J. L. Bean, K. B. Stevenson, N. M. Batalha, Z. Berta-Thompson, L. Kreidberg, N. Crouzet, B. Benneke, M. R. Line, D. K. Sing, H. R. Wakeford, H. A. Knutson, E. M.-R. Kempton, J.-M. Désert, I. Crossfield, N. E. Batalha, J. de Wit, V. Parmentier, J. Harrington, J. I. Moses, M. Lopez-Morales, M. K. Alam, J. Blecic, G. Bruno, A. L. Carter, J. W. Chapman, L. Decin, D. Dragomir, T. M. Evans, J. J. Fortney, J. D. Fraine, P. Gao, A. García Muñoz, N. P. Gibson, J. M. Goyal, K. Heng, R. Hu, S. Kendrew, B. M. Kilpatrick, J. Krick, P.-O. Lagage, M. Lendl, T. Louden, N. Madhusudhan, A. M. Mandell, M. Mansfield, E. M. May, G. Morello, C. V. Morley, N. Nikolov, S. Redfield, J. E. Roberts, E. Schlawin, J. J. Spake, K. O. Todorov, A. Tsiaras, O. Venot, W. C. Waalkes, P. J. Wheatley, R. T. Zellem, D. Angerhausen, D. Barrado, L. Carone, S. L. Casewell, P. E. Cubillos, M. Damiano, M. de Val-Borro, B. Drummond, B. Edwards, M. Endl, N. Espinoza, K. France, J. E. Gizis, T. P. Greene, T. K. Henning, Y. Hong, J. G. Ingalls, N. Iro, P. G. J. Irwin, T. Kataria, F. Lahuis, J. Leconte, J. Lillo-Box, S. Lines, J. D. Lothringer, L. Mancini, F. Marchis, N. Mayne, E. Palle, E. Rauscher, G. Roudier, E. L. Shkolnik, J. Southworth, M. R. Swain, J. Taylor, J. Teske, G. Tinetti, P. Tremblin, G. S. Tucker, R. van Boekel, I. P. Waldmann, I. C. Weaver, and T. Zingales. The Transiting Exoplanet Community Early Release Science Program for JWST. , 130(11):114402, 2018. [ bib | DOI | arXiv | PDF version | ADS link ]
The James Webb Space Telescope (JWST) presents the opportunity to transform our understanding of planets and the origins of life by revealing the atmospheric compositions, structures, and dynamics of transiting exoplanets in unprecedented detail. However, the high-precision, timeseries observations required for such investigations have unique technical challenges, and prior experience with Hubble, Spitzer, and other facilities indicates that there will be a steep learning curve when JWST becomes operational. In this paper, we describe the science objectives and detailed plans of the Transiting Exoplanet Community Early Release Science (ERS) Program, which is a recently approved program for JWST observations early in Cycle 1. We also describe the simulations used to establish the program. The goal of this project, for which the obtained data will have no exclusive access period, is to accelerate the acquisition and diffusion of technical expertise for transiting exoplanet observations with JWST, while also providing a compelling set of representative data sets that will enable immediate scientific breakthroughs. The Transiting Exoplanet Community ERS Program will exercise the timeseries modes of all four JWST instruments that have been identified as the consensus highest priorities, observe the full suite of transiting planet characterization geometries (transits, eclipses, and phase curves), and target planets with host stars that span an illustrative range of brightnesses. The observations in this program were defined through an inclusive and transparent process that had participation from JWST instrument experts and international leaders in transiting exoplanet studies. The targets have been vetted with previous measurements, will be observable early in the mission, and have exceptional scientific merit. Community engagement in the project will be centered on a two-phase Data Challenge that culminates with the delivery of planetary spectra, timeseries instrument performance reports, and open-source data analysis toolkits in time to inform the agenda for Cycle 2 of the JWST mission.
