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.