The PLATO space mission (PLAnetary Transits and Oscillation of stars) will detect terrestrial exoplanets at orbits up to the habitable zone of solar-type stars and characterise their bulk properties. PLATO will provide key information (planetary radii, mean densities, ages, stellar irradiation, and architecture of planetary systems) needed to determine the habitability of these unexpectedly diverse new worlds. PLATO capitalises on tremendous developments in high-precision photometry from space and ultra-stable ground-based spectroscopy techniques that have largely been led by Europe over the last 20 years.

PLATO will answer the scientific questions:

·       How do planets and planetary systems form and evolve?

·       Is our solar system special or are there other systems like ours?

·       Are there potentially habitable planets?

PLATO is the only mission either approved or in advanced planning that will be able to address these questions. For this purpose, it focuses on the small planets orbiting in the habitable zone of stars, including stars like our Sun. Furthermore, PLATO provides a huge database for all kinds of planets and planetary systems with well determined parameters. This database will complement the Gaia mission and provide a unique data set on planetary systems and stars for generations of scientists to come.

Science objectives

PLATO will explore the diversity of planetary system by accurately determining their bulk properties, by constraining planet formation models, and by better understanding the temporal evolution of planets and planetary systems. While the structure and mass distributions of bodies in our solar system are well known, we only have indirect and partial knowledge of its formation and evolution processes. To place our system into context, we must look at other systems and study their architectures and compositions at different stages of their evolution. Current observations have established that the bulk compositions of exoplanets and the architectures of exoplanet systems can differ substantially from the solar system, and this must be indicative of the complexity and diversity of formation process and evolutionary paths. PLATO will provide ground-breaking insight into these fundamental questions, by constraining with unprecedented accuracy radius, masses, and ages of a large number of planetary systems.

PLATO will provide unprecedented accurate planetary parameters to constraint the interior composition of terrestrial and gas planets. Many confirmed exoplanets fall into new classes unknown from our solar system, for example “hot Jupiters”, “mini-Neptunes” (planets with masses comparable with the terrestrial planets in our solar system, but with much smaller bulk densities), or “super-Earths” (rocky planets with masses below 10 ME). It came as a surprise that gaseous planets can be as small (or light) as a few Earth radii (or masses). As a result, many of the smallest (or lightest) exoplanets known today cannot be classified precisely as either rocky (required for habitability) or gaseous, because their mean densities remain unknown for lack of mass or radius measurements. PLATO will help us overcoming these limitations and for the first time, we will be able to determine accurate ages of a large number of stars that can be investigated by asteroseismology. Planetary (system) properties can then be correlated with temporal evolution processes once a sufficiently large database of planet and stellar properties is available from PLATO.

PLATO will characterise for the first time terrestrial planets in the habitable zone of solar like stars. Habitabilityis defined as the potential of an environment to sustain life of any kind. In this context, the existence of liquid water for long periods of time on the surface of a terrestrial planets is one of the requisites for life. The detection of small planets, similar to Earth, around stars like our Sun, orbiting at distances such that liquid water can exist for long periods of time, and potentially sustain life on the planetary surface, is challenging. The detection of these planets, which is the driver for the design of the PLATO mission, will be a breakthrough for our understanding of the conditions that led to the emergence of life on Earth.The investigation of these small planets with long period orbits will be complemented with planets of all sizes, in all kinds of systems, on all kind of orbits, e.g. binary (or multiple) stars with planets, planets around intermediate-mass and giant stars, post RGB-stars, planets on co-planar or inclined orbits, exomoons, planets with rings, disintegrating planets, and many more, exploring the diversity of planetary systems in the Galaxy. Since the planets well characterised by PLATO orbit bright stars (<11 mag), they will also be Rosetta-stone candidates for follow-up transit spectroscopy investigating their atmospheres, e.g. via JWST or E-ELT.

PLATO photometry is designed to allow the detection of planets via photometric transits. But planets will also be detected by timing variations of transits (TTV) and via the reflected stellar light on the planet. The latter, together with the orbital planet movement, cause periodic variations in the light curves, or phase curves, which provide a means to investigate planet atmospheres and planetary albedos. PLATO’s long pointings and high photometric precision are unique for enabling the observation of thousands of photometric planet phase curves.

PLATO will determine stellar properties with asteroseismology and complement information available from ESA’s Gaia mission. Asteroseismology of a large number of different types of stars at different stages of their evolution will provide a unique, revolutionary, understanding of stellar interior and evolution models. PLATO will be the first mission to make systematic use of asteroseismology to characterise planet host stars allowing us to link planetary and stellar evolution. The core program focuses on stars showing oscillations similar to those of the Sun, which are intrinsically stable and excited stochastically by the near-surface convection, providing strict requirements for the photometric performance of the payload.

The PLATO legacy database will provide a unique resource that will be crucial to test our models of planetary and stellar evolution. The large number of PLATO light curves possible as complementary science allows for a wide science program reaching far beyond the exoplanet science community. This program will include topics like, e.g., structure and evolution of Red Giant stars, hot OB sub-dwarf stars, massive stars, asymptotic giant branch stars and supergiants, white dwarfs, pre-main sequence stars, variable stars like eclipsing binaries or classical pulsators, as well as stellar clusters of various ages and metallicity. PLATO data in combination with Gaia results will help significantly to better understand processes in our Milky Way. The PLATO catalogue of thousands of characterised planets and between 300,000 and ~1,000,000 high precision stellar light curves (depending on the final observing strategy) will provide the basis of a huge legacy for stellar and (extra)galactic science, which will be explored by the community in the years to come during and after the PLATO mission.

