Habitability is defined as the potential of an environment to sustain life of any kind (e.g. Steele et al. 2006). Life as we know it has the following requirements: liquid water, availability of nutrients, an energy source, the possibility for complex carbon chemistry, as well as protection from hazardous radiation.
From the pre-requisite of liquid water comes the concept of the habitable zone (HZ), i.e., the region around a star where liquid water on the surface of an Earth-like planet is in principle possible. The classical HZ as calculated by Kasting et al. (1993), and updated by e.g. Kopparapu et al. (2013), assumes an Earth-sized planet with an Earth-like water reservoir and a cloud-free atmosphere consisting of molecular nitrogen, carbon dioxide and water vapour. An Earth-sized water reservoir allows for the assumption that the water vapour in the atmosphere is in phase equilibrium with the surface (though the assumption of a saturated atmosphere overestimates the amount of water vapour in the atmosphere as shown by Leconte et al. 2013).
Most super-Earths (1<Mplanet ≤10ME or Rplanet ≤ 2 RE) have been found at orbital distances to the star closer than the HZ. Detections in the HZ have been made by RV or transit measurements (red and blue dots). However, only a small number of super-Earths have both mass and radius determinations, and these do not lie in the HZ.
PLATO will provide terrestrial planets in the HZ of Solar-like stars (up to about 1 AU) with accurately and precisely determined bulk parameters, which necessitates direct transit and RV measurements, hence planets orbiting bright host stars. In addition, PLATO’s bright target stars allow for asteroseismology studies, increasing not only the accuracy of stellar, and thus planet parameters, but also providing the age of the systems detected.