In the thirties of last century, the French astronomer Bernard Lyot masks developed a system for simulation, optimization, the effect of a total eclipse, in order to study better and longer than the solar corona. He called his instrument "coronagraph". With the advancement of technology, it was decided to "miniaturize" and refine the coronagraph to mask no more sunlight, but that of distant stars and explore so close to check the presence of one or more planets.
Nowadays there are many different types of coronagraph: some people try to concentrate all the light from the star in its central area, to conceal it better, others are trying to exploit the fact that light is composed of waves, and it is possible them to interfere to cancel each other: if the crests of these waves are perfectly aligned, then the sum of a light wave will be more intense if the contrary, the crest of a wave is aligned with the throat of another, the sum of these waves out of phase nothing will be a wave of intensity.
Additional types of coronagraph are under study, and expect to recover with a lens system focused the light and masked, and record the image on a hologram. The hologram is a device that allows you to record not only the intensity of light, as they would in a normal photo, but also the information on the position of ridges and grooves of the light waves, which may be more or less out of phase. These waves of light recovered from the mask, which belong to the star's light will divert the light passed through the coronagraph, which belongs to both star in a possible planet. The waves of light the star vanish as the light of the planet, which is displaced compared to that of the star, is not canceled and the planet becomes visible. so far been discovered over 400 extrasolar planets using several methods. Some of them, those orbiting relatively nearby stars have already been observed using the coronagraph: the case of planets like our own Jupiter in terms of distance from the star and intensity of the reflected light. The difficulty in observing Earth-like planets is precisely the small distance and the weakness of light.
For example: at 36 light years away is a star named Gliese 436, equipped with a planet. Imagine that this planet home to a colony of aliens who are keen of the cosmos like ourselves, will be equipped with a telescope similar to the Hubble Space Telescope . If these aliens put their powerful tool in the direction of our Sun, the Earth would be ten billion times fainter than it. And if that were not enough to make extremely difficult the observation of the Earth around the Sun, one might add that the distance of the two stars in heaven would be very small: only four ten-thousandths of a degree (for comparison, the Moon is about half grade). It would be like trying to watch a firefly mail from Corsica to ten inches from the Lantern Genoa! However, if these aliens had available a coronagraph last generation, surely you would get, if not us, at least in our small planet.
waiting to receive any news from the curious alien planet of Gliese 436B, is up to us to try to improve the performance of the coronagraph and other interesting instruments (such as interferometers, ie networks of telescopes linked together to have pictures with a resolution greater), all to go even deeper and look for planets even closer to their stars, much less bright, more distant star around, and try to understand how other solar systems are similar to ours. DAVID RICCI
In the images, the pattern of operation of a coronagraph to detect planets around other stars by using a hologram (top), and an image of direct observation of three planets around the star HR8799 (below), made using a particular type of coronagraph called "vortex vector, exploiting the phenomenon of interference to cancel the light from the star (NASA / JPL-Caltech / Palomar Observatory). The discovery of E. Serabyn, D. Mawet & R. Burruss, was announced April 15, 2010 issue of the journal Nature .
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