Free Astronomy Magazine July-August 2016

PLANETOLOGY In the best scenario, this plan- et’s atmosphere could be heat- ed just like that of the Earth, and thus offering interesting ground conditions. Nonethe- less, it is also possible that it is significantly colder, but if the object's orbit is not circular, it is very likely that the gravita- tional pull exerted on it by the star may contribute to elevate its internal temperature, and accordingly that of the surface. Understanding whether TRAP- PIST-1d can be considered a habitable planet (in the broad sense) is at the limits of current technology. If it were to orbit around a brighter star, e.g. of the solar-type, like the many Earth-sized planets discovered by Kepler, we would have no chance of being able to investi- gate its atmosphere in search of particularly interesting mol- ecules. But the stellar light fil- tering through the atmosphere of TRAPPIST-1d, which conse- quently acquires the ‘signa- tures’ of the elements compos- ing it, is not swamped by the light emitted by the dwarf, due to the small size and low brightness of the latter. Thus on Earth are received read- able information on the atmo- sphere composition and Gil- lon’s team has already started to collect them with instru- ments operating at different wavelengths, in order to char- acterize with sufficient accu- racy that gaseous envelope and identify possible biomark- ers. The Hubble's Space Tele- scope Imaging Spectrograph (STIS) should already be able to provide data on the extention and composition of the atmo- sphere along the terminator (which during the transits coin- cides with the edge of the plan- A suggestive repre- sentation of TRAPPIST-1d’s surface. The environment is probably bitterly cold, but it might not be totally hostile to the emergence of basic forms of life. [ESO/M. Kornmesser]