Are there other planets suitable for life in the Universe? The PLATO satellite will set off at the end of 2026 to search for Earths similar to ours, determining with unprecedented precision the size, mass and age of exoplanets.
Since the discovery of the first exoplanet, 51-Pegasi-b, in 1995 at the Haute-Provence Observatory (south-east of France), some 5,700 planets have been identified outside our solar system, and the list continues to grow.
“The detection of exoplanets is very important […]but at the moment we really don’t know what they are made of, whether they are terrestrial or not, whether they are water planets, whether they are super-Earths,” explains Ana Heras, PLATO programme scientist at the European Space Agency (ESA).
However, it is essential to know more about them to know if some of them are located in a “habitable” zone, that is to say at a distance from their star such that water can exist in a liquid state on their surface. An essential condition for the emergence of life.
PLATO (for PLAnetary Transits and Oscillations of stars) aims to find exoplanets similar to ours, orbiting around nearby bright stars similar to the Sun, by determining their size with an accuracy of 3%, their mass (accuracy better than 10%) and their age (accuracy of 10%).
The space telescope, currently under construction, is due to be launched in December 2026. It will be positioned 1.5 million kilometres from Earth, at the Lagrange-2 point, a very stable area from a gravitational and thermal point of view, where other scientific observatories already operate, such as the James Webb telescope.
“Grain of sand”
From there, he will use 26 cameras to observe a very large portion of the sky in the southern hemisphere, or 200,000 stars located approximately 1,000 light years away, which he will photograph every 25 seconds for two years.
“It’s like pointing a laser at a grain of sand a kilometre away without moving” for months, underlines Catherine Vogel, head of the programme at Thales Alenia Space, which is building the satellite with the German company OHB and the Swiss company Beyond Gravity, during a press visit to its facilities in Cannes.
The goal is to detect slight variations in the brightness of stars. These phenomena, called transits, are a sign that a planet is passing in front of the star, temporarily reducing the latter’s light intensity.
These observations will make it possible to know, if necessary, how long it takes for the planet to orbit its star, and to know its inclination and its size.
The unusually long observation period will allow astronomers to spot and observe the most interesting exoplanets at least twice: those that orbit their star in about a year and which are likely, like Earth, to be located in a habitable zone.
On Earth, telescopes will then be able to determine the mass of these exoplanets by capturing variations in wavelengths emitted by the star using high-resolution spectrographs.
When a planet orbits a star, it exerts a small gravitational force on the star that causes it to wobble and move closer to or further away from Earth. The larger the wobble, the more massive the planet.
By knowing the size and mass of the planet, scientists will be able to determine its density, and for example whether it is rocky.
Finally, to determine the age of stars — and therefore of planets — PLATO will use asteroseismology by measuring vibrations on the surface of stars.
The PLATO mission, initially scheduled to last four years, will be followed in 2029 by that of Ariel, another ESA satellite which will aim to study the atmosphere of exoplanets in detail.