The PLATO space probe (PLAnetary Transits and Oscillations of Stars) will soon allow us to delve deeper into the fascinating realm of extrasolar planets. The mission aims to discover Earth-like planets harbouring water, offering the potential of some form of life. Such planets need to be in the 'habitable zone' – at the right distance from their star, in a region with moderate temperatures. The PLATO mission was selected by the European Space Agency (ESA) around ten years ago and is progressing steadily as its 2026 target launch date grows closer.
Tracking down planets with the transit method
PLATO will use the transit method, which has already been used successfully by missions like CoRoT, to find planets orbiting other stars. When a planet passes between us and its star, it obscures part of the star's disc. As a result, the brightness of the star regularly diminishes with each 'transit' in front of the disc. Planets can be detected through such periodic yet tiny changes in brightness. The sensitivity and resolution of the instruments and stable alignment of the telescope are extremely important for finding and evaluating such small signals. It is possible that differences in brightness as small as one-tenth of a thousandth of the starlight can be measured.
PLATO study areas with long observation periods
Two selected PLATO study areas with an observation period of at least two years (blue). PLATO will examine the area in the southern hemisphere first. The observation fields of other missions are shown in red (CoRoT) and magenta/green (Kepler/K2).
Credit:
Nascimbeni et al., A&A, 658, A31 (2022)
PLATO: a new telescope design with 26 cameras
Instead of a single telescope with a large mirror, PLATO uses 26 individual optical cameras that measure the light intensity of thousands of individual stars. These images can then be combined. Each camera contains four large-format CCD sensors. Their job is to convert the light signals into electrical charge.
Cameras with different jobs
The optical elements in front of PLATO's cameras are identical across all cameras, but the sensors in each read out data at different rates. Two 'fast' cameras have CCDs with readouts every 2.5 seconds, while the 24 'normal' cameras do so every 25 seconds. The information from the fast cameras is required to align and track the satellite precisely.
The 24 normal cameras are arranged in groups of six. These workhorses of the mission will observe thousands of stars and determine their light curves, from which planetary transits can be found. The cameras are aligned so that they all cover the centre of the study area, while the areas further out are only covered by 18, 12 or six cameras. This means that stars at the centre of the image field are measured with the highest accuracy.
Illustration of the PLATO cameras' field of view
The field of view of PLATO's 26 cameras covers a section of the sky measuring approximately 49 by 49 degrees. This corresponds to about five percent of the entire celestial sphere. The shades of blue indicate the different number of cameras observing the region. The centre is observed by all 26 cameras, and further out there are only 18, 12 and 6 respectively.
Credit:
Nascimbeni et al., A&A, 658, A31 (2022)
The extremely sophisticated readout and data processing electronics of the fast cameras, which also determine the satellite's precise orientation, were developed and made ready for PLATO's launch by DLR's two space institutes in Berlin: the Institute of Planetary Research and the Institute of Optical Sensor Systems. The fast cameras also feature blue and red filters, allowing for the detection of transit events in both the shorter blue and longer red wavelengths. Variations in the transit signals across these wavelengths can provide clues about the surface of observed exoplanets, and the possible presence of an atmosphere.
Integration of the first cameras
The first camera is mounted on PLATO's special platform
The first of 26 PLATO cameras was integrated on the optical bench at OHB in Oberpfaffenhofen in June 2024. It sits on the top row and has a distinctive black, cylindrical body. The white covers mark the locations of the remaining cameras.
Credit:
OHB System AG
The first ten cameras have now been thoroughly tested and qualified as suitable for use in space. They have been handed over to OHB System AG in Oberpfaffenhofen, the main contractor for the PLATO mission, where they are being mounted and adjusted with high precision on a special platform called an optical bench. The first set of cameras were integrated in June 2024, with the remaining cameras due to be added by the end of the year.
European cameras through and through
The individual components, optical elements, detectors and electronic modules were built in various European countries and are now being assembled in Belgium. The 26 cameras have already been assembled at the Centre Spatial de Liège (CSL) in Liège, Belgium. The technology will be qualified for use in space at three test facilities: SRON Netherlands Institute for Space Research in Groningen, the Netherlands; the Institut d'Astrophysique Spatiale (IAS) in Paris, France; and the National Institute of Aerospace Technology (INTA) in Madrid, Spain.
The cameras undergo a precisely defined procedure to assess their structural strength (a vibration test), their behaviour in a vacuum and their response to changes in temperature. Repair work, such as that carried out on the Hubble Space Telescope due to its low flying altitude and accessibility via space shuttle, has been ruled out with PLATO: there is no room for anything to go wrong here. PLATO's final position will be Lagrange point 2, which is 1.5 million km from Earth, along the extended Sun–Earth axis. The James Webb Space Telescope and the European Gaia probe are also located here.
European cooperation on the PLATO mission
PLATO is an M-class mission in the European Space Agency's (ESA's) Cosmic Vision 2020–2025 programme. The aim of the mission is to find and characterise Earth-like extrasolar planetary systems in the Milky Way. PLATO is scheduled to launch at the end of 2026 on an Ariane 6 rocket from Europe’s Spaceport in Kourou, French Guiana.
The payload of the PLATO mission is being developed by an international scientific consortium working in tandem with ESA. The international PLATO payload consortium is financed by national agencies from over 13 participating countries in Europe and Brazil, including the German Space Agency at DLR with funding from the Federal Ministry for Economic Affairs and Climate Action (BMWK). The PLATO payload consortium is headed by Heike Rauer, Director of the DLR Institute of Planetary Research in Berlin-Adlershof. A team from the DLR Institute of Planetary Research and the DLR Institute of Optical Sensor Systems is supporting the mission, especially with its calibration, operation and execution. This includes the development and construction of the readout electronics for the two fast cameras, the payload computer and parts of the data processing technology onboard the space telescope.
