The search for extrasolar planets is a real challenge for humanity, the underlying question that scientists are trying to answer is "Are we alone in the Universe?". Already in the 16th century, Giordano Bruno (1548-1600), in the footsteps of Copernicus challenges so visionary, anthropocentric framework (geocentrism) of the man (see nota).
Since the first discovery of extrasolar planets in 1995 (51 Pegasi b), a large number of space projects such as HARPS (2003), CoRoT (2006), Kepler (2009), JWST (2018) to Darwin (2020) have obtained financing. Until 2020, the methods used to find exoplanets, are indirect methods (transit effect, effect on the radial velocity, microlensing effect...), that is to say that exoplanets are detected only by the effects they cause on their star. But from 2020, with DARWIN, all mankind will see an extrasolar planet directly and perhaps an exoterre.
In 2011, NASA has confirmed the discovery of the first extrasolar planet (2.4 Earth masses) in the "habitable zone" of its star, a region where liquid water could exist on the surface of a planet.
But see the reflected light from a planet has no particular interest if not, see the move in its orbit, which is much more interesting is to analyze the composition of its atmosphere. The main objective of the DARWIN space project is to analyze the light spectrum of the atmosphere of a planet around a few Earth masses. In other words, it is possible to detect the presence of chemical compounds such as carbon dioxide (CO2), water (H2O), ozone (O3), oxygen (O2) or other ionic assembly and why not organic compounds based on a carbon skeleton. The CO2 in large quantities and the water would be the signature of photosynthetic activity.
The oxygen has a specific feature, this aggressive atom can not remain in a gaseous envelope around a planet because it binds easily with rocks and metals to make oxides. If there is oxygen in quantity around a planet that means biotic phenomena exist. These biotic phenomena reveal the presence of LIFE. No physical or chemical mechanism explains the abundance of oxygen outside biochemical mechanisms.
If all the biomass on Earth disappeared, then the oxygen would disappear as in only a few million years.
Each atom, each ion, and even every molecule has a characteristic spectral signature (a set of lines in the electromagnetic spectrum).
The chemical species are mainly detected in the infrared spectral band (9.7 microns for ozone, 8 microns for water, 7.6 microns for methane). This signature is used to detect the presence and concentration of a chemical compound in the medium traversed by the radiation between the source and the detection by the spectroscopic instrument.
But to see the atmosphere of a planet will require "turn off his star." DARWIN mission was designed for this, turn off the stars. Its instrument is called an interferometric coronagraph, it will hide the light of the star thanks to an ingenious optical system that puts in phase opposition correlated several light rays arriving on several telescopes. In other words, if two telescopes observe the same star, both receive the same electromagnetic wave, if the signal is returned to a focus of the instrument which shifts the second wavelength of one half-wavelength, the addition of two light sources of the star is equal to zero. It has turned out the light from the star.
Any light that is not perfectly aligned with the telescopes will not be extinguished on the contrary it will add up, that's exactly what they want the mission scientists, they want to see the planet without being dazzled by the nearby star. The light of the planet is out of phase with that of the star, which is why it is not extinct. The combined use of multiple telescopes provides a high angular resolution that depends on the distance between the telescopes. The small flotilla of telescopes will be positioned on the L2 Lagrange point in the Earth's shadow, hidden from sunlight.
NB: « The stars are suns like ours and there is a multitude of suns freely suspended in space without limit surrounded by planets, like Earth, populated by living beings... The sun is just one star among others , particular because very close to us. The sun has no central position in infinity without borders. »
Giordano Bruno (1548-1600), philosopher, theologian and scientist accused of atheism and heresy by the Inquisition, and executed 17 February 1600 in Rome.
Image: Transit (periodic micro- eclipse) occurs each time the planet passes between the star and the observer , at that time, the planet obscures some of the light from the star, producing a detectable periodic dimming. This remarkable idea is used to detect the planet and directly determine its size and orbit. The diameter of Jupiter is 10 times smaller than the Sun, Jupiter therefore mask 1% of the luminosity of the Sun when it passes in front of to a distant observer. Earth it is 100 times smaller than the Sun, it therefore mask 1/10 000th of its luminosity is 0.01 % as it passes between the Sun and the observer. But what we see is a signal brightness which decrease and not the planet. With Darwin we will see the planet or rather its atmosphere that reflects light from its star extinguished by an interferometric coronagraph. Any light that is perfectly aligned with the telescopes (that of the Star) will be turn off and so the planet will be seen without being dazzled by the nearby star.