PLATO will explore the diversity of planetary system by accurately determining their bulk properties, by constraining planet formation models, and by better understanding the temporal evolution of planets and planetary systems. 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 processes. To place our system into context, we must look at other systems and study their architectures and compositions at different stages of their evolution. Current observations have established that the bulk compositions of exoplanets and the architectures of exoplanet systems can differ substantially from the solar system, and this must be indicative of the complexity and diversity of formation process and evolutionary paths. PLATO will provide groundbreaking insight into these fundamental questions, by constraining with unprecedented accuracy radius, masses, and ages of a large number of planetary systems.

PLATO will provide unprecedented accurate planetary parameters to constraint the interior composition of terrestrial and gas planets. Many confirmed exoplanets fall into new classes unknown from our solar system, for example “hot Jupiters”, “mini-Neptunes” (planets with masses comparable with the terrestrial planets in our solar system, but with much smaller bulk densities), or “super-Earths” (rocky planets with masses below 10 ME). 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 precisely as either rocky (required for habitability) or gaseous, because their mean densities remain unknown for lack of mass or radius measurements. PLATO will help us overcoming these limitations and for the first time, we will be able to determine accurate ages of a large number of stars that can be investigated by asteroseismology. Planetary (system) properties can then be correlated with temporal evolution processes once a sufficiently large database of planet and stellar properties is available from PLATO.

PLATO will characterise for the first time terrestrial planets in the habitable zone of solar like stars. Habitability is defined as the potential of an environment to sustain life of any kind. In this context, the existence of liquid water for long periods of time on the surface of a terrestrial planets is one of the requisites for life. The detection of small planets, similar to Earth, around stars like our Sun, orbiting at distances such that liquid water can exist for long periods of time, and potentially sustain life on the planetary surface, is challenging. The detection of these planets, which is the driver for the design of the PLATO mission, will be a breakthrough for our understanding of the conditions that led to the emergence of life on Earth. The investigation of these small planets with long period orbits will be complemented with planets of all sizes, in all kinds of systems, on all kind of orbits, e.g. binary (or multiple) stars with planets, planets around intermediate-mass and giant stars, post RGB-stars, planets on co-planar or inclined orbits, exomoons, planets PLATO Definition Study Report page 10 with rings, disintegrating planets, and many more, exploring the diversity of planetary systems in the Galaxy. Since the planets well characterised by PLATO orbit bright stars (<11 mag), they will also be Rosetta-stone candidates for follow-up transit spectroscopy investigating their atmospheres, e.g. via JWST or E-ELT.

PLATO photometry is designed to allow the detection of planets via photometric transits. But planets will also be detected by timing variations of transits (TTV) and via the reflected stellar light on the planet. The latter, together with the orbital planet movement, cause periodic variations in the light curves, or phase curves, which provide a means to investigate planet atmospheres and planetary albedos. PLATO’s long pointings and high photometric precision are unique for enabling the observation of thousands of photometric planet phase curves.

PLATO will determine stellar properties with asteroseismology and complement information available from ESA’s Gaia mission. Asteroseismology of a large number of different types of stars at different stages of their evolution will provide a unique, revolutionary, understanding of stellar interior and evolution models. PLATO will be the first mission to make systematic use of asteroseismology to characterise planet host stars allowing us to link planetary and stellar evolution. 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, providing strict requirements for the photometric performance of the payload.

The PLATO legacy database will provide a unique resource that will be crucial to test our models of planetary and stellar evolution. The large number of PLATO light curves possible as complementary science allows for a wide science program reaching far beyond the exoplanet science community. This program will include topics like, e.g., structure and evolution of Red Giant stars, hot OB sub-dwarf stars, massive stars, asymptotic giant branch stars and supergiants, white dwarfs, pre-main sequence stars, variable stars like eclipsing binaries or classical pulsators, as well as stellar clusters of various ages and metallicity. PLATO data in combination with Gaia results will help significantly to better understand processes in our Milky Way. The PLATO catalogue of thousands of characterised planets and between 300,000 and ~1,000,000 high precision stellar light curves (depending on the final observing strategy) will provide the basis of a huge legacy for stellar and (extra)galactic science, which will be explored by the community in the years to come during and after the PLATO mission.

PLATO will detect thousands of new planetary systems of all kinds, including hundreds of small planets suitable for precise characterisation (RV and asteroseismology). Depending on the actual occurrence rate, which is still debated in the literature, PLATO will detect and characterise tens of small/low-mass planets in the HZ of bright, Sun-like stars for which accurate radii, masses, mean densities, and ages can be derived. This goal is unique to PLATO. In addition, PLATO will be able to detect exomoons, planetary ring systems, Trojan-planets, exo-comets, etc., thereby expanding our knowledge about the diversity of planetary systems.
The mission aims not only at a statistical approach to studying the frequency of terrestrial planet occurrences, but also asks about the nature of these planets, their bulk properties, atmospheres, and ultimately whether they could harbour life. Answering these questions require a new approach and impose new requirements on planet detection surveys, because they need detailed follow-up observations at high SNR. To address these science questions, PLATO will:

• Detect planets around bright stars (V≤ 11) to determine accurate and precise bulk densities and ages, and allow for follow-up spectroscopy of planetary atmospheres;
• Detect and characterise terrestrial planets at intermediate orbital distances up to the habitable zone around Solar-like stars to place our solar system in context;
• Detect and characterise planets in statistically significant numbers for a broad range of planet and planetary system classes to constrain planet formation scenarios.

These requirements are at the core of the design of PLATO, and define its main target range and observational strategy.

 

References

PLATO – Revealing habitable worlds around solar-like stars
Definition Study Report, ESA-SCI(2017)1, April 2017