Size matters ... for exoplanets. The European space telescope Cheops was launched on Wednesday 18 December to observe hundreds of exoplanets from space and get a better idea of the size of these stars which do not orbit around the Sun.
Embarked on a Soyuz rocket, Cheops positioned in orbit some 700 km from Earth at around 1 pm (French time) to scan “hundreds of star systems with exoplanets”, explains to France 24 Anthony Boccaletti, deputy director of the Laboratory of Space Studies and Instrumentation in Astrophysics (Lesia) of the Observatoire de Paris.
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The telescope will focus on observing stars close to our solar system "some of which are even visible to the naked eye," said Magali Deleuil, astrophysicist at the Marseille Technopole Astrophysics Laboratory who is participating in the Cheops mission, contacted by France. 24.
The density of exoplanets
This new telescope, jointly developed by the European Space Agency (ESA) and the Swiss Space Office, is not yet another finder of extrasolar planets. Since the 1990s, nearly 4,000 exoplanets have been identified, mainly thanks to the American Kepler and Tess telescopes and the European Corot.
For its part, Cheops should allow a better understanding of the nature of the planets already discovered. So far, the observations made have given us a fairly precise idea of the mass of exoplanets. But that is not enough to determine what type of planet we are dealing with.
This is where the new telescope comes in. "It will calculate more precisely the size of these planets. And when we have the size and the mass, we can deduce the density", explains Anthony Boccaletti. And density is one of the keys to studying exoplanets. It gives information on the composition of a planet and makes it possible in particular to determine if it rocky (or telluric) - like the Earth - or rather gaseous, like Neptune. Indeed, a planet made of rock, like stone, metal, has a much higher density than a gaseous star.
“Super-Terre” or “mini-Neptune”
The Cheops mission will focus on studying a very specific category of exoplanets: the “super-Earth” or “mini-Neptune”. These are stars that have a mass significantly greater than that of our planet but less than that of a gas giant. “There is none in our solar system and we know very little about them”, underlines Magali Deleuil.
By discovering their density, it will be possible to make hypotheses on the composition of these strange “Osmi” (Misidentified space objects) and to know if they are rather rocky, gaseous, “or even perhaps oceanic, these exoplanets whose existence has been theorized but which have never been observed ”, notes the French researcher.
Beyond these “super-Earths”, Cheops will also provide more precise information on the composition of tens or hundreds of telluric or gaseous exoplanets which populate our universe. "This will allow us to compare this data with our physical models to better understand the formation of these planets and their evolution," says Anthony Boccaletti. And, for him, these observations, which will last more than three years, will help answer one of the big questions of astrophysics: "How many other Earths are there in the universe?”.
In this sense, the Cheops mission, which belongs to the “small” European space programs with its budget of around 100 million euros, is preparing the ground for the Plato program. This space observatory, which is to be deployed in 2026 and will cost 600 million euros, will have the specific task of finding exoplanets similar to Earth. Cheops may have shown where to look first.
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