Free Astronomy Magazine March-April 2018

E xTrA telescopes at the ESO Observatory in La Silla, Chile, will search for and study Earth-sized planets in orbit around nearby red dwarfs. The innovative design of ExTrA allows a higher sensitivity than previous tools. Here we see one of the three ExTrA telescopes inside its dome. [ESO/Petr Horálek] A planet of the same diameter, on the other hand, transiting a larger, solar-type star’s disc produces an eclipse about a hundred times less evident. This is another reason we cannot examine in detail the at- better S/N ratios are achieved with stars that appear brighter, meaning those that are closer to us (on average). This implies that the atmospheres of thousands of ex- trasolar planets discovered around very dis- tant stars, such as the ones monitored by the Kepler space telescope, are currently out of reach for a detailed spectroscopic in- vestigation. It follows that if we want to discover ter- restrial-like atmospheres and even poten- tial biomarkers inside them, we must look for planets transiting in front of close stars. Because the S/N ratio improves as the star size decreases, our best chance of discover- ing planets is surveys of databases contain- ing M-type red dwarfs, the most common stars in the galaxy and relatively abundant in the part of the galaxy near the Sun. Calculations tell us that Earth-sized planet that passes in front of a red dwarf with a diameter one-tenth that of the Sun will eclipse 0.8% of the star’s surface.