L. Puig, G. Pilbratt, A. Heske, I. Escudero, P.-E. Crouzet, B. de Vogeleer, K. Symonds, R. Kohley, P. Drossart, P. Eccleston, P. Hartogh, J. Leconte, G. Micela, M. Ollivier, G. Tinetti, D. Turrini, B. Vandenbussche, and P. Wolkenberg. The Phase A study of the ESA M4 mission candidate ARIEL. Experimental Astronomy, 46:211-239, 2018. [ bib | DOI | PDF version | ADS link ]
ARIEL, the Atmospheric Remote sensing Infrared Exoplanet Large survey, is one of the three M-class mission candidates competing for the M4 launch slot within the Cosmic Vision science programme of the European Space Agency (ESA). As such, ARIEL has been the subject of a Phase A study that involved European industry, research institutes and universities from ESA member states. This study is now completed and the M4 down-selection is expected to be concluded in November 2017. ARIEL is a concept for a dedicated mission to measure the chemical composition and structure of hundreds of exoplanet atmospheres using the technique of transit spectroscopy. ARIEL targets extend from gas giants (Jupiter or Neptune-like) to super-Earths in the very hot to warm zones of F to M-type host stars, opening up the way to large-scale, comparative planetology that would place our own Solar System in the context of other planetary systems in the Milky Way. A technical and programmatic review of the ARIEL mission was performed between February and May 2017, with the objective of assessing the readiness of the mission to progress to the Phase B1 study. No critical issues were identified and the mission was deemed technically feasible within the M4 programmatic boundary conditions. In this paper we give an overview of the final mission concept for ARIEL as of the end of the Phase A study, from scientific, technical and operational perspectives.
G. Tinetti, P. Drossart, P. Eccleston, P. Hartogh, A. Heske, J. Leconte, G. Micela, M. Ollivier, G. Pilbratt, L. Puig, D. Turrini, B. Vandenbussche, P. Wolkenberg, J.-P. Beaulieu, L. A. Buchave, M. Ferus, M. Griffin, M. Guedel, K. Justtanont, P.-O. Lagage, P. Machado, G. Malaguti, M. Min, H. U. Nørgaard-Nielsen, M. Rataj, T. Ray, I. Ribas, M. Swain, R. Szabo, S. Werner, J. Barstow, M. Burleigh, J. Cho, V. C. du Foresto, A. Coustenis, L. Decin, T. Encrenaz, M. Galand, M. Gillon, R. Helled, J. C. Morales, A. G. Muñoz, A. Moneti, I. Pagano, E. Pascale, G. Piccioni, D. Pinfield, S. Sarkar, F. Selsis, J. Tennyson, A. Triaud, O. Venot, I. Waldmann, D. Waltham, G. Wright, J. Amiaux, J.-L. Auguères, M. Berthé, N. Bezawada, G. Bishop, N. Bowles, D. Coffey, J. Colomé, M. Crook, P.-E. Crouzet, V. Da Peppo, I. E. Sanz, M. Focardi, M. Frericks, T. Hunt, R. Kohley, K. Middleton, G. Morgante, R. Ottensamer, E. Pace, C. Pearson, R. Stamper, K. Symonds, M. Rengel, E. Renotte, P. Ade, L. Affer, C. Alard, N. Allard, F. Altieri, Y. André, C. Arena, I. Argyriou, A. Aylward, C. Baccani, G. Bakos, M. Banaszkiewicz, M. Barlow, V. Batista, G. Bellucci, S. Benatti, P. Bernardi, B. Bézard, M. Blecka, E. Bolmont, B. Bonfond, R. Bonito, A. S. Bonomo, J. R. Brucato, A. S. Brun, I. Bryson, W. Bujwan, S. Casewell, B. Charnay, C. C. Pestellini, G. Chen, A. Ciaravella, R. Claudi, R. Clédassou, M. Damasso, M. Damiano, C. Danielski, P. Deroo, A. M. Di Giorgio, C. Dominik, V. Doublier, S. Doyle, R. Doyon, B. Drummond, B. Duong, S. Eales, B. Edwards, M. Farina, E. Flaccomio, L. Fletcher, F. Forget, S. Fossey, M. Fränz, Y. Fujii, Á. García-Piquer, W. Gear, H. Geoffray, J. C. Gérard, L. Gesa, H. Gomez, R. Graczyk, C. Griffith, D. Grodent, M. G. Guarcello, J. Gustin, K. Hamano, P. Hargrave, Y. Hello, K. Heng, E. Herrero, A. Hornstrup, B. Hubert, S. Ida, M. Ikoma, N. Iro, P. Irwin, C. Jarchow, J. Jaubert, H. Jones, Q. Julien, S. Kameda, F. Kerschbaum, P. Kervella, T. Koskinen, M. Krijger, N. Krupp, M. Lafarga, F. Landini, E. Lellouch, G. Leto, A. Luntzer, T. Rank-Lüftinger, A. Maggio, J. Maldonado, J.-P. Maillard, U. Mall, J.-B. Marquette, S. Mathis, P. Maxted, T. Matsuo, A. Medvedev, Y. Miguel, V. Minier, G. Morello, A. Mura, N. Narita, V. Nascimbeni, N. Nguyen Tong, V. Noce, F. Oliva, E. Palle, P. Palmer, M. Pancrazzi, A. Papageorgiou, V. Parmentier, M. Perger, A. Petralia, S. Pezzuto, R. Pierrehumbert, I. Pillitteri, G. Piotto, G. Pisano, L. Prisinzano, A. Radioti, J.