How did planetary systems form, and how did they evolve? What are the conditions for life to be able to exist on a planet? Is our Earth unique, or has life evolved anywhere else in the universe? To approach the answers to these questions the science mission PLATO (PLAnetary Transits and Oscillations of stars) will conduct a search for planetary systems outside our solar system starting in 2026.
Launch of Mission: 2026
Planets in such systems are also called "exoplanets". PLATO shall detect these planets, and determine their physical parameters such as masses, diameters, and the orbits of these celestial bodies. The goal of the mission is to considerably expand our knowledge about the origin and the evolution of planetary systems. One of the most important issues of the mission is the search for exoplanets that are only slightly larger than the Earth.
Applying 26 cameras mounted on a common platform PLATO will investigate up to a million stars for possible planets. It is expected that it will detect many thousands of rocky, icy, and gas planets circling around stars similar to the sun, but also around red dwarf stars, binaries and white dwarfs. Types of planets which cannot be found in our solar system are of specific scientific interest. Among these are, for example, "Hot Jupiters", giant planets which orbit their stars on very close orbits within a few days, "Mini Neptunes", i.e. gas planets that have diameters smaller of that of Neptune, and "Super-Earths", rocky planets with masses of more than one and less than ten Earth masses.
In order to trace and find exoplanets PLATO uses the observational method of planetary transits. With this method the spacecraft registers the temporal eclipse of a star caused by a planet transiting in front of it. The decline of light intensity induced by small planets is in most cases only a few thousandths. Planetary "candidates" detected in this way will later be rechecked using radial velocity measurements made with telescopes from the Earth’s surface. Beyond that, PLATO will measure stellar vibrations in order to fathom the inner structure of the stars by means of asteroseismology. This, then, allows to deduce further parameters such as the ages of the stars, and therefore of their planets.
Planetary Transits
In order to achieve its scientific objectives PLATO utilizes the method of photometric transit observations of planets in front of their stars. This allows planets to be found which, when viewed from the Earth, or from the position of a spacecraft, pass in front of the respective star, and therefore obscuring it a little bit. If at least two consecutive transits are observed it is possible to directly determine the orbital parameters of the planet (orbital period and distance from the star), and its diameter from the duration and interval between the transits, and the drop in brightness of the star.
The two long-term observations by PLATO will each last two years to find planets with orbital periods up to one year, and maybe also detect twins of the Earth. So called "step-and-stare-observations" will follow monitoring selected fields in the sky for two to four months each. In this way almost half of the entire sky can be covered within a total mission duration of about six years.
Observations of Radial Velocities
The orbital motion of a planet around its star makes its possible to verify the existence of an otherwise invisible exoplanet by other means than the transit method. Gravitation causes rotation of a star and its planet about their common center of gravity. During this motion not only the planet, but also the bright star moves at times a little bit towards Earth and the observer, and, half a period later, in the opposite direction.
If you split up the light of the star into its components (colors) with the help of a spectrograph attached to a big ground-based telescope the orbital motion of the star about its center of gravity becomes noticeable as a periodic motion of narrow lines in its spectrum. If the star in its orbit moves away from the observer these spectral lines will be shifted towards the red end of the spectrum; if it moves towards him there will be a blue shift (Doppler effect). The closer the planet is to its star, and the higher its mass, the larger is the effect.
In this way the orbit of the exoplanet and a lower limit for its mass can be derived. When, in addition, the diameter of the planet is known from measurements with the transit method a (mean) mass density can also be determined. Thus, it is finally possible to draw conclusions on the nature of the planet since rocky planets like the Earth tend to have higher densities, whereas gas planets like Jupiter and Saturn have lower ones.
Subsequent ground-based observations are an important part of the mission and require a coordinated approach with astronomical observatories and institutions around the world such as the European Southern Observatory (ESO). Furthermore, stellar masses, radii and ages will be derived with high accuracy using asteroseismology.
