But how do they get OFF planet?

The new discussion about protoforms led me to ask something that I've wondered about all summer:
I can accept how the Autobots arrived on Earth. But how are they supposed to get OFF the Earth if they have to return to space? How do Autobots anywhere else in the galaxy leave their respective planets, too?
The only thing I can come up with (and it's mentioned in the books) is that they have their ship out there somewhere. So are they going to remote-control it down if they ever need an egress from Earth?
I can accept how the Autobots arrived on Earth. But how are they supposed to get OFF the Earth if they have to return to space? How do Autobots anywhere else in the galaxy leave their respective planets, too?
The only thing I can come up with (and it's mentioned in the books) is that they have their ship out there somewhere. So are they going to remote-control it down if they ever need an egress from Earth?