There is only one reason governments form: to protect what is inside from what is outside and they are based on the *identity* of 'us' vs 'them' (or in autocracy "me" vs "them"). This is the very basis of state and power in human species.

While there is nothing 'outside' of Earth, there is no "us vs them", and there will be no Earth government, there can only be governments within.

Once there is outside, which could be aliens of course, but more realistic is self-sufficient human colonies on other planets, then there are 2 ways governments form:

1. Forced by power-wielding authority, a.k.a. World conquered by force
2. Willful unification based on potential threat of "them" to "us", that would warrant a creation of the government as discussed from the start.