Asteroseismology is the study of the global oscillations of stars (see Aerts et al. 2010, Chaplin & Miglio 2013). These oscillations are due either to trapped acoustic waves (p-modes) or to internal gravity waves (g-modes), or to a mixture of the two. Their frequencies depend on the radially varying density and internal sound speed of the star. Thus, measurements of oscillation frequencies can be used to infer both the internal structure of stars and their bulk properties. The precision of stellar bulk parameters determines the precision of related parameters for its orbiting planets. Asteroseismology of planet host stars is therefore of key importance for derivation of accurate and precise planet parameters (Gizon et al. 2013; Van Eylen et al. 2014). For instance in an early study on the CoRoT target HD46375 hosting a Saturn-like planet, the seismic mass determination of the host star improved the mass estimate of the planet by a factor of 2 (Gaulme et al. 2010). Furthermore, asteroseismology is the only method that provides accurate and precise ages for planetary systems (Lebreton & Goupil 2014; Silva Aguirre et al. 2015; Huber 2015).
Asteroseismology is therefore part of the core science of PLATO, which, thanks to its bright target sample, will be the first mission to make systematic use of asteroseismology to characterise planet host stars. The core program focuses on stars showing oscillations similar to those of the Sun, which are intrinsically stable and excited stochastically by the near-surface convection. Extremely small amplitudes generally characterise solar-like oscillations, which makes their detection difficult. Nonetheless, data acquired by the CoRoT (Baglin et al. 2013, Baglin and the CoRoT team 2016) and Kepler (Borucki et al. 2010; Gilliland et al. 2010; Borucki 2016) missions have clearly demonstrated the power of this technique. While asteroseismic analyses are currently carried out for tens to hundreds of Sun-like oscillating dwarf and subgiant stars, PLATO will enable the technique to be applied to tens of thousands of stars. Moreover, PLATO will focus on bright stars and will therefore fully benefit from the strong synergies with the Gaia mission as well as ground-based high-resolution spectroscopy. Asteroseismology of cool dwarf stars with PLATO will thus drastically improve our understanding of stellar evolution beyond what has been achieved with previous missions, and significantly enhance the quality of current stellar evolution models.
PLATO will measure the oscillation frequencies of at least 15,000 dwarf and subgiant stars with V>11. In total, more than 300,000 stellar photometric light curves will be obtained for stars with V>13. It will thus be a powerful new tool for the characterisation and study of the evolution of star-planet systems.