-M. Réess, L. Rezac, M. Rocchetto, A. Rosich, N. Sanna, A. Santerne, G. Savini, G. Scandariato, B. Sicardy, C. Sierra, G. Sindoni, K. Skup, I. Snellen, M. Sobiecki, L. Soret, A. Sozzetti, A. Stiepen, A. Strugarek, J. Taylor, W. Taylor, L. Terenzi, M. Tessenyi, A. Tsiaras, C. Tucker, D. Valencia, G. Vasisht, A. Vazan, F. Vilardell, S. Vinatier, S. Viti, R. Waters, P. Wawer, A. Wawrzaszek, A. Whitworth, Y. L. Yung, S. N. Yurchenko, M. R. Z. Osorio, R. Zellem, T. Zingales, and F. Zwart. A chemical survey of exoplanets with ARIEL. Experimental Astronomy, 46:135-209, 2018. [ bib | DOI | PDF version | ADS link ]
Thousands of exoplanets have now been discovered with a huge range of masses, sizes and orbits: from rocky Earth-like planets to large gas giants grazing the surface of their host star. However, the essential nature of these exoplanets remains largely mysterious: there is no known, discernible pattern linking the presence, size, or orbital parameters of a planet to the nature of its parent star. We have little idea whether the chemistry of a planet is linked to its formation environment, or whether the type of host star drives the physics and chemistry of the planet's birth, and evolution. ARIEL was conceived to observe a large number ( 1000) of transiting planets for statistical understanding, including gas giants, Neptunes, super-Earths and Earth-size planets around a range of host star types using transit spectroscopy in the 1.25-7.8 μm spectral range and multiple narrow-band photometry in the optical. ARIEL will focus on warm and hot planets to take advantage of their well-mixed atmospheres which should show minimal condensation and sequestration of high-Z materials compared to their colder Solar System siblings. Said warm and hot atmospheres are expected to be more representative of the planetary bulk composition. Observations of these warm/hot exoplanets, and in particular of their elemental composition (especially C, O, N, S, Si), will allow the understanding of the early stages of planetary and atmospheric formation during the nebular phase and the following few million years. ARIEL will thus provide a representative picture of the chemical nature of the exoplanets and relate this directly to the type and chemical environment of the host star. ARIEL is designed as a dedicated survey mission for combined-light spectroscopy, capable of observing a large and well-defined planet sample within its 4-year mission lifetime. Transit, eclipse and phase-curve spectroscopy methods, whereby the signal from the star and planet are differentiated using knowledge of the planetary ephemerides, allow us to measure atmospheric signals from the planet at levels of 10-100 part per million (ppm) relative to the star and, given the bright nature of targets, also allows more sophisticated techniques, such as eclipse mapping, to give a deeper insight into the nature of the atmosphere. These types of observations require a stable payload and satellite platform with broad, instantaneous wavelength coverage to detect many molecular species, probe the thermal structure, identify clouds and monitor the stellar activity. The wavelength range proposed covers all the expected major atmospheric gases from e.g. H2O, CO2, CH4 NH3, HCN, H2S through to the more exotic metallic compounds, such as TiO, VO, and condensed species. Simulations of ARIEL performance in conducting exoplanet surveys have been performed - using conservative estimates of mission performance and a full model of all significant noise sources in the measurement - using a list of potential ARIEL targets that incorporates the latest available exoplanet statistics. The conclusion at the end of the Phase A study, is that ARIEL - in line with the stated mission objectives - will be able to observe about 1000 exoplanets depending on the details of the adopted survey strategy, thus confirming the feasibility of the main science objectives.
