PUBLISHED: 27 MARCH 2017 | VOLUME: 1 | ARTICLE NUMBER: 0104 news & views EXOPLANETS Why should we care about TRAPPIST-1? The discovery of a system of six (possibly system we’ve found with terrestrial planets seven) terrestrial planets around the tightly packed together and in resonance. TRAPPIST-1 star, announced in a paper Their orbits are also all co-planar, as the by Michaël Gillon and collaborators image shows, and luckily for us this orbital (Nature 542, 456–460; 2017) attracted a plane passes through the line of sight great deal of attention from researchers between us and TRAPPIST-1: all these as well as the general public. Reactions planets are ‘transiting’. This is much more ranged from the enthusiastic — already than a simple technical detail, as it allows us hypothesizing a string of inhabited to characterize all their atmospheres — an planets — to those who felt that it essential step to understand their climate didn’t add anything substantial to our and propensity for habitability. current knowledge of exoplanets and Finally and maybe most importantly, was ultimately not worth the hype. The these planets are distributed over all of cover of the Nature issue hosting the the three zones related to different phases paper (pictured), made by Robert Hurt, (IPAC) HURT NASA/JPL-CALTECH/ROBERT of water, as cleverly represented by the a visualization scientist at Caltech, is a illustration. However, this information, fine piece of artwork, but it also helps us They can thus be called ‘terrestrial’ without linked to the concept of habitable or in assessing the actual relevance of the any qualms about terminology. In addition, temperate zone and the possible presence of TRAPPIST-1 system. even if the whole system is much less liquid water on the surface, must be taken Firstly, the cover shows a very packed massive than our Solar System, the ratio with caution, since we do not yet know system. This corresponds to reality: all of the total planetary mass to the mass whether any of the TRAPPIST-1 planets seven planets would fit well within the of the star is comparable (≈>1/4,000 for have either an atmosphere or the albedo orbit of Mercury. In fact, the closest TRAPPIST-1 versus ~1/5,500 for us). This required for liquid water to survive. analogue of the TRAPPIST-1 system is is puzzling as, according to our current In conclusion, the TRAPPIST-1 system not our Solar System, but Jupiter and its models of planetary formation, the will provide a planetary-scale laboratory to Galilean moons. TRAPPIST-1 itself is total amount of matter in the system test and constrain all sorts of theories and only 1.1 times bigger than Jupiter, and should play a role in the efficiency of models concerning planetary formation the distance between its innermost and planetary accretion. and evolution, atmospheres, interplanetary outermost planets, expressed in orbital The TRAPPIST-1 planets also form a interaction and potential for habitability. period, is almost coincident with that quasi-resonant chain of orbits (where their So yes, we should definitely care about between Io and Callisto. orbital periods can be expressed as a ratio of TRAPPIST-1, even if we won’t be able to Then, at least six of the planets have two small numbers), again not dissimilar to establish a colony there anytime soon. radii and masses — and, consequently, what happens in the Jovian system between densities — close to those of the Earth. Io, Europa and Ganymede. This is the first LUCA MALTAGLIATI NATURE ASTRONOMY 1, 0104 (2017) | DOI: 10.1038/s41550-017-0104 | www.nature.com/natureastronomy 1 ©2017 Mac millan Publishers Li mited, part of Spri nger Nature. All ri ghts reserved. .