Find "statistically significant" Earth-mass planets in the habitable regions of several kinds of main-sequence stars;
Figure out the radius and mass of the star and any planets with 1% accuracy, and estimate the age of exoplanet systems with 10% accuracy;
Better determine the parameters of different kinds of planets, ranging from brown dwarfs (failed stars) to gas giants to rocky planets, all the way down to those that are smaller than Earth.