Fig. 2. Orbital period versus planetary radius for Kepler planetary candidates. The colored symbols represent how many planets orbit a given star, and legend lists how many of this type of system are in the catalogue. (For example, the catalogue lists 2967 stars with only one planet orbiting them.) Planets with shorter orbital periods are over-represented because geometric factors and frequent transits make them easier for Kepler to detect. The upward slope in the lower envelope of these points is caused by the difficulty in detecting small planets with long orbital periods, for which transits are shallow and few are observed. Credit: Rebekah Dawson, University of California, Berkeley