The nature of the TRAPPIST-1 exoplanets

May 2018 • 2018A&A...613A..68G

Authors • Grimm, Simon L. • Demory, Brice-Olivier • Gillon, Michaël • Dorn, Caroline • Agol, Eric • Burdanov, Artem • Delrez, Laetitia • Sestovic, Marko • Triaud, Amaury H. M. J. • Turbet, Martin • Bolmont, Émeline • Caldas, Anthony • de Wit, Julien • Jehin, Emmanuël • Leconte, Jérémy • Raymond, Sean N. • Van Grootel, Valérie • Burgasser, Adam J. • Carey, Sean • Fabrycky, Daniel • Heng, Kevin • Hernandez, David M. • Ingalls, James G. • Lederer, Susan • Selsis, Franck • Queloz, Didier

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%.


IPAC Authors

Sean Carey

Senior Scientist

Jim Ingalls

Associate Scientist