Free Astronomy Magazine March-April 2018

11 MARCH-APRIL 2018 EXOPLANETS T hese spectra show the chemical makeup of the atmospheres of four Earth-size planets orbiting within or near the habitable zone of the nearby star TRAPPIST-1. The habitable zone is a region at a distance from the star where liquid water, the key to life as we know it, could exist on the planets’ surfaces. To obtain the spectra, astronomers used the Hubble Space Telescope to collect light from TRAPPIST-1 that passed through the exoplanets’ atmospheres as the alien worlds crossed the face of the star. The purple curves show the predicted signatures of gases such as water and methane that absorb certain wavelengths of light. These gases would be found in a puffy hydrogen-dominated atmosphere similar to gaseous planets such as Neptune. The Hubble results, noted by the green crosses, reveal no evidence of an extended atmosphere in three of the exoplanets (TRAPPIST-1d, f, and e). Additional observations are needed to rule out a hydrogen-dominated atmosphere for the fourth planet (TRAPPIST-1g). The evidence indicates that the atmospheres are more compact than could be measured by the Hubble observations. [NASA, ESA, and Z. Levy (STScI)] time the mass of the star has surely managed to syn- chronise the rotation periods of all of the planets with their respective revolution periods, making the hemi- spheres facing the star too hot, and those in the shade too cold (without considering the powerful atmo- spheric winds that situation can generate). On the pos- itive side, however, an old red dwarf produces less flares than a younger one, and it’s, therefore, less dev- astating for any possible life forms living in the system. Moreover, if the planets are still where we see them