The nature of the TRAPPIST-1 exoplanets

¹ University of Bern, Center for Space and Habitability, Gesellschaftsstrasse 6, 3012 Bern, Switzerland
e-mail: brice.demory@csh.unibe.ch
² Space Sciences, Technologies and Astrophysics Research (STAR) Institute, Université de Liège, Allée du 6 août 19C, 4000 Liège, Belgium
³ Astronomy Department, University of Washington, Seattle, WA 98195, USA
⁴ NASA Astrobiology Institute’s Virtual Planetary Laboratory, Seattle, WA 98195, USA
⁵ Cavendish Laboratory, J J Thomson Avenue, Cambridge CB3 0HE, UK
⁶ Institute of Astronomy, Madingley Road, Cambridge CB3 0HA, UK
⁷ School of Physics & Astronomy, University of Birmingham, Edgbaston, Birmingham B15 2TT, UK
⁸ Laboratoire de Météorologie Dynamique, IPSL, Sorbonne Universités, UPMC Univ. Paris 06, CNRS, 4 place Jussieu, 75005 Paris, France
⁹ Université Paris Diderot, AIM, Sorbonne Paris Cité, CEA, CNRS, 91191 Gif-sur-Yvette, France
¹⁰ Laboratoire d’astrophysique de Bordeaux, Univ. Bordeaux, CNRS, B18N, Allée Geoffroy Saint-Hilaire, 33615 Pessac, France
¹¹ Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139, USA
¹² Center for Astrophysics and Space Science, University of California San Diego, La Jolla, CA 92093, USA
¹³ IPAC, Mail Code 314-6, Calif. Inst. of Technology, 1200 E California Blvd, Pasadena, CA 91125, USA
¹⁴ Department of Astronomy and Astrophysics, Univ. of Chicago, 5640 S Ellis Ave, Chicago, IL 60637, USA
¹⁵ Department of Physics and Kavli Institute for Astrophysics and Space Research, Massachusetts Institute of Technology, 77 Massachusetts Ave., Cambridge, MA 02139, USA
¹⁶ NASA Johnson Space Center, 2101 NASA Parkway, Houston, TX 77058, USA
¹⁷ Guggenheim Fellow
¹⁸ Institut d’Astrophysique de Paris, 98 bis Boulevard Arago, 75014 Paris, France
¹⁹ University of Zurich, Institute of Computational Sciences, Winterthurerstrasse 190, 8057 Zurich, Switzerland

Received: 3 November 2017
Accepted: 21 January 2018

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%.