T. Zingales, G. Tinetti, I. Pillitteri, J. Leconte, G. Micela, and S. Sarkar. The ARIEL mission reference sample. Experimental Astronomy, 46:67-100, 2018. [ bib | DOI | arXiv | PDF version | ADS link ]
The ARIEL (Atmospheric Remote-sensing Exoplanet Large-survey) mission concept is one of the three M4 mission candidates selected by the European Space Agency (ESA) for a Phase A study, competing for a launch in 2026. ARIEL has been designed to study the physical and chemical properties of a large and diverse sample of exoplanets and, through those, understand how planets form and evolve in our galaxy. Here we describe the assumptions made to estimate an optimal sample of exoplanets - including already known exoplanets and expected ones yet to be discovered - observable by ARIEL and define a realistic mission scenario. To achieve the mission objectives, the sample should include gaseous and rocky planets with a range of temperatures around stars of different spectral type and metallicity. The current ARIEL design enables the observation of 1000 planets, covering a broad range of planetary and stellar parameters, during its four year mission lifetime. This nominal list of planets is expected to evolve over the years depending on the new exoplanet discoveries.
D. Turrini, Y. Miguel, T. Zingales, A. Piccialli, R. Helled, A. Vazan, F. Oliva, G. Sindoni, O. Panić, J. Leconte, M. Min, S. Pirani, F. Selsis, V. Coudé du Foresto, A. Mura, and P. Wolkenberg. The contribution of the ARIEL space mission to the study of planetary formation. Experimental Astronomy, 46:45-65, 2018. [ bib | DOI | arXiv | PDF version | ADS link ]
The study of extrasolar planets and of the Solar System provides complementary pieces of the mosaic represented by the process of planetary formation. Exoplanets are essential to fully grasp the huge diversity of outcomes that planetary formation and the subsequent evolution of the planetary systems can produce. The orbital and basic physical data we currently possess for the bulk of the exoplanetary population, however, do not provide enough information to break the intrinsic degeneracy of their histories, as different evolutionary tracks can result in the same final configurations. The lessons learned from the Solar System indicate us that the solution to this problem lies in the information contained in the composition of planets. The goal of the Atmospheric Remote-Sensing Infrared Exoplanet Large-survey (ARIEL), one of the three candidates as ESA M4 space mission, is to observe a large and diversified population of transiting planets around a range of host star types to collect information on their atmospheric composition. ARIEL will focus on warm and hot planets to take advantage of their well-mixed atmospheres, which should show minimal condensation and sequestration of high-Z materials and thus reveal their bulk composition across all main cosmochemical elements. In this work we will review the most outstanding open questions concerning the way planets form and the mechanisms that contribute to create habitable environments that the compositional information gathered by ARIEL will allow to tackle.
E. Ducrot, M. Sestovic, B. M. Morris, M. Gillon, A. H. M. J. Triaud, J. De Wit, D. Thimmarayappa, E. Agol, Y. Almleaky, A. Burdanov, A. J. Burgasser, L. Delrez, B.-O. Demory, E. Jehin, J. Leconte, J. McCormac, C. Murray, D. Queloz, F. Selsis, S. Thompson, and V. Van Grootel. The 0.8-4.5 μm Broadband Transmission Spectra of TRAPPIST-1 Planets. , 156:218, 2018. [ bib | DOI | arXiv | PDF version | ADS link ]
The TRAPPIST-1 planetary system provides an exceptional opportunity for the atmospheric characterization of temperate terrestrial exoplanets with the upcoming James Webb Space Telescope (JWST). Assessing the potential impact of stellar contamination on the planets transit transmission spectra is an essential precursor to this characterization. Planetary transits themselves can be used to scan the stellar photosphere and to constrain its heterogeneity through transit depth variations in time and wavelength. In this context, we present our analysis of 169 transits observed in the optical from space with K2 and from the ground with the SPECULOOS and Liverpool telescopes. Combining our measured transit depths with literature results gathered in the mid-/near-IR with Spitzer/IRAC and HST/WFC3, we construct the broadband transmission spectra of the TRAPPIST-1 planets over the 0.8-4.5 μm spectral range. While planet b, d, and f spectra show some structures at the 200-300 ppm level, the four others are globally flat. Even if we cannot discard their instrumental origins, two scenarios seem to be favored by the data: a stellar photosphere dominated by a few high-latitude giant (cold) spots, or, alternatively, by a few small and hot (3500-4000 K) faculae. In both cases, the stellar contamination of the transit transmission spectra is expected to be less dramatic than predicted in recent papers. Nevertheless, based on our results, stellar contamination can still be of comparable or greater order than planetary atmospheric signals at certain wavelengths. Understanding and correcting the effects of stellar heterogeneity therefore appears essential for preparing for the exploration of TRAPPIST-1 with JWST.
