  We suggest the existence of a correlation between the planetary radius and
orbital period for planets with radii smaller than 4 R_Earth. Using the Kepler
data, we find a correlation coefficient of 0.5120, and suggest that the
correlation is not caused solely by survey incompleteness. While the
correlation coefficient could change depending on the statistical analysis, the
statistical significance of the correlation is robust. Further analysis shows
that the correlation originates from two contributing factors. One seems to be
a power-law dependence between the two quantities for intermediate periods
(3-100 days), and the other is a dearth of planets with radii larger than 2
R_Earth in short periods. This correlation may provide important constraints
for small-planet formation theories and for understanding the dynamical
evolution of planetary systems.
