With 90% clear skies over 3 months of winter with very little scintillation, Dome C's exceptional observing conditions are ideal for another method of detecting exoplanets, which requires high precision over a long duration: gravitational microlensing. Once again, the challenge is to measure precisely a variation in the brightness of a star but this time the brightness increases. According to the General Relativity, the mass of a star curves space around itself, so the light passing near it is bent. Thus, a massive object placed between a star and an observer act as a lens which magnifies the light of the star. This phenomenon can be detected by continuously observing many bright stars, chiefly towards the galactic bulge, and waiting for a small cool star (weighing some tenths of a solar mass) to move in front of one of them. (These ?red dwarf? stars are the most common ones in our galaxy.) When this happens, there is a characteristic `light curve' (see curve). This method particularly interests exoplanet hunters because current theories of the formation of planetary systems predict that these cool stars could be orbited by many telluric planets (a few tenths to ten times the mass of the Earth). These cold super-Earths should give a second lensing signal. This gravitational microlensing technique is currently the only way to detect objects this small, and is therefore crucial to our understanding of the formation of planetary systems.
? NASA/JPL ? Caltech/R. Hurt (SSC)
Our Sun is located near the periphery of our galaxy, the Milky Way, nearly 25.000 light-years from the galactic center.
To detect exoplanets by gravitational microlensing effect, the astronomers point their telescope towards the galactic bulge area which contains a great concentration of bright stars, while they are patiently waiting for the passage of red dwarfs on their line of sight. Red dwarfs are the most common stars within our galaxy and one suspects them of being orbited by Earth-like planets.
Detecting this magnification signal, as a red dwarf star passes in front of a bright star in the galactic bulge, requires uninterrupted weeks or even months of observations because the signal is expected to last for about 5 to 100 days. From the southern hemisphere, the galactic bulge area is visible during winter. This is why astronomers have set up a network of telescopes throughout the southern hemisphere, which take turns, providing continuous coverage of the bulge. More than 600 detections of red dwarf transits are recorded annually with this program, giving more than 600 opportunities to detect telluric planets. Because of the excellent conditions, a single telescope at Dome C could do the work of many telescopes elsewhere.
? IMAGE - NASA JP. Beaulieu et al (2206) ? C. Baudouin
When a red dwarf-like star passes between a bright star located in the galactic bulge and an observer, it acts as a lens by magnifying the light detected by the astronomers. This passage results in a peak of brightness across the time. If a dwarf red is orbited by an exoplanet, its presence results in a weaker brightness magnification than the previous one. This is the case on this curve which shows the presence of a terrestrial type planet, of 5.5 earth masses, located at some 20.000 light-years from our solar system.
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