  The Kepler Mission has discovered thousands of exoplanets and revolutionized
our understanding of their population. This large, homogeneous catalog of
discoveries has enabled rigorous studies of the occurrence rate of exoplanets
and planetary systems as a function of their physical properties. However,
transit surveys like Kepler are most sensitive to planets with orbital periods
much shorter than the orbital periods of Jupiter and Saturn, the most massive
planets in our Solar System. To address this deficiency, we perform a fully
automated search for long-period exoplanets with only one or two transits in
the archival Kepler light curves. When applied to the $\sim 40,000$ brightest
Sun-like target stars, this search produces 16 long-period exoplanet
candidates. Of these candidates, 6 are novel discoveries and 5 are in systems
with inner short-period transiting planets. Since our method involves no human
intervention, we empirically characterize the detection efficiency of our
search. Based on these results, we measure the average occurrence rate of
exoplanets smaller than Jupiter with orbital periods in the range 2-25 years to
be $2.0\pm0.7$ planets per Sun-like star.
