First of all, since it takes 20 years for light from the rockets to arrive to us and the same amount of time for our signals to get there, they won't be affected for at least 40 years and we won't see a response for at least 40 years after initially sending the signal.
Expecting a response sooner than 40 years would mean expecting they have FTL response methods.
Expecting a response sooner than 20 years would mean expecting they have time travel technology.
Now, here are a few possible scenarios for the 89-year delay (some of which are already mentioned in other answers):
There's really nobody there, and the structures are just an interesting newfound natural phenomenon.
There's nobody there...any more. The structures are all that's left, automatically completing tasks that have been unnecessary for centuries.
There's someone there, but they aren't listening. Perhaps they had their own SETI a few hundred years ago and gave up after a while.
They saw the signal, but refused to respond. Aside from possible cultural and political reasons, interstellar contact is very risky - for what they know, we could be a galactic empire, broadcasting the signal so species living on habitable worlds would give up their location to us. (We should reconsider sending the signal for the same reason - the signal can be traced back to Earth. We've been noisy enough with radio signals already.) Not to mention they could be preparing to invade.
What we should do:
- Turn off the signal. The best time to do this would be 89 years ago. The second best time is now.
- Determine whether there are signs of these structures around other planets in the system and whether they are new. (Would be best)
- Send probes to survey the system. It's the only way to be sure.
- Prepare for invasion, just in case.