Figure 1: Artist's impression of an exoplanet.
Kepler is a planet-hunting telescope that searches for planets that transit their host stars. It is most sensitive to planets with short orbital periods. Nevertheless, Foreman-Mackey et al. (2016) performed a fully automated search for long period planets with only one or two transits in the archival Kepler light curves. The search was done for ~40,000 Sun-like stars and it led to the detection of 16 long period planet candidates. These planet candidates have orbital periods between 2 to over 50 years. Based on this finding, the occurrence rate of planets with orbital periods in the range 2 to 25 years and radii in the range 0.1 to 1 times the radius of Jupiter is estimated to be 2.00 ± 0.72 planets per Sun-like star.
Figure 2: The catalogue of long period transiting planet candidates (green points with error bars) compared to the Kepler planet candidates (blue points) and confirmed planets, and the Solar System planets (orange squares). The vertical solid line shows the absolute maximum period accessible to transit searches that require at least three transits in the Kepler data. Foreman-Mackey et al. (2016)
Reference:
Foreman-Mackey et al. (2016), "The population of long-period transiting exoplanets", arXiv:1607.08237 [astro-ph.EP]