Free Astronomy Magazine March-April 2018

MARCH-APRIL 2018 T his diagram compares the sizes, masses and estimated tem- peratures of the TRAPPIST-1 plan- ets with Solar System planets. The colours indi- cate temperatures and the black line matches the den- sities and compo- sition of the ter- restrial planets in the Solar System. Planets above the line are less dense and plan- ets below are denser. [ESO/S. Grimm et al.] planet, TRAPPIST-1h, a fundamental para- meter to understand whether liquid water can exist in that world (admitted and not granted that there is water). A team of re- searchers, led by Rodrigo Luger of the Uni- versity of Washington, intervened to solve the problem. In an article published in Na- ture Astronomy last May, the team reports the results of processing the photometric data of TRAPPIST-1 collected by the Kepler space telescope between December 2016 and March 2017. In this period, known as K2 Campaign 12, Kepler monitored the small star for 74 days, providing the longest, nearly contin- uous set of observations of TRAPPIST-1. This data allowed the researchers to study the mutual gravitational interactions be- tween planets and to better characterise the individual orbits. The orbital period of TRAPPIST-1h was found to be 18.76 days, which corresponds to an average distance from the star of 9.27 million km, too exces- sive to suppose that liquid water can exist on that planet. Thus, the Luger team ex- cluded the farthest planet from the TRAP- PIST-1’s habitable zone, after the two closest ones, TRAPPIST-1b and TRAPPIST-1c, had already been considered uninhabit- able, not only for the maximum surface temperatures (higher than 100°C), but also because their rotation periods are surely synchronized with their revolution peri- ods, which leads them to always expose the same hemisphere to the star radiation, with non-positive consequences. At that point, many exoplanet specialists focused their attention on the four planets almost certainly orbiting in the habitable zone: TRAPPIST-1d, TRAPPIST-1e, TRAPPIST-1f