Since the discovery of the first exoplanet 51 Pegasi b by the Swiss Michel Mayor and Didier Queloz, Nobel Prize in physics 2019 for this discovery, the list of these mysterious extrasolar planets has grown considerably. The limit of 4,000 having been briskly crossed this year, it now looks like a directory of fire PTT! As for the new entrants, unless they are particularly original, they hardly do any more talk about them. What good… since we basically know very little about these planets and that everything we can say about it is still written in the conditional tense. With the Cheops satellite, Switzerland and the European Space Agency nevertheless hope to bring exoplanetology into a new era. In fact, unlike its predecessors Corot and Kepler, Cheops does not have the mission to scan a particular area of the sky to discover new exoplanets whose abundance is no longer in doubt. No, as its name in full CHaracterising ExOplanets Satellite indicates, its role is to observe already identified planetary systems to characterize them and start to bring order and a little more sense in all this, under the responsibility scientist from Didier Queloz, of the Geneva observatory.
A three and a half year mission
What are these planets made of? Do they have an atmosphere? And, if so, what is its composition? Do they have liquid water on their surface? Finally, what do they tell us about the formation of our own solar system? To try to answer these burning questions, scientists first need to know the density of these bodies. Data which they generally do not have and which, at such a distance, can only be obtained by comparing their mass and their size. This is where Cheops comes in. Positioned in low orbit about 700 kilometers above our heads, its telescope equipped with a 30 cm mirror must allow it to measure a maximum of exoplanets with unequaled precision, of the order of 10%. To do this, he will watch them pass in front of their star in order to observe very precisely the drop in luminosity induced and will thus determine their radius, that is to say by applying the method of transits. This will make it possible to obtain the density of those whose mass has already been determined during their detection by the method of radial velocities. Which consists in measuring the oscillation of the star resulting from the gravitational attraction exerted by a planet in orbit around it, this one resulting directly from the mass of the object. However, the task will not be easy, since most exoplanets can only be observed with one or the other of the two methods described above.
Scheduled to last three and a half years, the mission nevertheless aims to provide a short list of well-documented terrestrial-type planets on which the next generation telescopes - such as the James Webb telescope, whose commissioning is scheduled for 2021 and the E-ELT (European Southern Observatory) expected for 2025 - will be able to concentrate on going further in the knowledge of these distant worlds. By the way, Cheops could obviously glean some new aspirants, but also perhaps get their hands on rings, moons and succeed in measuring the temperature of hot Jupiters which intrigue astrophysicists so much since, in our case, in the solar system, giants are more of the frozen kind…
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