Earth = the ball-shaped object that was live on, usually seen as a planet in space:  ‘The earth moves round the sun.’ ‘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 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.’
On (the) earth (NOT in (the) earth): ‘Peace on the earth is still just a dream.’
