Does a second Earth exist in the Universe? Planet hunter Plato will focus on the properties of rocky planets orbiting Sun-like stars. In particular, Plato will discover and characterise planets in orbits up to the habitable zone – the ‘goldilocks’ region around a star where the temperature is just right for liquid water to exist on a planet’s surface. Plato will characterise hundreds of rocky (including Earth twins), icy or giant planets by providing exquisite measurements of their radii (3% precision), masses (better than 10% precision) and ages (10% precision).
This will revolutionise our understanding of planet formation and the evolution of planetary systems, as well as the potential habitability of these diverse worlds. As well as looking at these planets, Plato will analyse their host stars. Using data from the mission, scientists hope to perform stellar seismology, gathering evidence of ‘starquakes’ in the imaged stars. This will give insight into the characteristics and evolution of the stars, improving our understanding of entire planetary systems. Plato will use 26 cameras at once to observe terrestrial planets in orbits up to the habitable zone of bright Sun-like stars, and to characterise these stars. Using such a large number of cameras will enable a combined higher ‘signal-to-noise’ ratio and larger field of view than has been possible with previous missions. Through the observations of bright stars, Plato will assemble the first catalogue of confirmed and characterised planets with known densities, compositions, and ages, which will include planets in the habitable zone of Sun-like stars.
PLAnetary Transits and Oscillations of stars (PLATO) was selected as the third medium (‘M-class’) mission in ESA's Cosmic Vision programm in February 2014. Its objective is to find and study a large number of extrasolar planetary systems, with emphasis on the properties of terrestrial planets in the habitable zone around solar-like stars. PLATO has also been designed to investigate seismic activity in stars, enabling the precise characterisation of the planet host star, including its age. Plato successfully passed the extra milestone review of the mission in January 2022, which was carried out because of the risks associated with development Plato’s complex set of 26 cameras. The next major milestone is the spacecraft critical design review in 2023–2024, which will verify the detailed design of the complete spacecraft before proceeding with assembly.
The PLATO 2.0 instrument consists of 34 small aperture telescopes (32 with 25 sec readout cadence and 2 with 2.5 sec candence) providing a wide field-of-view (2232 deg2) and a large photometric magnitude range (4-16 mag). It focusses on bright (4-11 mag) stars in wide fields to detect and characterize planets down to Earth-size by photometric transits, whose masses can then be determined by ground-based radial-velocity follow-up measurements. Asteroseismology will be performed for these bright stars to obtain highly accurate stellar parameters, including masses and ages. The combination of bright targets and asteroseismology results in high accuracy for the bulk planet parameters: 2%, 4-10% and 10% for planet radii, masses and ages, respectively. The planned baseline observing strategy includes two long pointings (2-3 years) to detect and bulk characterize planets reaching into the habitable zone (HZ) of solar-like stars and an additional step-and-stare phase to cover in total about 50% of the sky. PLATO 2.0 will observe up to 1,000,000 stars and detect and characterize hundreds of small planets, and thousands of planets in the Neptune to gas giant regime out to the HZ.
It will therefore provide the first large-scale catalogue of bulk characterized planets with accurate radii, masses, mean densities and ages. This catalogue will include terrestrial planets at intermediate orbital distances, where surface temperatures are moderate. Coverage of this parameter range with statistical numbers of bulk characterized planets is unique to PLATO 2.0. The PLATO 2.0 catalogue allows us to e.g.:
The catalogue will allow us to study planets and planetary systems at different evolutionary phases. It will further provide a census for small, low-mass planets. This will serve to identify objects which retained their primordial hydrogen atmosphere and in general the typical characteristics of planets in such low-mass, low-density range. Planets detected by PLATO 2.0 will orbit bright stars and many of them will be targets for future atmosphere spectroscopy exploring their atmosphere. Furthermore, the mission has the potential to detect exomoons, planetary rings, binary and Trojan planets. The planetary science possible with PLATO 2.0 is complemented by its impact on stellar and galactic science via asteroseismology as well as light curves of all kinds of variable stars, together with observations of stellar clusters of different ages. This will allow us to improve stellar models and study stellar activity. A large number of well-known ages from red giant stars will probe the structure and evolution of our Galaxy. Asteroseismic ages of bright stars for different phases of stellar evolution allow calibrating stellar age-rotation relationships. Together with the results of ESA’s Gaia mission, the results of PLATO 2.0 will provide a huge legacy to planetary, stellar and galactic science.