there are nine, if you look at the gate you can see that where its normally hidden in the stands on planets it has 2 extra chevrons under the walkway. 8 and 9. 8 is galaxy
9, ok yes galaxy could be it but wouldnt it be hard getting back to the right one? also we have the mirror from season one for that.
and it can't be time travel as thats what the machine on the planet from 'Window of Opportunity' was for and the ancients messed it up.