PLATO: PLAnetary Transits and Oscillations of stars
by Ruth Titz-Weider (DLR), August 2024
Mission idea
To date, we have not found an Earth-like exoplanet in the life-friendly, habitable zone around a Sun-like star, even though there were occasional sensational media reports in the last few decades. Earth-like in this case means that it resembles Earth in a lot of ways. So, if it has a similar radius and mass, it is a rock planet with certainty, and if its orbit time and distance to its star – which is similar to our Sun – such that its surface exhibit moderate temperatures. We may have found some planets with similar mass or radius, but often both values are not well known simultaneously, or we cannot clearly classify the star and its age.
Artist’s depiction of the PLATO-satellite. Credit: ESA / ATG medialab
NASA’s Kepler mission has discovered thousands of planets and planet candidates with the transit method, but for many of those, follow-up measurements to determine mass and stellar type are not possible because their star appears to dim. This, however, is the explicit goal of PLATO: to find Earth-sized planets in the habitable zone of Sun-like stars and to characterize them further. But the mission also includes an accurate study of the host stars, including determining its age.
Organisation
PLATO is a satellite mission of ESA, the European Space Agency, which was chosen in 2014 as a so-called M-class-mission as part of the “Cosmic Vision” program. The scientific instrument and the mission execution are the responsibility of the international PLATO consortium, led by Professor Heike Rauer, DLR. Fourteen European countries and Brazil are represented in the consortium. The nominal mission duration is four years, and the technical specifications allow service of 8.5 years in total.
Sticker of the PLATO Mission Consortiums with flags of participating countries. Credit: DLR (CC BY-NC-ND 3.0)
Instrument
The PLATO instrument features a novel telescope design with 26 cameras. Each camera is equipped with a wide-angle lens with an aperture of 12 cm and four large-format CCD sensors in the focal plane. The sensors are sensitive in visible light and near infrared. 24 of these cameras, the ‘normal’ cameras, will observe more than 100,000 stars to detect planetary transits. The other two cameras have a special task. They read out the CCD very frequently – every 2.5 seconds versus 25 seconds – to ensure the satellite’s exact orientation. These two ‘fast’ cameras are equipped with a special red or blue filter, so that transit events can be recorded in both wavebands and thus obtain indications of the surface or a possible atmosphere.
Illustration of a PLATO camera: the optics are located in the tube, the protective shield at the top is intended to prevent stray light from entering the camera. Credit: PLATO Mission Consortium
Where and how is PLATO looking?
Six cameras each form a group whose fields of view are slightly offset from each other. The central field is covered by all 26 cameras, while the outer areas are covered by 18, 12 and six cameras. Stars in the central field are therefore measured with a higher photometric accuracy than those in the periphery. This large field, approximately 49° x 49°, corresponds to around 5% of the entire celestial sphere.
A depiction of PLATO’s field of view can be seen here: Nascimbeni et al. A&A, 658, A31 (2022)
The selection of the observation fields is based on a long and in-depth optimization process. For example, the stars had to be clearly measurable, i.e. no photometric impurities. Last year, the first observation field was selected, which PLATO will look at for a long time, more than a year.The selection of the fields is crucial for the mission in order to get the right stars in view.
One observation field is located in the northern hemisphere, the other in the southern hemisphere. The first field to be selected is the southern two observation fields, which are located near the star Canopus in the constellation Carina. The image shows the two PLATO fields in blue and, for comparison, the fields of the Kepler mission (magenta), K2 mission (green) and CoRoT (red).
The picture can be seen here: Nascimbeni et al. A&A, 658, A31 (2022)
The southern field was chosen as the first observation target, because there are more and better opportunities in the southern hemisphere to make follow-up observations with ground-based telescopes, e.g. to determine the mass using the radial velocity method.
Orbit
Like the James Webb Space Telescope, PLATO will work at the so-called Lagrange point 2. This point, or rather area, is about 1.5 million kilometers away from Earth in the extension of the line connecting the Sun and Earth. Here, the gravitational forces of the sun and the earth cancel each other out and the space probe can “rest” without using any energy. At this distance, no repair is possible, unlike the Hubble Space Telescope, which orbits at an altitude of around 500 km above the Earth and could be approached by the space shuttle (which, however, is slowly losing altitude due to the friction of the air molecules still present there and can no longer be raised in its orbit due to the end of the space shuttle program).
Schematic “top down” view of the Earth’s orbit around the Sun, with the 5 stable Lagrange points L1 to L5 shown. PLATO will orbit around L2 (on the right). Credit: NASA, STScI
Expectations
PLATO will observe bright stars. The most interesting ones form a group of about 15,000 dwarf stars and subgiants whose spectral type lies between F5 and K7 with magnitudes of less than 11 magnitudes. For these objects, the planetary and stellar parameters will be determined with the highest accuracy.
A second group is primarily used for statistical purposes. The light curves are already calculated on board to reduce the amount of data. This group comprises around 250,000 stars, including dwarf stars and subgiants, but also includes fainter stars with a brightness of less than 13 magnitudes. For most of these stars there will be a good radius determination, but not necessarily astroseismology or follow-up measurements by the radial velocity method.
Of course, estimates have also been made of how many planets PLATO can find. According to a study from 2022, PLATO could find eight to 34 Earth-sized planets in the habitable zone of a sun-like star.
It is exciting to wait for PLATO’s launch end of 2026 with an Ariane-6 launcher, and for the first measurements to come in.