P. Auclair-Desrotour, S. Mathis, J. Laskar, and J. Leconte. Oceanic tides from Earth-like to ocean planets. Astronomy Astrophysics, 615:A23, 2018. [ bib | DOI | arXiv | PDF version | ADS link ]
Context. Oceanic tides are a major source of tidal dissipation. They drive the evolution of planetary systems and the rotational dynamics of planets. However, two-dimensional (2D) models commonly used for the Earth cannot be applied to extrasolar telluric planets hosting potentially deep oceans because they ignore the three-dimensional (3D) effects related to the ocean's vertical structure. <BR /> Aims: Our goal is to investigate, in a consistant way, the importance of the contribution of internal gravity waves in the oceanic tidal response and to propose a modelling that allows one to treat a wide range of cases from shallow to deep oceans. <BR /> Methods: A 3D ab initio model is developed to study the dynamics of a global planetary ocean. This model takes into account compressibility, stratification, and sphericity terms, which are usually ignored in 2D approaches. An analytic solution is computed and used to study the dependence of the tidal response on the tidal frequency and on the ocean depth and stratification. <BR /> Results: In the 2D asymptotic limit, we recover the frequency-resonant behaviour due to surface inertial-gravity waves identified by early studies. As the ocean depth and Brunt-Väisälä frequency increase, the contribution of internal gravity waves grows in importance and the tidal response becomes 3D. In the case of deep oceans, the stable stratification induces resonances that can increase the tidal dissipation rate by several orders of magnitude. It is thus able to significantly affect the evolution time scale of the planetary rotation.
S. L. Grimm, B.-O. Demory, M. Gillon, C. Dorn, E. Agol, A. Burdanov, L. Delrez, M. Sestovic, A. H. M. J. Triaud, M. Turbet, É. Bolmont, A. Caldas, J. de Wit, E. Jehin, J. Leconte, S. N. Raymond, V. Van Grootel, A. J. Burgasser, S. Carey, D. Fabrycky, K. Heng, D. M. Hernandez, J. G. Ingalls, S. Lederer, F. Selsis, and D. Queloz. The nature of the TRAPPIST-1 exoplanets. Astronomy Astrophysics, 613:A68, 2018. [ bib | DOI | arXiv | PDF version | ADS link ]
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. <BR /> 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. <BR /> 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. <BR /> 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%.
P. Auclair-Desrotour and J. Leconte. Semidiurnal thermal tides in asynchronously rotating hot Jupiters. Astronomy Astrophysics, 613:A45, 2018. [ bib | DOI | arXiv | PDF version | ADS link ]
Context. Thermal tides can torque the atmosphere of hot Jupiters into asynchronous rotation, while these planets are usually assumed to be locked into spin-orbit synchronization with their host star. <BR /> Aims: In this work, our goal is to characterize the tidal response of a rotating hot Jupiter to the tidal semidiurnal thermal forcing of its host star by identifying the structure of tidal waves responsible for variation of mass distribution, their dependence on the tidal frequency, and their ability to generate strong zonal flows. <BR /> Methods: We develop an ab initio global modelling that generalizes the early approach of Arras Socrates (2010, ApJ, 714, 1) to rotating and non-adiabatic planets. We analytically derive the torque exerted on the body and the associated timescales of evolution, as well as the equilibrium tidal response of the atmosphere in the zero-frequency limit. Finally, we numerically integrate the equations of thermal tides for three cases, including dissipation and rotation step by step. <BR /> Results: The resonances associated with tidally generated gravito-inertial waves significantly amplify the resulting tidal torque in the range 1-30 days. This torque can globally drive the atmosphere into asynchronous rotation, as its sign depends on the tidal frequency. The resonant behaviour of the tidal response is enhanced by rotation, which couples the forcing to several Hough modes in the general case, while the radiative cooling tends to regularize it and diminish its amplitude.
