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PLATO's primary goal is to detect terrestrial exoplanets in the habitable zone of bright, solar-type stars, and to characterise their bulk properties. By providing key information required to determine habitability, PLATO will provide the answer to one of the most profound questions in modern astrophysics: how common are worlds like ours, and are they suitable for the development of life?

Determining the Habitability of Exoplanets

The primary goal of PLATO is to open a new avenue of exoplanetary science research, by detecting terrestrial exoplanets in the habitable zone of solar-type stars, and by characterising their bulk properties en masse. PLATO will provide key information nedded to determine the habitability of these undoubtedly diverse new worlds:planet radii; mean densities; stellar irradiation levels, and planetary system architecture. Understanding planet habitability is a truly multi-disciplinary endeavour. It requires knowledge of planetary composition, to distinguish terrestrial planets from non-habitable gaseous mini-Neptunes, and of the atmospheric properties of planets.

PLATO will lead this drive to determine habiltability by combining:

  • planet detection and radius determination from photometric transits
  • determination of planet masses from ground-based radial velocity follow-up
  • determination of accurate stellar masses, radii, and ages from asteroseismology
  • identification of bright targets for atmospheric spectroscopy.

PLATO will assemble the first catalogue of confirmed and characterised planets in habitable zones with known mean densities, compositions, and evolutionary ages/stages. In total, the PLATO catalogue will consist of thousands of characterised planets of all types, 85,000 stars with accurately known ages and masses, and 1,000,000 high-precision stellar light curves.

The Uniqueness of Our Solar System

While the structure and mass distributions of bodies in our Solar System are well known, we only have indirect and partial knowledge of its formation and evolution. To place our system in context we must look to other systems, and study their architectures and composition. From current observations, it has become obvious that the bulk compositions of exoplanets can differ substantially from those of Solar System planets; this must be indicative of the formation process.

Thanks to PLATO, the bulk density and composition of thousands of exoplanets will be obtained from their measured masses and radii. The mission will characterise thousands of rocky, icy or gaseous planets, ranging from giants to Earth-twins, by providing exquisite measurements of their radii (at 2% precision), masses (better than 10% precision), and ages (10% precision). This will revolutionise our understanding of planet formation, and of the evolution of planetary systems. In addition, important properties of host stars, such as chemical composition and activity levels, will be measured by PLATO and the associated ground-based follow-up for a large sample of systems. Extending the bulk characterisation towards cool, terrestrial, Earth-sized planets on Earth-like orbits will be unique to PLATO and key to answering the question: how unique is our Solar System?

Interiors of Terrestrial and Gas Planets

Many confirmed exoplanets fall into classes that are unknown from our Solar System, for example "hot Jupiters", "mini-Neptunes", or "super-Earths" (rocky planets with masses below 10 MEarth). It came as a surprise that gaseous planets can be as small (or light) as a few Earth radii (or masses). As a result, many of the smallest (or lightest) exoplanets known today cannot be classified as either rocky (required for habitability) or gaseous, because their mean densities remain unknown for lack of mass or radius measurements. PLATO will be unique in providing vital constraints for planetary interior models.

Evolution of Planetary Systems

Planets and their host stars evolve. Giant gas planets cool and contract, a process which can last up to several billion years: this process will be studied by PLATO through accurate measurements of stellar ages. Using accurate radius and mass measurements, we will determine how planets form and evolve by observationally building evolutionary tracks for gaseous exoplanets as functions of stellar properties. Over time, terrestrial planets lose their primary hydrogen atmospheres, develop secondary atmospheres, and may develop life. PLATO will provide key data on terrestrial planets at intermediate orbital distances, including in the habitable zones of solar-like stars with different ages, allowing us to study Earth-like planets at different epochs. Furthermore, the architecture of planetary systems is shaped through physical and dynamical processes on time scales accessible to PLATO asteroseismic dating.

Planetary Atmosphers and Star-Planet Interactions

Planets discovered around the bright PLATO stars (mV=4–11 mag) will be prime targets for spectroscopic transit follow-up observations of their atmospheres (using JWST, E-ELT, etc.). Small planets with low mean density are particularly interesting, as they are likely to have a primordial hydrogen atmosphere. Small planets with high densities are likely to be terrestrial planets with secondary atmospheres. The PLATO catalogue will therefore play a key role in identifying small planet targets of interest at intermediate orbital distances. It will also provide information on planetary albedos and the stratification of planetary atmospheres. Finally, the close-in planets found around stars of different types and ages will provide a huge sample to study the interaction between stars and planets due to, for example, stellar winds or tides.

Structure and Evolution of the Milky Way

The intrinsic luminosity of red giant stars allows us to probe distances of up to 10kpc into our Galaxy, and to determine accurate ages from asteroseismology. Red giants can thus be efficiently used to map and date the Galactic disc. These data will complement the information on distances and chemical composition obtained by the Gaia mission. In addition, asteroseismic ages provided for PLATO targets can be compared to age determinations by other means, for example to calibrate gyrochronology - the method of estimating stellar ages based on their rotational rates.