The ability to derive the age of planetary systems is one of the key assets of PLATO. The age of stars is traditionally poorly constrained, to within (at best) only a few Gyr for stars on the main-sequence. Furthermore, young planets, that are the most important when trying to decipher the conditions under which planetary systems are formed, orbit around young, active stars; the determination of their parameters has thus far remained elusive (e.g. Gillon et al. 2010; Czesla et al. 2009; Guillot & Havel 2011).
With relative ages of a large sample of main-sequence stars known to 10% precision, PLATO will essentially remove the age ambiguity in planet evolution. A large sample of planetary systems with well-determined ages will allow us to search for archetypes of planet and planetary system evolution, and identify correlations with host star parameters, planet interior composition, and planetary structure.
Planets and planetary systems evolve with age in several different ways:
- Gas giant planets progressively cool and contract, a process that lasts up to several Gyr (see Section 2.1.6). An accurate knowledge of age is therefore crucial for the interpretation of measured radii and a determination of interior structure.
- Planet formation theories predict rocky planets with primordial hydrogen atmospheres, but terrestrial planets evolve with time, as exemplified by the planets in our solar system (see also Section 2.1.5). The atmospheres of the terrestrial planets in our solar system are secondary atmospheres produced by impacts and outgassing from the interior, both processes having been more intense in the young solar system. In the case of Mars, a possibly denser young atmosphere has since been lost to space. In the case of the Earth the atmosphere has been further modified by the development of oxygen-producing life (tertiary atmosphere) since the planet was about 2.5 Gyr old.
- Host stars evolve with time and expose young planets to much higher UV and high-energy radiation levels than found on Earth today (see Section 2.1.13). This affects processes such as atmospheric losses, and also increases radiation levels at the surface of terrestrial planets, affecting the prospects for life. Therefore, good characterisation and dating of planet host stars is crucial if we are to obtain an understanding of the evolution of planetary atmospheres, and of habitable conditions. The significance of atmospheric loss processes is also crucial for understanding whether small planets are able to retain their extended, hydrogen-dominated, primary atmospheres over a significant part of the lifetime of a planetary system (see Section 2.1.12). An observational constraint on the presence of primary atmospheres for planets with different ages will allow us to test planet synthesis models (e.g. Mordasini et al. 2009, 2012b; Section 2.1.2). Crucial here are planets at intermediate orbital distances, which are less affected by loss processes caused by strong stellar radiation.
- The architecture of planetary systems is shaped through planet formation and subsequent dynamical processes that cover a wide range of timescales, up to billions of years. The comparison of planetary system populations with different ages will allow us to investigate whether typical evolution scenarios exist. In the case of hot Jupiters, precise ages of different systems will allow us to assess their evolution (disk or Kozai migration) and their fate. For small size planets, Kepler has discovered several compact multiple systems which show significant dynamical interactions, and are very interesting for the understanding of planetary formation: Kepler-90 with 7 planets (Cabrera et al. 2014); Kepler-11 with 6 planets (Lissauer et al. 2011), and Kepler-154 with 6 planets (Ofir & Dreizler 2013; Rowe et al. 2014).
The distinct evolution of the terrestrial planets in our solar system is far from being fully understood. Exoplanets can complement our investigations of terrestrial planet evolution by contributing information not accessible in the solar system, namely a large number of planets covering a wide range of bulk parameters, with different ages. This will allow us to search for systems to be used as case studies. Such a sample will also allow investigation of possible correlations of planetary evolution processes with stellar and planetary system parameters, which will provide a breakthrough in our understanding of the evolution of atmospheric composition and habitability. PLATO will provide key steps towards this ultimate goal.
Terrestrial planets at intermediate orbital distances play a crucial role; PLATO will measure their bulk densities and masses, necessary to estimate outgassing efficiencies and atmospheric scale heights, and will determine accurate and precise ages. Since PLATO target stars are bright, they will be ideal targets for future large missions that will spectroscopically characterise the atmospheres of nearby Earth-like planets for signatures of life.
PLATO will be able to provide accurate and precise masses for complex planetary systems. Furthermore, TTV observations over long time periods, e.g. by combining PLATO with already available Kepler observations, would allow measurement of the Q-factor describing internal tidal energy dissipation of planets, a factor crucial to understand the tidal evolution of close-in planets.
The accurate and precise determination of planetary system ages for thousands of systems is one of the key features of the PLATO mission. This crucial goal will not be achieved by anyother current or planned transit mission. Key science questions PLATO can answer include:
- What are the ages of planetary systems?
- How do planet parameters (e.g. mean densities, radii of gas giants, planet star distance distributions, and, if combined with spectroscopic follow-up, atmosphere properties) correlate with age?
- How many super-Earths retain their primary atmosphere? Is there a correlation of small planet primary atmospheres with system age? What are the main parameters governing the presence of primary atmospheres (e.g. formation mechanism, stellar type, orbital distance, age, metallicity)?
- What is the planetary evolution timescale compared to the lifetime of the system?
How does the structure/architecture of planetary systems vary and evolve with age?