Since the discovery of the first extrasolar planet orbiting a solar type star in 1995 (Mayor & Queloz 1995), just over 3700 (according to exoplanet.eu updated at May 2018) extrasolar planets have been detected and confirmed as planets.
For many of these planets only one of their fundamental parameters (radius or mass) has been determined directly. In those cases where planets have been observed with both the transit and RV methods, their mass and radius, and thus bulk density, have been measured. This has led to exciting discoveries, including new classes of intermediate planets called “super-Earths” (Rplanet ≤ 2RE) and “mini-Neptunes” (2RE≤ Rplanet≤ 4RE).
Many transiting planet hosts, including the majority of CoRoT, Kepler and K2 discoveries, are too faint to permit full characterisation of the transiting planet.
PLATO’s main detection range is however V≤11, and it will thus provide large numbers of targets that are suitable for follow-up spectral characterisation.
PLATO was designed to maximise the yield of detected small planets around bright stars that could be characterised with asteroseismology and followed-up with RV from the ground.
There are no planets from Kepler or TESS with orbital periods beyond 150 days, that is, no small planets in the habitable zone of dwarf and sub-giant stars characterised with asteroseismology with Kepler or TESS (unless by single transit detections). PLATO outnumbers the performance of Kepler and TESS for planets with bright host stars for asteroseismology.
While TESS will have a sizeable impact on the detection of small planets around stars close to our solar system, it will not address the science case of characterising rocky planets at intermediate distances (a > 0.3 AU) around Sun-like stars. This goal remains unique to PLATO. Whether our solar system is typical or special will thus remain unclear until we can reliably detect and characterise Earth-like planets, in Earth-like orbits, around all kinds of bright host stars. Detecting these planets, and accurately and precisely determining their radii, masses, bulk densities, and ages, is the primary objective of PLATO.
PLATO – Revealing habitable worlds around solar-like stars
Definition Study Report, ESA-SCI(2017)1, April 2017