Mission design

The measurement principle of PLATO is to carry out high precision, long(months to years),uninterruptedphotometric monitoringinthevisiblebandofverylargesamplesofbright(V≤11–13)stars. The resulting light curves will be used for the detection of planetary transits, from which the planetary radii will be determined, and for the asteroseismology analysis to derive accurate stellar parameters and ages. Thanks to the brightness of the PLATO targets, the masses of the detected planets will be determined from radial velocity observations at ground-based observatories.

PLATO comprises the spacecraft, provided by ESA, and the payload, provided by the PLATO Mission Consortium. The payload consists of 24 “normal” telescopes with CCD (ESA provided) based focal planes, operating in white light and providing a very wide field of view (FoV). They are read out with a cadence of 25 s and will monitor stars with V > 8. Two additional “fast” cameras with high read-out cadence (2.5 s) will be used for stars with V~4–8 and as fine guidance sensor. The paucity of bright stars necessitates a wide FoV, while the science drivers dictate the required sensitivity (numbers of cameras). Hence, the multi-telescope design allows for a large photometric dynamic range of 4 ≤ V≤ 16 (4 ≤ V≤ 11 for the core sample) and an extremely wide field (~2232 deg2). The ensemble of instruments is mounted on an optical bench. The cameras are based on a fully dioptric design with 6 lenses. Each camera has an 1100 deg2FoV and a pupil diameter of 120 mm and is equipped with a focal plane array of 4 CCDs each with 4510² pixels of 18 μm size, working in full frame mode for the “normal” camera and in frame transfer mode for the “fast” cameras.

The satellite will be built and verified for an in-orbit lifetime of 6.5 years and to accommodate consumables for 8 years. The duration of the nominal science operations phase is 4 years. The current baseline strategy is to carry out a long duration observation phase including two single sky fields monitored for two years each. An alternative scenario is a split into 3 years long duration pointing and 1 year “step-and-stare” phase with several pointings. The final observing scenario will be decided two years before launch. PLATO will be launched by Soyuz-Fregat2-1b from Kourou in 2025, into a large amplitude libration orbit around L2.

PLATO performance

The analysis of the PLATO performance has shown compliance with the science requirements related to the accuracy of the planetary parameters. In particular, for the reference star (G0V, V=10), PLATO will allow for the determination of the radius of a planet of the same size as the Earth with an accuracy of 3%, and of the stellar age with an accuracy of 10%. The main PLATO targets will also be bright enough (V< 11) such that the planetary mass can be derived through radial velocity measurements with < 10% uncertainty at existing or in development ground-based facilities.

Considering the current observing baseline, the estimated planet yield for all planet sizes and orbital periods (V< 13) is ~4,600. In bright sources (V< 11), the planet yield for small planets (R< 2RE) in all orbital periods is 770 and, in the habitable zone of solar like stars, the estimated value lies between 6 and 280, depending on the planet occurrence rate assumed, which is strongly debated in the literature.

The mass determination with radial velocities observations can be carried out for a significant number of the detected small planets using resources comparable to those available through a large ESO programme (55 nights per year at 8m class telescopes, 65 nights per year at 4m class telescopes). In particular, with these resources it will be possible to measure the mass of 100 super Earths (9 of them with semi-major axis about 1 au), 22 Earth-sized planets (7 of them with semi-major axis comparable to 1 au) and some tens of Neptunes. Using facilities in La Palma (32 nights per year) additional 52 super-Earths (including 5 with semi-major axis about 1 au) could be followed up.

PLATO data products

The baseline telemetry budget yields a daily data volume of 435 Gb. This will enable downloading the imagettes of all high precision PLATO targets for an enhanced processing on the ground. The remaining targets will be processed on-board. The prime PLATO data product (Level-1) is a large sample of high precision calibrated stellar light curves and centroids for all bright targets. The Level-2 product consists of the list of planetary transit candidates and their parameters, the results of the asteroseismic analysis, the stellar rotation periods and stellar activity properties, the seismically-determined stellar masses, radii and ages of stars, and the list of planetary systems confirmed through the detection of Transit Time Variations (TTVs). The Level-3 product consists of the list of confirmed planetary systems, which will be fully characterised by combining information from the planetary transits, the seismology of the planet-hosting stars, and the results of ground-based observations. The public release of L0, L1 and L2 products for each three-month observing period will be made within one year after the L1 product has been validated.

PLATO ground segment

The PLATO Ground Segment consists of six main elements: i) an ESA provided Mission Operations Centre (MOC), in charge of satellite operations; ii) an ESA provided Science Operations Centre (SOC), in charge of the scientific mission planning and the generation and archival of data products; iii) a PLATO Data Centre (PDC), provided by the PLATO Mission Consortium (PMC), which will provide the pipeline algorithms and modules and generate the high level scientific data products; iv) a PMC Science Management team (PSM), which will carry out scientific preparatory and operational activities, as well as support ESA in public relations and outreach activities; v) a PMC Calibration/Operation Team (PCOT); and vi) a Ground-based Observations Programme Team, that will carry out the ground-based observation of the core sample targets.

Guest observer programme

Members of the scientific community may participate in the PLATO mission by becoming Guest Observers (GOs) selected by ESA through calls for proposals. The calls will ask for complementary science programmes targeting objects within the PLATO sky fields that have been defined for the core science, therefore not requiring dedicated repointing of the spacecraft.

The search for planets similar to our Earth, potentially suitable for the development of life, is one of the greatest scientific, technological, and philosophical undertakings of our time, which is captivating public interest. The PLATO results will have a profound influence on our understanding of the Universe and our place in the Cosmos. PLATO will accurately measure the radii, masses, and ages of Earth-like planets in the habitable zones of stars similar to our own. This is unique to PLATO and will lay the foundations for exoplanetary research in the following decades.

Excerpt from PLATO – Revealing habitable worlds around solar-like stars, Definition Study Report, ESA-SCI(2017)1, April 2017