Paris (AFP)

Is there life possible beyond our solar system? The Cheops space telescope will fly from Kourou on Tuesday to try to understand what exoplanets are made of, a step in the long quest for conditions of extraterrestrial life forms, but also of the origins of Earth.

Nearly 4,000 exoplanets - orbiting a star other than the Sun - have been detected since the discovery of the first, 51 Pegasi b, 24 years ago.

"We have since known that there are planets everywhere, that about one star in two has its procession of planets. Now we want to go beyond statistics and study them in detail," said David Ehrenreich, scientific manager, to AFP. of the mission led by Switzerland and the European Space Agency (ESA).

The objective of Cheops (CHaracterising ExOPlanet Satellite) is therefore not to seek out new planets, but to analyze those already identified. Embedded in a satellite, the telescope will orbit 700 km above the Earth so as not to be disturbed by the atmosphere, and will access the whole sky, Sun in the back.

Its target: Proxima Centauri, 55 Cancri, Koro 1 ... at least 400 planetary systems, a few hundred light years away - the "near suburb" of the Sun on the scale of the Milky Way.

The planets being too far away to be visible, it is the powerful brilliance of their star which will serve as a gauge, thanks to the method of transits: Cheops will collect the minute variations in luminosity caused by the passage of a planet in front of its host star, like a micro-eclipse.

By comparing the light emitted by the star before, during and after transit, astrophysicists will be able to deduce the size and radius of the planet, with unprecedented precision.

This new radius data, combined with information gathered by ground-based telescopes on the ground, will make it possible to measure density, an essential parameter for determining the composition of the planet. And this last criterion is fundamental to define the probability that a planet can harbor life.

- "Sort" -

We already know that certain planets are located in the habitable zone of their star, that is to say just where the temperature allows water to exist in a liquid state and where life, such as knows it, could develop.

The interest of Cheops is to be able to differentiate two planets of identical mass which are in this habitable zone: "if the density of the first is high, that means that it is essentially composed of rocks, with a fine solid atmosphere. If the density of the second is low, it will be a planet composed of gases, with an extremely thick atmosphere ", anticipates David Ehrenreich.

The presence of liquid water will only be possible on the first. "So if the goal is to seek traces of life, Cheops will tell me that it is on this that I will focus" more powerful telescopes like the future James-Webb, he adds.

"Cheops will be useful for us to sort it out. We will not go to probe at random", agrees Pierre Ferruit, in charge of the James Webb mission at ESA, who welcomes a "brick" in the "long quest conditions for life in the Universe. "

But the mission will also study the "non-habitable" planets, to understand all their diversity. "By observing the exoplanets, we realize that the solar system is completely atypical", notes Francis Rocard, planetologist at CNES: elsewhere, there are "everywhere" objects that do not exist here, mini-Neptune, super-Earth with large envelopes of gas, "hot Jupiter" ....

Cheops will take off from Kourou, in French Guiana, aboard a Soyuz rocket at 5.54 local time (10.54 French time). Duration of the mission: 3.5 years.

© 2019 AFP