Free Astronomy Magazine July-August 2016

PLANETOLOGY therefore outside the habitable zone, which in the case of TRAPPIST-1 extends between 0.024 and 0.049 AU. The period of revolution and the distance from the star of the third planet, TRAPPIST-1d, are certainly greater but less definite, due to the limited number of registered transits: Gillon and colleagues have indicated as possible about a dozen different orbital periods ranging from 4.5 and 72.8 days. Since this range includes the values for the semi-major orbital axis of between 0.022 and 0.146 AU, there is some likelihood that TRAPPIST-1d is orbiting in the hab- itable zone, or not much outside it. If that were so, this planet would be the best place where to look for traces of life outside our solar system. This assertion may seem odd, given the diversity of that dwarf compared to a more ‘straightfor- ward’ star of the solar-type. In reality, what really count in the end is the surface temperature and rocky nature of a planet, along with the possibility to verify the existence of an atmosphere and accu- rately characterize its composition. Iden- tifying the gases that make up the atmo- sphere of an Earth-sized exoplanet is an es- sential step in the search for certain mole- cules, referred to as biomarkers (H 2 0, CO 2 , CH 4 , O 3 and others), which could indicate the presence of extraterrestrial life forms. But are TRAPPIST-1’s planets rocky? Do they have an atmosphere? Do they have tem- peratures suitable for life as we know it? Theoretical models show that these planets have a high probability of being rocky and A scale compari- son of TRAP- PIST-1 with the Sun. The ultracool dwarf is only slightly larger than Jupiter. [ESO] T his video com- plements what has been said in the text about the TRAP- PIST-1 system and the search for possible signs of life around ultra- cool dwarfs. [ESO]