Plato — which stands for planetary transits and oscillations of stars — was selected as a future medium (M) class mission by the agency’s science programme committee at a meeting in Paris.
The observatory will monitor relatively bright and nearby stars, looking for Earth-sized planets and super-Earths at distances from their parent star that would allow them to be habitable. Plato will use an array of 32 small, identical telescopes and cameras, plus two specialized cameras, to look for planets around up to 1 million stars. The mission will also carry out sensitive ‘astroseismology’ — using minute changes in starlight caused by vibrations within a star to determine characteristics such as its age.
Like the NASA planet-hunter Kepler, Plato will look for the dips in stars’ brightness that signals a planet is travelling in front of them, known as the transit method. When combined with ground-based radial-velocity observations — measurements of the star’s ‘wobble’ due to the planet’s gravity — these let physicists calculate planets’ density and likely composition.
Plato is the latest in a growing line of exoplanet missions. Following its launch in 2009, Kepler detected more than 3,000 exoplanet candidates. Mechanical failures ended the mission in its current form in 2013 and NASA is still considering its future.
But by the time Plato gets off the ground, ESA’s CHEOPS (Characterizing Exoplanet Satellite) mission, set for launch in 2017, should already be in the sky, investigating systems known to host planets. NASA also plans to launch its new exoplanet hunter, the Transiting Exoplanet Survey Satellite (TESS) in 2017. Both TESS and Plato will be upgrades to Kepler, surveying a much greater portion of the sky. Compared to TESS, Plato will be able to look for smaller planets and those with longer orbital periods.
Plato will have an initial six-year mission, operating from the second Lagrangian point (L2), a stable position 1.5 million kilometres outside Earth’s orbit around the Sun. It is the third medium-scale mission selected under ESA’s 20-year Cosmic Vision programme. The Solar Orbiter, which will study the Sun and solar wind, is planned for launch in 2017, and Euclid, mapping the geometry of the ‘dark universe’, will go up in 2020.
The observatory beat four other mission ideas for the spot. These were EChO (the Exoplanet Characterisation Observatory), designed to study exoplanet atmospheres; LOFT (the Large Observatory for X-ray Timing) a mission to study matter near black holes and neutron stars; MarcoPolo-R, which would return a sample from an asteroid; and STE-Quest (Space-Time Explorer and Quantum Equivalence Principle Space Test), which would test Einstein’s general theory of relativity.