It can become a planetary currency, very possible. If astronauts on the Moon or robots on Mars in NASA missions want to send signal and details to broadcast their Bitcoin transactions from Moon/ Mars to the Earth, we will see multi-planetary transactions via Bitcoin. Yeah it can still be transactions between human or human-related equipments, not between human and aliens.

The bottom line is IF you have both private key and can send signals (via Internet connection or sort of other signals that can transmit details of the transaction you want to broadcast), your transaction can be broadcasted everywhere.