M. Turbet, E. Bolmont, J. Leconte, F. Forget, F. Selsis, G. Tobie, A. Caldas, J. Naar, and M. Gillon. Modeling climate diversity, tidal dynamics and the fate of volatiles on TRAPPIST-1 planets. Astronomy Astrophysics, 612:A86, 2018. [ bib | DOI | arXiv | PDF version | ADS link ]
TRAPPIST-1 planets are invaluable for the study of comparative planetary science outside our solar system and possibly habitability. Both transit timing variations (TTV) of the planets and the compact, resonant architecture of the system suggest that TRAPPIST-1 planets could be endowed with various volatiles today. First, we derived from N-body simulations possible planetary evolution scenarios, and show that all the planets are likely in synchronous rotation. We then used a versatile 3D global climate model (GCM) to explore the possible climates of cool planets around cool stars, with a focus on the TRAPPIST-1 system. We investigated the conditions required for cool planets to prevent possible volatile species to be lost permanently by surface condensation, irreversible burying or photochemical destruction. We also explored the resilience of the same volatiles (when in condensed phase) to a runaway greenhouse process. We find that background atmospheres made of N2, CO, or O2 are rather resistant to atmospheric collapse. However, even if TRAPPIST-1 planets were able to sustain a thick background atmosphere by surviving early X/EUV radiation and stellar wind atmospheric erosion, it is difficult for them to accumulate significant greenhouse gases like CO2, CH4, or NH3. CO2 can easily condense on the permanent nightside, forming CO2 ice glaciers that would flow toward the substellar region. A complete CO2 ice surface cover is theoretically possible on TRAPPIST-1g and h only, but CO2 ices should be gravitationally unstable and get buried beneath the water ice shell in geologically short timescales. Given TRAPPIST-1 planets large EUV irradiation (at least 103 × Titan's flux), CH4 and NH3 are photodissociated rapidly and are thus hard to accumulate in the atmosphere. Photochemical hazes could then sedimentate and form a surface layer of tholins that would progressively thicken over the age of the TRAPPIST-1 system. Regarding habitability, we confirm that few bars of CO2 would suffice to warm the surface of TRAPPIST-1f and g above the melting point of water. We also show that TRAPPIST-1e is a remarkable candidate for surface habitability. If the planet is today synchronous and abundant in water, then it should very likely sustain surface liquid water at least in the substellar region, whatever the atmosphere considered.
L. Delrez, M. Gillon, A. H. M. J. Triaud, B.-O. Demory, J. de Wit, J. G. Ingalls, E. Agol, E. Bolmont, A. Burdanov, A. J. Burgasser, S. J. Carey, E. Jehin, J. Leconte, S. Lederer, D. Queloz, F. Selsis, and V. Van Grootel. Early 2017 observations of TRAPPIST-1 with Spitzer. , 475:3577-3597, 2018. [ bib | DOI | arXiv | PDF version | ADS link ]
The recently detected TRAPPIST-1 planetary system, with its seven planets transiting a nearby ultracool dwarf star, offers the first opportunity to perform comparative exoplanetology of temperate Earth-sized worlds. To further advance our understanding of these planets' compositions, energy budgets, and dynamics, we are carrying out an intensive photometric monitoring campaign of their transits with the Spitzer Space Telescope. In this context, we present 60 new transits of the TRAPPIST-1 planets observed with Spitzer/Infrared Array Camera (IRAC) in 2017 February and March. We combine these observations with previously published Spitzer transit photometry and perform a global analysis of the resulting extensive data set. This analysis refines the transit parameters and provides revised values for the planets' physical parameters, notably their radii, using updated properties for the star. As part of our study, we also measure precise transit timings that will be used in a companion paper to refine the planets' masses and compositions using the transit timing variations method. TRAPPIST-1 shows a very low level of low-frequency variability in the IRAC 4.5-μm band, with a photometric RMS of only 0.11 per cent at a 123-s cadence. We do not detect any evidence of a (quasi-)periodic signal related to stellar rotation. We also analyse the transit light curves individually, to search for possible variations in the transit parameters of each planet due to stellar variability, and find that the Spitzer transits of the planets are mostly immune to the effects of stellar variations. These results are encouraging for forthcoming transmission spectroscopy observations of the TRAPPIST-1 planets with the James Webb Space Telescope.
