Free Astronomy Magazine March-April 2018

13 MARCH-APRIL 2018 EXOPLANETS tional. In the mean- time, we can try to understand which of the planets is less hos- tile to life. A large team of researchers, led by Simon Grimm (Universität Bern), went in this direction, and applied very com- plex models to all of the available data of TRAPPIST-1’s planets, to define their densi- ties with a precision never reached before. The results of this study, recently pub- lished on Astronomy & Astrophysics , suggest that all those plan- ets may have retained water reserves equal to 5% of their mass, a huge amount if we consider that in the case of the Earth only 0.02% of the mass comes in the form of water. In the scenario that takes shape from Grimm’s team’s study, if TRAPPIST-1b and TRAPPIST-1c still have an atmosphere, water should be present in a vapour state, which would help make those worlds in- hospitable. The outer planets, “f”, “g” and “h”, are perhaps a little too far from the star to ensure liquid water on the surface. The planets “d” and “e” are more interest- ing. The former is the lightest of the sys- tem, it has indeed only 30% of Earth’s mass, and we do not know in what state the water (perhaps) present on it is. TRAP- PIST-1e is instead the most promising planet: it is just a little smaller than the Earth, but it is slightly denser, features compatible with the presence of an iron core, a rocky surface alternating with ex- panses of water, and an atmosphere that is not necessarily thick. In addition to the size, density and per- haps the consistency of the atmosphere, TRAPPIST-1e is comparable to Earth also for the amount of radiation it receives from its star. All of this, of course, does not mean habitability or presence of life, but we can bet that this planet will be one of Webb’s first targets. T his video shows the TRAPPIST-1 sys- tem from the most distant planet (TRAPPIST- 1h). In the video, the transits of two inner planets can be seen, with the large disc of TRAPPIST-1g transiting last. The artist’s im- pression in this video is based on the known phys- ical parameters for the planets and stars seen, and uses a vast database of ob- jects in the Uni- verse. [ESO/L. Calçada/spaceeng ine.org . Music: Johan B. Monell] the heat accumulated in the hemispheres perpetually exposed towards the star. On the contrary, however, very dense atmo- spheres could easily trigger an irreversible greenhouse effect, similar to the one pre- sent on Venus. Correct characterisation of the atmo- spheres is therefore essential to under- stand how much those planets can re- semble ours, rather than Venus’ or, con- versely, Mars’. In this regard, the results of the first spectroscopic analysis of the plan- ets “d”, “e”, “f” and “g”, i.e. those that orbit in the TRAPPIST-1’s habitable zone, were published in Nature Astronomy on 5 February. This new study, led by a team of astronomers from the Space Telescope Sci- ence Institute (STScI), revealed that at least the first three of those four planets are not surrounded by a puffy, particularly hydro- gen-rich atmosphere, like that of a mini- Neptune. This is a good sign, as hydrogen is a greenhouse gas that makes the planets inhospitable. If in the atmospheres there is so little hydrogen not to be easily de- tectable, this increases the likelihood that heavier atoms and molecules are prepon- derant, like some potential biomarkers − more interesting for researchers − that could reveal the presence of life on those planets. But to confirm this, we have to wait until next year, when NASA’s James Webb Space Telescope will become opera- !