  How might an advanced alien civilization manipulate the orbits within a
planetary system to create a durable signpost that communicates its existence?
While it is still debated whether such a purposeful advertisement would be
prudent and wise, we propose that mean-motion resonances between neighboring
planets -- with orbital periods that form integer ratios -- could in principle
be used to encode simple sequences that one would not expect to form in nature.
In this Letter we build four multi-resonant planetary systems and test their
long-term orbital stability. The four systems each contain 6 or 7 planets and
consist of: (i) consecutive integers from 1 to 6; (ii) prime numbers from 2 to
11; (iii) the Fibonacci sequence from 1 to 13; and (iv) the Lazy Caterer
sequence from 1 to 16. We built each system using N-body simulations with
artificial migration forces. We evaluated the stability of each system over the
full 10 Gyr integration of the Sun's main sequence phase. We then tested the
stability of these systems for an additional 10 Gyr, during and after post-main
sequence evolution of the central stars (assumed to be Sun-like) to their
final, white dwarf phase. The only system that was destabilized was the
consecutive integer sequence (system i). The other three sequences therefore
represent potential SETI beacons.