J. de Wit, H. R. Wakeford, N. K. Lewis, L. Delrez, M. Gillon, F. Selsis, J. Leconte, B.-O. Demory, E. Bolmont, V. Bourrier, A. J. Burgasser, S. Grimm, E. Jehin, S. M. Lederer, J. E. Owen, V. Stamenković, and A. H. M. J. Triaud. Atmospheric reconnaissance of the habitable-zone Earth-sized planets orbiting TRAPPIST-1. Nature Astronomy, 2:214-219, 2018. [ bib | DOI | arXiv | PDF version | ADS link ]
Seven temperate Earth-sized exoplanets readily amenable for atmospheric studies transit the nearby ultracool dwarf star TRAPPIST-1 (refs 1,2). Their atmospheric regime is unknown and could range from extended primordial hydrogen-dominated to depleted atmospheres3-6. Hydrogen in particular is a powerful greenhouse gas that may prevent the habitability of inner planets while enabling the habitability of outer ones6-8. An atmosphere largely dominated by hydrogen, if cloud-free, should yield prominent spectroscopic signatures in the near-infrared detectable during transits. Observations of the innermost planets have ruled out such signatures9. However, the outermost planets are more likely to have sustained such a Neptune-like atmosphere10, 11. Here, we report observations for the four planets within or near the system's habitable zone, the circumstellar region where liquid water could exist on a planetary surface12-14. These planets do not exhibit prominent spectroscopic signatures at near-infrared wavelengths either, which rules out cloud-free hydrogen-dominated atmospheres for TRAPPIST-1 d, e and f, with significance of 8σ, 6σ and 4σ, respectively. Such an atmosphere is instead not excluded for planet g. As high-altitude clouds and hazes are not expected in hydrogen-dominated atmospheres around planets with such insolation15, 16, these observations further support their terrestrial and potentially habitable nature.
J. Leconte. Continuous reorientation of synchronous terrestrial planets due to mantle convection. Nature Geoscience, 11:168-172, 2018. [ bib | DOI | arXiv | PDF version | ADS link ]
Many known rocky exoplanets are thought to have been spun down by tidal interactions to a state of synchronous rotation, in which a planet's period of rotation is equal to that of its orbit around its host star. Investigations into atmospheric and surface processes occurring on such exoplanets thus commonly assume that day and night sides are fixed with respect to the surface over geological timescales. Here we use an analytical model to show that true polar wanderwhere a planetary body's spin axis shifts relative to its surface because of changes in mass distributioncan continuously reorient a synchronous rocky exoplanet. As occurs on Earth, we find that even weak mantle convection in a rocky exoplanet can produce density heterogeneities within the mantle sufficient to reorient the planet. Moreover, we show that this reorientation is made very efficient by the slower rotation rate of a synchronous planet when compared with Earth, which limits the stabilizing effect of rotational and tidal deformations. Furthermore, a relatively weak lithosphere limits its ability to support remnant loads and stabilize against reorientation. Although uncertainties exist regarding the mantle and lithospheric evolution of these worlds, we suggest that the axes of smallest and largest moment of inertia of synchronous exoplanets with active mantle convection change continuously over time, but remain closely aligned with the star-planet and orbital axes, respectively.
J. Leconte. Why Compositional Convection Cannot Explain Substellar Objects Sharp Spectral-type Transitions. Astrophysical Journal, 853:L30, 2018. [ bib | DOI | arXiv | PDF version | ADS link ]
As brown dwarfs and young giant planets cool down, they are known to experience various chemical transitionsfor example, from {CO} rich L-dwarfs to methane rich T-dwarfs. Those chemical transitions are accompanied by spectral transitions with sharpness that cannot be explained by chemistry alone. In a series of articles, Tremblin et al. proposed that some of the yet-unexplained features associated with these transitions could be explained by a reduction of the thermal gradient near the photosphere. To explain, in turn, this more isothermal profile, they invoke the presence of an instability analogous to fingering convectioncompositional convectiontriggered by the change in mean molecular weight of the gas due to the chemical transitions mentioned above. In this Letter, we use existing arguments to demonstrate that any turbulent transport, if present, would in fact increase the thermal gradient. This misinterpretation comes from the fact that turbulence mixes/homogenizes entropy (potential temperature) instead of temperature. So, while increasing transport, turbulence in an initially stratified atmosphere actually carries energy downward, whether it is due to fingering or any other type of compositional convection. These processes therefore cannot explain the features observed along the aforementioned transitions by reducing the thermal gradient in the atmosphere of substellar objects. Understanding the microphysical and dynamical properties of clouds at these transitions thus probably remains our best way forward.