The primary goal of PLATO is to open a new way in exoplanetary science, by providing a full statistical analysis of exoplanetary systems around stars that are bright and nearby enough to allow for simultaneous and/or later detailed studies of their host stars.
The possibility of observing this sample of host stars with a photometric precision of 1ppm will allow a seismic analysis leading to determination of stellar and planetary masses of around 1%. Ages of the systems will be possible to determine to within several hundred million years allowing for an order of magnitude increase in our knowledge of planetary and stellar evolution.
The mission will also observe a very large number of stars to a slightly lesser precision. All of this will lead to the extending by an order of magnitude the knowledge of exoplanetary systems and their host stars initiated with CoRoT and followed up with Kepler, by surveying many more stars down to a magnitude allowing for the detection of Earth-sized planets.
As a secondary important objective, PLATO will also perform seismic analysis for a very large sample of stars all across the HR diagram, also without detected exoplanetary systems.
This will lead to an understanding of the processes of stellar and planetary evolution, by the study of stellar interiors and the distribution of planetary systems which will constitute a major step for future progress in most areas of astrophysics and in the scientific and philosophical approaches towards the origin of life in the Universe.