A prime goal of PLATO will be to detect a large number of planets, down to the terrestrial regime in size, and with well-determined masses and radii, and hence bulk densities, with unprecedented precision. Bulk density is a testable quantity to probe the input physics, constrain planet formation models, and evaluate simulated planet population distributions.
There are a good number of planets for which the bulk density has been measured till today, as shown in the drawing below.
Mean planet density versus mass with density lines for different bulk compositions. Dots are all currently known planets with measured radius and mass (cf. PLATO RedBook).
The ‘Standard Model’ of planet formation predicts formation of Eart-like rocky planets within the ice line (~ 3-4 AU) and giant planets beyond, where the amount of condesable solids is sufficient to reach the critical core mass for rapid gas capture. Subsequent evolution includes inward migration by interaction with the gaseous disk and, possibly, eccentricity excitation and migration due to planet-planet scattering, potentially followed by circularization due to tidal interaction with the star.
Most of the planets with measured bulk density are close-in planets. In fact, by selecting those of them with orbital period longer than 80 days (i.e., the period of the planet Mercury), we remain with a small sample made with only two transiting exoplanets with measured RV masses (orange dots), and an additional five (red dots) with TTV mass determinations.
Here Dots are all currently known planets with P > 80 days, with measured radius and mass (cf. PLATO RedBook).
Only a few additional planets from K2, TESS, and CHEOPS are expected to fill this diagram. Thus, while we will be able to compare planet population synthesis models with observational data for planets at small orbital distances, the picture will remain very limited for planets on longer orbits (i.e. orbits undisturbed by their host star and with potentially temperate surface conditions) until PLATO provides the capability to probe these large orbital distances.
PLATO will be the first mission to cover the parameter range of small, characterised (mass, radius, bulk density, age) planets with sufficiently large detection statistics to provide direct observational constraints on formation models and their predictions, as well as the dynamical evolution of young planetary systems.
The capability of discovering and get the density of small planets with period longer than 80 days, makes PLATO unique among the other projects. PLATO will allow us to study planets in regions where they form.
References
PLATO – Revealing habitable worlds around solar-like stars
Definition Study Report, ESA-SCI(2017)1, April 2017
platomission.com 
