:
• a safer future for the planet
• —planetary adjective [only before noun]
• the planetary system
• WORD FOCUS: space 
• vehicles used in space: spaceship, spacecraft, rocket, (space) shuttle,probe, satellite, space station
• someone who travels in space: astronaut, cosmonaut Russian
• parts of a rocket's journey: countdown, launch, blast-off/take-off/lift-off, leaving the earth's atmosphere, going into orbit, re-entering the earth's atmosphere, splashdown/touchdown
• places and things in space: planet, moon, star, sun, satellite, solar system, constellation, galaxy, universe, the cosmos, black hole,quasar, comet, meteor, asteroid
• the study of space: astronomy
• space
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• ➔ See also space

Definition from the Longman Dictionary of Contemporary English
Advanced Learner's Dictionary.