The nature of the TRAPPIST-1 exoplanets
Triaud, Amaury H. M. J.; Burgasser, Adam J.; Gillon, Michaël; de Wit, Julien; Delrez, Laetitia; Jehin, Emmanuël; Queloz, Didier; Carey, Sean; Demory, Brice-Olivier; Leconte, Jérémy; Heng, Kevin; Turbet, Martin; Agol, Eric; Grimm, Simon L.; Ingalls, James G.; Fabrycky, Daniel; Burdanov, Artem; Dorn, Caroline; Hernandez, David M.; Raymond, Sean N.; Selsis, Franck; Sestovic, Marko; Van Grootel, Valérie; Bolmont, Émeline; Caldas, Anthony; Lederer, Susan
Switzerland, Belgium, United States, France, United Kingdom
Abstract
Context. The TRAPPIST-1 system hosts seven Earth-sized, temperate exoplanets orbiting an ultra-cool dwarf star. As such, it represents a remarkable setting to study the formation and evolution of terrestrial planets that formed in the same protoplanetary disk. While the sizes of the TRAPPIST-1 planets are all known to better than 5% precision, their densities have significant uncertainties (between 28% and 95%) because of poor constraints on the planet's masses.
Aims: The goal of this paper is to improve our knowledge of the TRAPPIST-1 planetary masses and densities using transit-timing variations (TTVs). The complexity of the TTV inversion problem is known to be particularly acute in multi-planetary systems (convergence issues, degeneracies and size of the parameter space), especially for resonant chain systems such as TRAPPIST-1.
Methods: To overcome these challenges, we have used a novel method that employs a genetic algorithm coupled to a full N-body integrator that we applied to a set of 284 individual transit timings. This approach enables us to efficiently explore the parameter space and to derive reliable masses and densities from TTVs for all seven planets.
Results: Our new masses result in a five- to eight-fold improvement on the planetary density uncertainties, with precisions ranging from 5% to 12%. These updated values provide new insights into the bulk structure of the TRAPPIST-1 planets. We find that TRAPPIST-1 c and e likely have largely rocky interiors, while planets b, d, f, g, and h require envelopes of volatiles in the form of thick atmospheres, oceans, or ice, in most cases with water mass fractions less than 5%.