noun
1
BAD : I have friends in different parts of the earth.
GOOD : I have friends in different parts of the world.
BAD : We must all work together to protect the earth.
GOOD : We must all work together to protect the planet.
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earth/Earth = the ball-shaped object that we live on, usually seen as a planet in space: 'The earth moves round the sun every 365 days.' 'On the journey back to Earth, one of the spaceship's computers failed.'
world = the ball-shaped object that we live on, usually seen in terms of people, countries, cities etc: 'Tuberculosis is still common in some parts of the world.' 'It's one of the largest countries in the world.' 'You'll be seeing some of the world's leading golfers.'
Note the expression all over the world (= everywhere in the world): 'I have friends all over the world.'
planet = the ball-shaped object that we live on, usually seen in terms of the natural environment: 'This disaster could affect the whole planet.' 'The planet is gradually getting warmer.'
2
BAD : The aliens were fascinated by life in earth.
GOOD : The aliens were fascinated by life on earth.
BAD : Mankind has lived in the earth for thousands of years.
GOOD : Mankind has lived on the earth for thousands of years.
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on (the) earth (NOT in (the) earth ): 'Australia has some of the strangest animals to be found anywhere on earth.' 'Peace on earth is still just a dream.'