I realize article 'the' should be used with unique things such as Mars, Sun, Mercury, etc. But is the a rule for when we shouldn't use 'the' to talk about the objects mentioned?
...and we write 'the moon' (uncapitalised) because it's a common noun meaning 'satellite' (our only one), unlike 'Ganymede' (one of the moons of Jupiter).
But shouldn't we use definite article if there is only one:
The Pope is visiting Russia.
The moon is very bright tonight.
why then can't we "The Mercury is a small planet that is the closest to the sun in the solar system."
Like 'the moon' we say 'the sun' as it's a common noun, unlike Sol.