PLATO (PLAnetary Transits and Oscillations of stars) is a proposed ESA mission within the Cosmic Vision 2015-2025 program. PLATO was proposed for a launch in 2018 / 2019 (M1/M2). It is now competing for launch in 2022 (M3).
PLATO is a satellite mission designed to find and characterize transiting exoplanets. The combination of asteroseismology and transit method will allow to determine the radius, mass, and age of the planet with high precision. This will allow to get first restrictions on the interior of planets. Additionaly PLATO will find thousands of transiting planets around bright stars. These planets will be targets for future spectroscopic characterization.
In my opinion PLATO will be the needed major step forward in the understanding of exoplanets and thus our own planetary system. The survey of most of the sky will finaly give a large statistical sample of exoplanets with know size and density.
One or even two of the factors in the famous Drake equation will be determined with high precission, and the planets in habitable zones around the brightest stars are future targets for spectroscopic follow up and might be the first exoplanets were biomarkers / biosignatures might be detected :-). If PLATO gets seleted one thing is for sure, the detections of this satellite will dominate the exoplanet research for decades to come.
PLATO in relation to other missions
Comparring PLATO with other missions is difficult, as no similar mission exists or is planned. In comparrisson to the existing CoRoT and Kepler satellite PLATO will have a distinct advantage, as PLATO has much larger field of view and larger magnitude range. In contrast to CoRoT and Kepler, PLATO will focus on much brighter stars, which can much easier followed up from ground to verify the planetary character of the object of interest and do spectroscopic analysis of their atmospheres.
As satellite mission, PLATO will be able to find small earth sized planets which will not be detected by ground based surveys.
Feb 2014, SPC final decission on M3 candidates
2015, Adoption by ESA