PLATO science objectives: evolution of stars and their planetary systems


Studying stars and their planets

In our conception of planetary systems, the formation and evolution of both components, stars and planets, are intricately related. Stars and planets are born together from the same parental material, and therefore share a common initial history. In particular the initial protoplanetary disc and its central stellar core have the same chemical composition and their respective angular momentum reflects the angular momentum distribution in the protostellar/protoplanetary nebula. Also, in the early phases of evolution, young stars exchange angular momentum with their accretion protoplanetary disks, which eventually evolve into planets and transfer their angular momentum to planet orbits.

Even later in the evolution, various processes occur that result in mutual interactions between the stars and their planets. The stellar radiation flux obviously impacts the planet atmospheres, while particle bombardment of the planet by the stellar winds can also affect the chemical and biological
evolution of the planets. Planets can also influence their parent stars, e.g. by colliding with them, and enriching them in various chemical elements.
Giant planets in close-in orbits can also influence their star's rotation via tidal effects.

The study of planetary system evolution thus must be considered as a whole and it cannot be separated from that of stellar evolution. We cannot understand how planets are formed and how they evolve without a proper knowledge of stellar formation and evolution. We cannot characterize planetary systems without characterizing  precisely their host star. The basic philosophy behind the PLATO mission is precisely to study complete planetary systems composed of planets and their host stars, these two components being observed together with the same technique.


Evolution of planetary systems

Our understanding of planetary system formation and evolution is insufficient. Detections of giant exoplanets have revealed a large variety and complexity of configurations in exoplanetary systems, which was totally unexpected. Major questions and uncertainties remain, which hamper our progress in this area.

The true distribution of characteristics of exoplanets and of their orbits is unknown, with current knowledge strongly biased from the  detectable sub-sample. In particular, we do not know the distribution of planets with sizes and masses significantly smaller than those of gaseous giant planets. The extension of our investigation  of exoplanets toward lower masses, down to terrestrial planets, may reveal further surprises. The first planets with masses corresponding to Neptune have been discovered recently, but their nature still remains obscure.

A full statistical description of exoplanetary systems, down to masses and sizes of a fraction of those of the Earth, is a prerequisite for any decisive advance in the field of planetary formation and evolution. It is therefore necessary to extend significantly the sample of detected exoplanets beyond CoRoT and Kepler. This constitutes one of the objectives of PLATO.

A basic goal of PLATO is also to provide a large sample of exoplanets around bright stars, spanning a wide range of orbits,  sizes and masses, and to measure precisely and reliably their orbital parameters, sizes, masses and ages. This requires a detailed characterization of their central stars, involving both seismic observations with PLATO and ground-based support observations, allowing us to measure all their fundamental parameters, including mass, radius, age, temperature, chemical composition, rotation. Exoplanetary transit techniques indeed give access to the ratio of  planet to star radii, so that the planet sizes cannot be determined if the star radii are not perfectly known. Similarly, radial velocity techniques, even when the inclination angle is known, provide the ratio of planet to star masses, and a good measurement of the star's mass is needed. Star radii and masses are usually estimated by locating the star in the HR diagramme, which is imprecise and often unreliable. Finally, the understanding of exoplanetary system evolution requires an estimate of their ages, which can only be obtained by a measurement of the age of their central stars.

Such an approach is beyond our capabilities for most of the planets that will be discovered by CoRoT and Kepler, which are orbiting stars that are too distant and too faint for such a detailed characterization, but is within reach of PLATO, which focuses on stars that are bright and nearby.

Some particular examples of investigations that require a precise characterization of exoplanets and their central stars are indicated briefly below:

- the full knowledge of the properties of planets, their orbits and their parent stars, in particular their ages, will allow us to understand the mechanisms controlling orbital eccentricities and planet migration.

- the investigation of the still mysterious connection between giant planets and the metallicity of their parent stars requires good statistical knowledge of planet and parent star properties, including stellar ages and metallicities, of the type PLATO will provide.

- the potential chemical composition difference between the inner part and the external convective zone of a star, that will be present if high metallicity hosts have ingested planetary material \cite{Bazot2004}, can be investigated via asteroseismology.


Stellar evolution

Theory of stellar evolution has undergone major progress in the last decades. However, in spite of the progress in our understanding of microscopic physics in stellar interiors, our description of some physical processes controlling stellar structure and evolution is subject to major uncertainties. Convection and various other mixing and transport processes are poorly understood and yet play a major role in stellar evolution, determining evolution timescales, and must be taken into account for measuring stellar ages. Our current poor knowledge of most of these processes is usually compensated in our modeling by some poorly constrained parameterisation, and therefore the resulting stellar ages are model dependent and unreliable.

One of the consequences of this unsatisfactory modeling is that the ages of the oldest globular clusters are still very uncertain, and for some values of the model free parameters can still be higher than the estimated age of the Universe. Additionally, the relatively large adopted value of the core overshooting parameter needed to fit young open cluster data is in contradiction with recent asteroseismic estimates for this parameter for field beta Cephei stars. This clearly points out that our current knowledge of convective and rotational mixing processes inside massive stars is very incomplete, resulting in huge uncertainties in stellar masses and ages of supernova progenitors. Uncertainties in convective overshooting can lead to uncertainties in the ages of open clusters up to a factor of 2. Considering these difficulties, it is clear that the age ladder of the Universe, which rests on stellar age estimates, is still highly unreliable.

Our modeling of stellar interiors and stellar evolution therefore needs to be seriously improved. The situation for the Sun has evolved considerably with the advent of helioseismology, which has provided precise insight into the properties of the solar interior. Based on this very positive experience, it is clear that asteroseismic investigations, i.e. measurements of oscillation frequencies, amplitudes and lifetimes of a large number
of stars of various masses and ages constitute the only and necessary tool to constrain efficiently our modeling of stellar interiors, and improve our understanding of stellar evolution.

The pioneering CoRoT space mission is bringing us essential information to progress in this area, by providing high precision asteroseismic measurements for a few dozen stars distributed in several regions of the HR diagramme. The Kepler mission will also include a limited asteroseismology programme. However, these first measurements will remain limited to small and strongly constrained samples, which do not contain for example members of open clusters, or old population II stars, which would constitute major targets for such investigations. A better and more complete exploration of seismic properties of various classes of stars, sampling all stellar parameters (mass, age, rotation, chemical composition) is necessary. Such is the goal of PLATO.