The first set of light-sensitive camera sensors for the European Space Agency (ESA) 'PLAnetary Transits and Oscillations of stars' (PLATO) space telescope was approved by ESA in March 2019. The delivery of these detectors is an important milestone in the construction of what will be a groundbreaking space telescope, which from 2026 will search for and study Earth-sized exoplanets orbiting nearby stars. The PLATO space telescope is the first of its kind. It will consist of 26 separate telescopes mounted on a spacecraft platform. "Together, the PLATO cameras will make scientific use of the largest light-sensitive CCD surface ever deployed in space," says Professor Heike Rauer, Director of the German Aerospace Center (Deutsches Zentrum für Luft- und Raumfahrt; DLR) Institute of Planetary Research and Leader of the PLATO Payload Consortium.
"These sensors are at the heart of the entire mission. The delivery of the first detectors is very important for further progress in constructing PLATO and maintaining the mission timeline. This approval will allow the first stage in the complex integration of the sensors to begin and the testing of the telescopes to continue," Rauer continues. PLATO is the third European mission designed to search for exoplanets – planets outside the Solar System. The first was the very successful Convection, Rotation and planetary Transits (CoRoT) mission, which was led by the French space agency (Centre National d'Études Spatiales; CNES) and in which DLR also participated. CoRoT observed stars in the Milky Way from 2007 to 2013. The other two missions are led by ESA. They are the CHaracterising ExOPlanet Satellite (CHEOPS) mission and PLATO, which are scheduled for launch in 2019 and 2026, respectively. The PLATO Payload Consortium, which is developing the scientific instruments that make up the spacecraft payload, is led by DLR.
The largest ever area of camera sensors will be used to search for Earth-like planets
PLATO's main task will be to search for Earth-like or rocky planets orbiting Sun-like stars. Over a period of months to years, PLATO will use highly sensitive detectors – very advanced versions of the Charge-Coupled Device (CCD) sensors used in digital cameras – to monitor changes in brightness of thousands of stars. Periodic dips in brightness could be caused by planets when these pass in front of their host star, called a transit.
To perform these observations, each of the 26 PLATO telescopes will be equipped with a set of four CCD sensors. CCDs are light-sensitive, 'charge-coupled' semiconductor devices that are arranged across a surface to form an integrated circuit or 'chip', and are used behind an optical system to acquire an image. This is basically the same technique used for every picture taken today with a mobile phone or digital camera. However, CCDs for cameras in space have to meet much higher requirements than those on Earth. "The delivery of the first detectors at this stage is important because it ensures the early availability of one of the key elements of the overall mission,” said Bengt Johlander, ESA Payload Manager for PLATO. “This is the first step in the complex testing of the telescope cameras."
Each of the 26 telescopes will have four CCDs specially developed and produced by Teledyne e2v in Chelmsford (United Kingdom). The first 20 CCDs for PLATO were delivered to ESA in mid-March 2019; the remaining 84 detectors will arrive in further batches up until the end of 2020. Each of the PLATO CCDs acquires 20-megapixel images, a size comparable with commercially available digital cameras. Most of the PLATO CCDs will acquire an image every 25 seconds to measure the light from the target stars. Eight of the CCDs will be installed in two 'fast' telescopes ('fast CCDs' or FCCDs), which will perform measurements on brighter stars every 2.5 seconds to fine-tune the performance of the spacecraft. The combined telescopes have a total image size of approximately two gigapixels. This is more than twice as much as ESA's Gaia mission, which currently has the largest camera ever flown in space. The requirements for fast readout of the CCDs and the subsequent automatic processing of the image data are correspondingly high. Approximately 435 gigabits of data will be transmitted to Earth each day.
More than 11,000 times the area subtended by a full Moon will be examined at once
With the telescopes' partially overlapping fields of view, the PLATO cameras can detect any slight dimming or brightening of a star caused by the movement of one or more planets in front of it. Extrasolar planets are extremely difficult to detect and study due to their small size and low brightness, proximity to their host star, and great distance from Earth. The CCDs are a key element of the scientific payload being provided by the consortium of European research centres and institutes led by DLR. PLATO will not only search for new planets, but also determine their masses and investigate the properties of their host stars. The mission will determine the sizes and ages of the stars with unprecedented accuracy. In this way, the scientists will be able to understand how planetary systems are structured and identify potentially habitable worlds.
The PLATO telescope system will have an extremely wide field of view and cover a total sky area of approximately 2250 square degrees. For comparison, a full Moon observed from Earth covers only approximately 0.2 square degrees of the sky. PLATO will be able to cover this large field of view because the telescopes have overlapping fields of view in groups. The large format of the CCDs, about eight by eight centimetres per detector, results in an optically sensitive total area of 0.74 square metres. The detectors will operate at a temperature of below minus 65 degrees Celsius in order to achieve increased sensitivity due to a particularly low level of ‘background noise’.
Depending on the final scientific operational plan, PLATO will observe between 10 and 50 percent of the sky from its orbit around the second Lagrange point of the Sun-Earth system. Lagrange or ‘equilibrium’ points are five points in a system of two celestial bodies where the gravitational forces due to the major bodies cancel out and a lightweight object such as a spacecraft can follow the smaller body with the same orbital period. The L2 point of the Sun-Earth system is located 1.5 million kilometres behind Earth as viewed from the Sun and allows a 24-hour-a-day view of space, undisturbed by solar radiation and with uninterrupted communications with its ground stations.
PLATO – an ESA M-class mission
PLATO is the third M-class (medium-sized) mission in ESA's long-term Cosmic Vision programme. Its goal is to find a large number of extrasolar planetary systems and, above all, to study terrestrial or rocky planets, some of which might lie in the habitable zone around Sun-like stars. During the planned four-year primary mission, PLATO will observe hundreds of thousands of stars, leading to the discovery and characterisation of thousands of new exoplanets. PLATO will survey and observe large portions of the sky, focusing on the brightest and closest stars. PLATO has also been designed to investigate seismic activity in stars, enabling the precise characterisation of the planets' host stars, and in particular their age.
ESA is providing the spacecraft, the CCDs, mission operations and part of the science operations. The PLATO Payload Consortium is financed by national agencies – including the DLR Space Administration, which receives funding for this purpose from the German Federal Ministry for Economic Affairs and Energy. The consortium is based in Berlin under the leadership of Heike Rauer of DLR; it is developing the science payload will participate in the programme's science operations.