I won't reprint the spell's text here, but the long and the short of it is that I don't see anything in the text of the spell that would prevent casting it on extremely big objects.
- The planet sits below the caster and is partially in range
- If dragon can be hit when partially in range, shouldn't the planet?
- The planet is an object. It may be covered in other objects and features but by the definition of object we commonly use in 5e, the planet is one
The background: I have a player who wants to solve the problem of an evil city by teleporting to the moon and enlarging it at just the right moment to cause it to be brought, by gravity, to crash into said city.
The question: Can you Enlarge/Reduce the planet you stand on?