26 cameras will survey the sky
PLATO’s 26 cameras will search the sky for planets around bright stars. Each of these cameras is equipped with a wide-angle optics, and four sensors (CCDs) with 4510 x 4510 pixels each. The combined fields-of-view of all cameras amounts to about five percent of the whole sky. Every 25 seconds 24 of the cameras take one picture each, while the two remaining cameras operate at a ten times shorter exposure time of 2,5 seconds. These "fast cameras" shall mainly observe the brightest stars that would be overexposed during long exposures. A comprehensive sensor and data processing electronics on board the spacecraft will extract images of the target stars from each picture of the sky, and produces a little image (imagette) of each star in the field. These will then be compressed and transmitted to Earth where the data will be further processed.
Concept studies of the PLATO probe
Concept study of the PLATO probe with its payload consisting of 26 separate cameras with overlapping fields of view (left). An alternative concept is shown on the right: The two "fast" cameras (coloured grey) are positioned in the middle. Both images still show an old design with 34 cameras.
PLATO is the third of the medium-sized science missions in ESA’s Cosmic-Vision-Programme 2015-2025. In contrast to its precursor missions CoRoT and Kepler, PLATO will monitor more, and mostly brighter stars over a more extended area of the sky, and over a longer period of time. In this way, a substantially larger number of newly discovered planets are to be expected. At present, the exoplanet missions CHEOPS (launch 18 December 2019) and TESS (launch 18 April 2018) are operating. CHEOPS carries out targeted photometric observations of already known exoplanets around bright stars, while TESS aims at a two-year survey for exoplanets of the brightest stars in the sky. Both missions, however, will only be able to find a considerably lower number of planet candidates owing to their shorter mission durations, and because of their much slower optics.
A comprehensive ground segment receives, processes and archives the data
The PLATO ground segment consists of the Mission Operations Centre (MOC) which will be located at the European Space Operations Centre (ESOC) in Darmstadt, Germany, and a worldwide network of ground stations (antennas) to send and receive the data. Moreover, a Science Operations Centre (SOC) is responsible for mission planning, the processing of the science data, and the generation of higher-level data products. The SOC will be established at the European Space Astronomy Centre (ESAC) in the vicinity of Madrid, Spain. A PLATO Data Centre, distributed over several European countries will provide all algorithms and tools necessary to process the information, check the data products, provide them to the scientists and the public, and finally archives them.
DLR coordinates international cooperation
Prototype of the wide-angle optics
In the run-up to the mission, a prototype of the wide-angle optics (telescope) for the camera system of the PLATO payload was built. The diameter of the entrance window (top) is around 20 centimetres.
The payloads for the PLATO mission are developed and built by an international consortium which will deliver contributions to the hard- and software as well as the data centre. This includes, inter alia, scientific institutes and companies from Germany, Italy, the United Kingdom, France, Spain, Switzerland, Belgium, Hungary, Portugal, the Netherlands, Austria, Sweden, Brazil and Denmark. The consortium is led by the DLR Institute of Planetary Research in Berlin, Germany, with its Principal Investigator (PI), Prof. Heike Rauer.
The overall mission implementation, i.e. the spacecraft, the launcher, the ground segment, and the operations falls under the responsibility of ESA. The setup and operation of the PLATO Data Center is headed by the Max-Planck-Institute for Solar System Research in Göttingen, Germany. A substantial participation in the scientific data reduction and processing completes the German contribution to the PLATO mission. Part of the payload development, the data center, and the payload operations after 2026 is funded by the German Federal Ministry for Economic Affairs and Energy (BMWi) via the DLR Space Administration.
Mission Data and Technical Parameters of PLATO:
Launch:
2026 from Kourou (French Guiana)
Launcher:
Soyus-Fregat2-1b
Orbit:
Halo orbit around Lagrangian point L2 (distance from Earth about 1.5 million kilometers)
Mission duration:
Minimum of 4 years (4 years of nominal operations and mission extension)
