  In order to gain possible hints for planet formation from the current data of
known extra-solar planets, the period-ratios and mass-ratios of adjacent planet
pairs in multi-planet systems are determined. A moderate
period-ratio-mass-ratio correlation is found to have a correlation coefficient
r=0.5779 with 99% confidence interval (0.464, 0.672). In contrast, for
non-adjacent planet pairs, the correlation coefficient is r=0.2820 with 99%
confidence interval (0.133, 0.419). Our results reveal the imprint of
planet-planet interactions of the adjacent planet pairs in a certain fraction
of the multi-planet systems during the stage of planet formation.
