Free Astronomy Magazine March-April 2018

MARCH-APRIL 2018 T his infographic lists the main properties of the seven TRAPPIST-1 planets, along with the four in- nermost planets in the Solar System at the same scale. [NASA/JPL-Cal- tech/R. Hurt, T. Pyle (IPAC)] Left, this video takes the viewer on a quick trip from Earth to TRAPPIST- 1 and its seven planets. The stars in the animation are accurately po- sitioned as in real- ity. The artist’s impression in this video is based on the known physi- cal parameters for the planets and stars seen, and uses a vast database of objects in the Universe. [ESO/ L. Calçada/ spaceengine.org ] after billions of years (they have migrated there in remote times from more external orbits), it means that the system is stable, and we know well how stability is neces- sary for life to appear and evolve. How- ever, the advanced TRAPPIST-1’s age raises some doubts about the actual existence of atmospheres around its planets. Indeed, even neglecting the fateful scenario of magnetic fields, the extreme proximity of the whole system to the star and the expo- sure for several billion years to its radiation could have led to the boiling of the atmo- sphere. Researchers calculated that each of the five innermost planets might have lost, through evapo- ration, an average amount of water (if any) equivalent to a terrestrial ocean. Only the two outer planets would have escaped that fate, but for other rea- sons, they are not less hostile. To be precise, there is no evidence to confirm the evaporation of atmospheres. On the contrary, the oppo- site seems to be true, and this is because despite those planets sizes being compara- ble to that of Earth’s, they have lower av- erage densities, which means that a significant part of their mass could be in a gaseous state. In other words, those plan- ets are more than likely surrounded by a rather thick atmosphere. Assuming that this scenario is realistic, we would have at least a couple of very posi- tive effects: the dense atmospheres would be able to shield a relevant part of the harm- ful stellar radiation; furthermore, they would favour the global redistribution of