A brief history of the TRAPPIST-1 system

Not so long ago we could still believe that our solar system was unique, that the planets surrounding our Sun were exceptions and that life only exists on planet Earth. Since the first discovery of an exoplanet (a planet orbiting a different star than the sun) in 1995 [1] our views changed drastically. First of all, we realised that hosting planets is more likely a rule for a star than an exception. Second, we discovered a large diversity of systems with extraordinary structure and history. In particular, we realised that the majority of planets discovered belong to a class of planets that does not even exist in our system. To date, we count more than 4000 confirmed exoplanets with almost one new planet being discovered every two days. In this article we review the detection and first characterisation of an exceptional system, the TRAPPIST-1 system. We explain what makes this system so special and all the work that has been archived since the first announcement of its discovery in 2015. © 2021 Societe Royale des Sciences de Liege. All rights reserved.