Free Astronomy Magazine March-April 2018

40 MARCH-APRIL 2018 EXOPLANETS of three Ritchey-Chrétien telescopes, 60 cm in diameter, f/8, equipped with two-lens field correctors. The telescopes are housed in three independent domes at the Euro- pean Southern Observatory’s La Silla Obser- vatory in Chile. The light collected by the mirrors, rather than being focused directly into a CCD cam- era, is transferred through optical fibre beams to a low-resolution multi-object spec- trograph and then recorded by a sensor. The reasons why French researchers adopted this innovative solution will be clearer after we explain what the ExTrA targets are and the problems involved in studying them. The mission of the new instrument is to monitor hundreds of M-type red dwarf stars, looking for temporary and very slight drops in brightness, typical of planets tran- siting the discs of stars. If the physical properties of a star are known with an excellent amount of confi- dence, a transit can provide information on the planet’s size, its orbit and, if it has an atmosphere, the composition of that atmo- sphere. The atmosphere itself filters a very small portion of starlight, adding the fin- gerprints of the molecules that compose it. Under optimal conditions, it is possible to recognise the nature of those fingerprints using spectroscopy and then to characterise the planet’s atmosphere with reasonable certainty. If the spectrum of a star is known with precision, the planetary atmosphere verview of the ExTrA project. Astronomers now have a new instrument for searching for potentially habitable worlds. [ESO] can theoretically be studied both when it passes in front of the star’s disc and when it passes behind it. In the second case, the very weak spectrum produced by the planet