Plato’s primary objective is to pave the way for a new kind of exoplanetary science by carrying out a comprehensive statistical analysis of extrasolar planetary systems. Specifically, systems which are relatively close to us and whose host star is bright enough to be studied in detail.
Plato will observe these stars with a photometric accuracy of 1 ppm and provide a seismic analysis to determine stellar mass with an accuracy of 1%. With this data, scientists will be able to determine these systems’ age with an accuracy of a few hundred million years and improve our understanding of planetary and stellar evolution.
A vast number of additional stars will also be observed, with a lower accuracy. This will extend our knowledge of extrasolar planetary systems and their host stars, building on the CoRot and Kepler missions’ discoveries. Plato will go further by observing more stars, more closely, in order to detect orbiting planets with Earth-like size and orbit.
An important secondary objective for Plato is to carry out seismic analysis for a large number of stars along the entire Hertzsprung-Russel diagram, even when no planetary system is detected.
This will help further our understanding of stellar and planetary evolution processes by studying stars’ internal structure and planetary system distribution. This represents a major step, preparing for future breakthroughs in most astrophysics-related fields as well as scientific and philosophical views on the origin of life and/or the Universe.
The current plan is to launch Plato using a Soyuz 2-1b launcher with an ST-type fairing for injection into orbit around the Sun-Earth system’s L2 Lagrangian point. This launcher has a payload capacity of just over 2,100 kg for this type of orbit. Launch is scheduled for 2024.
Plato will follow a Lissajous orbit ranging from 400,000 to 500,000 km from L2. L2 was selected for its stable environment in terms of temperature and radiation, as well as eclipse-free orbit possibilities and an unobstructed view of a large part of the sky (the Sun, the Earth and the Moon are allocated in one relatively small angle as seen from the satellite). Once in orbit, the satellite will periodically rotate around its pointing axis in order to keep its solar shield towards the Sun. Orbital station-keeping operations will be carried out once a month.
The mission’s nominal lifetime is 6 years, divided into three stages. The first two will be long-duration observations, focused on two sky fields thought to hold a high density of cold dwarf stars and designed to detect transits with an orbital period similar to Earth. One of these will most likely be centred on longitude λ = 210° and latitude β = -60° from the ecliptic, and the second around λ = 306° and β = 67°, close to the galactic plane. The first observation phase will last 2 to 3 years and the second 2 years. Long observation periods are necessary in order to reduce the risk of false positives; these can be caused naturally, as star brightness can vary and background objects can cause interference, or artificially, by the satellite’s payload. The third and last part of the mission will be a “step-and-stare” phase, during which different parts of the sky with scientifically relevant cycles will be monitored for a few months. This last stage will last for at least 1 year.
Areas of the sky already observed by Corot and Kepler and future Plato targets. The lighter “step-and-stare” target areas are indicative, as their exact location is yet to be determined.Each area will contain at least 20,000 dwarf and sub-giant stars with a magnitude under 8. The satellite’s photometric accuracy is 3.4 x 10-5. Maximum magnitude for target stars is 11, in order to work with observatories for optimal monitoring and characterisation conditions.