I've recenly been told that we should say 'on the Earth" when referring to the planet and 'on earth" when using a fixed phrase, for instance,
-Where on earth have you been all this time?
- In 500 years' time there will be no life on the Earth.
I'm not sure this is correct. 'On earth' seems to cover both usages, or... Do we say 'on the Earth'?