TRAPPIST-1: Global results of theSpitzerExploration Science Program Red Worlds

Abstract

Context. With more than 1000 h of observation from Feb. 2016 to Oct. 2019, the Spitzer Exploration Program Red Worlds (ID: 13067, 13175 and 14223) exclusively targeted TRAPPIST-1, a nearby (12 pc) ultracool dwarf star, finding that it is orbited by seven transiting Earth-sized planets. At least three of these planets orbit within the classical habitable zone of the star, and all of them are well-suited for a detailed atmospheric characterization with the upcoming JWST. Aims. The main goals of the Spitzer Red Worlds program were (1) to explore the system for new transiting planets, (2) to intensively monitor the planets’ transits to yield the strongest possible constraints on their masses, sizes, compositions, and dynamics, and (3) to assess the infrared variability of the host star. In this paper, we present the global results of the project. Methods. We analyzed 88 new transits and combined them with 100 previously analyzed transits, for a total of 188 transits observed at 3.6 or 4.5 μm. For a comprehensive study, we analyzed all light curves both individually and globally. We also analyzed 29 occultations (secondary eclipses) of planet b and eight occultations of planet c observed at 4.5 μm to constrain the brightness temperatures of their daysides. Results. We identify several orphan transit-like structures in our Spitzer photometry, but all of them are of low significance. We do not confirm any new transiting planets. We do not detect any significant variation of the transit depths of the planets throughout the different campaigns. Comparing our individual and global analyses of the transits, we estimate for TRAPPIST-1 transit depth measurements mean noise floors of ~35 and 25 ppm in channels 1 and 2 of Spitzer/IRAC, respectively. We estimate that most of this noise floor is of instrumental origins and due to the large inter-pixel inhomogeneity of IRAC InSb arrays, and that the much better interpixel homogeneity of JWST instruments should result in noise floors as low as 10 ppm, which is low enough to enable the atmospheric characterization of the planets by transit transmission spectroscopy. Our analysis reveals a few outlier transits, but we cannot conclude whether or not they correspond to spot or faculae crossing events. We construct updated broadband transmission spectra for all seven planets which show consistent transit depths between the two Spitzer channels. Although we are limited by instrumental precision, the combined transmission spectrum of planet b to g tells us that their atmospheres seem unlikely to be CH4-dominated. We identify and model five distinct high energy flares in the whole dataset, and discuss our results in the context of habitability. Finally, we fail to detect occultation signals of planets b and c at 4.5 μm, and can only set 3-σ upper limits on their dayside brightness temperatures (611 K for b 586 K for c).

A Review of Possible Planetary Atmospheres in the TRAPPIST-1 System

“…K2; Howell et al 2014). Follow-up observations Ducrot et al 2018;Burdanov et al 2019;Ducrot et al 2020) later not only confirmed the existence of at least seven temperate, terrestrial-size planets around the star TRAPPIST-1, but also helped to better constrain their main properties (summarized in Table 1).…”

“…of . References: (1) , (2) , (3) Burgasser and Mamajek (2017), ( 4) Kane (2018) using Gaia DR2 parallaxes, (5) , ( 6) , ( 7) Ducrot et al (2020) for a null albedo, a synchronous rotation, and no atmosphere at all Fig. 1 Architecture of the TRAPPIST-1 system and evolution of the runaway greenhouse/atmospheric collapse limit for water (a.k.a.…”

“…Since the initial discovery of the TRAPPIST-1 system, many ground and space-based largeaperture telescopes have been used to measure transits of all the seven TRAPPIST-1 planets in a large range of wavelengths. Figure 5 summarizes all the transit observations that have been published as of December 2019 Bourrier et al 2017b,a;Ducrot et al 2018;Luger et al 2017b;Burdanov et al 2019;Wakeford et al 2019;Ducrot et al 2020) as a function of the wavelength of observation, producing the most complete transmission spectra of TRAPPIST-1 planets to date. At least three effects can explain radius variations with wavelength: (i) atomic/molecular absorptions by TRAPPIST-1 planetary atmospheres, (ii) instrumental biases and (iii) contamination by the stellar activity (e.g.…”

“…5 This figure shows transit spectra (in Earth radius units) of the seven TRAPPIST-1 planets. These spectra were constructed using the transit depth measurements obtained with Spitzer Ducrot et al 2018Ducrot et al , 2020, HST (de Wit et al 2016Wakeford et al 2019), K2 (Luger et al 2017b;Ducrot et al 2018), SPECULOOS-South Observatory aka SSO and Liverpool telescope (Ducrot et al 2018), VLT/HAWK-I, UKIRT and AAT (Burdanov et al 2019). These transit depths were then converted into transit radii using the TRAPPIST-1 stellar radius estimate of , i.e.…”

Characterisation of the hydrospheres of TRAPPIST-1 planets

“…However, the atmosphere of planet d could be dominated by other atmospheric gases different from H 2 O-based mixtures, which could produce an extended atmosphere and increase the total planetary radius. Hydrogen-dominated atmospheres have been deemed unlikely as one of the possible atmospheric compositions for all planets in the TRAPPIST-1 system, both cloudfree (de Wit et al 2016(de Wit et al , 2018 and with high-altitude clouds and hazes (Grimm et al 2018;Ducrot et al 2020). Similarly, CH 4 -dominated atmospheres are not probable according to the photometry data of the Spitzer Space Telescope (Ducrot et al 2020).…”