The TRAPPIST-1 JWST Community Initiative Michael¨ Gillon1, Victoria Meadows2, Eric Agol2, Adam J. Burgasser3, Drake Deming4, Rene´ Doyon5, Jonathan Fortney6, Laura Kreidberg7, James Owen8, Franck Selsis9, Julien de Wit10, Jacob Lustig-Yaeger2, Benjamin V. Rackham10 1Astrobiology Research Unit, University of Liege,` Belgium 2Department of Astronomy, University of Washington, USA 3Department of Physics, University of California San Diego, USA 4Department of Astronomy, University of Maryland at College Park, USA 5Institute for Research in Exoplanets, University of Montreal, Canada 6Other Worlds Laboratory, University of California Santa Cruz, USA 7Center for Astrophysics — Harvard and Smithsonian, USA 8Department of Physics, Imperial College London, United Kingdom 9Laboratoire d’Astrophysique de Bordeaux, University of Bordeaux, France 10Department of Earth, Atmospheric, and Planetary Sciences, MIT, USA February 17, 2020 he upcoming launch of the James Webb analysis techniques, complementary space-based Space Telescope (JWST) combined with and ground-based observations) and theoretical T the unique features of the TRAPPIST-1 levels (e.g. model developments and comparison, planetary system should enable the young field retrieval techniques, inferences). Depending on of exoplanetology to enter into the realm of tem- the outcome of the first phase of JWST observa- perate Earth-sized worlds. Indeed, the proximity tions of the planets, this initiative could become of the system (12pc) and the small size (0.12 the seed of a major JWST Legacy Program devoted R ) and luminosity (0.05% L ) of its host star to the study of TRAPPIST-1. should make the comparative atmospheric char- acterization of its seven transiting planets within Keywords: planetary systems, star and planet reach of an ambitious JWST program. Given the formation, stars and stellar evolution limited lifetime of JWST, the ecliptic location of the star that limits its visibility to 100d per year, the large number of observational time required 1 JWST & TRAPPIST-1: towards arXiv:2002.04798v2 [astro-ph.EP] 13 Feb 2020 by this study, and the numerous observational and theoretical challenges awaiting it, its full the spectroscopic study of po- success will critically depend on a large level of tentially habitable exoplanets coordination between the involved teams and on the support of a large community. In this context, we present here a community initiative aiming Since the seminal discovery of 51 Pegasi b (Mayor and to develop a well-defined sequential structure Queloz, 1995), more than four thousands exoplanets for the study of the system with JWST and to have been detected at an ever increasing rate, includ- coordinate on every aspect of its preparation ing a steeply growing fraction of planets significantly and implementation, both on the observational smaller than Uranus and Neptune, some even smaller (e.g. study of the instrumental limitations, data than our Earth. In parallel, many projects aiming to characterize exoplanets have developed during the last The TRAPPIST-1 JWST Community Initiative two decades, bringing notably first pieces of informa- tion on the atmospheric properties of several dozens of exoplanets. Most of these atmospheric studies have been made possible by the transiting configuration of the probed planets. Indeed, the special geometrical configuration of transiting planets offers the detailed study of their atmosphere without the cost of spatially resolving them from their host stars (Winn, 2010). The first atmospheric studies of transiting "hot Jupiters" (e.g. Charbonneau, 2009; Sing et al., 2016) and "hot Neptunes" (e.g. Deming et al., 2007; Wakeford et al., 2017) have provided initial glimpses at the atmospheric chemical composition, vertical pressure-temperature profiles, albedos, and circulation patterns of extrasolar Figure 1: Transmission spectra for three photochemically self- worlds (see Madhusudhan, 2019 for a recent review). consistent Earth-like planets around three M dwarfs Within the last decade, such atmospheric studies have with different radii. Original references for the trans- been extended to smaller and/or cooler planets like mission spectra simulations are Segura et al., 2005 the hot rocky worlds 55 Cnc e (Demory et al., 2012; and Schwieterman et al., 2016 for AD Leo, Meadows Demory et al., 2016) and LHS3844 b (Kreidberg et al., et al., 2018 for Proxima Cen, and Meadows et al. 2019) or the more temperate mini-Neptunes GJ1214 b (in prep) for TRAPPIST-1. (Kreidberg et al., 2014) and K2-18 b (Benneke et al., 2019). Extending further such studies to rocky worlds or- the star as one moves down the main-sequence. For biting within the circumstellar habitable zone (HZ) of the latest-type red dwarfs, this zone corresponds to their star (e.g. Kasting, Whitmire, and Reynolds, 1993) orbital periods of only a few days, making possible to would make it possible to probe the atmospheric and stack dozens if not hundreds of eclipse observations surface conditions of potentially habitable worlds, to within the JWST lifetime, and thus to maximize the explore their atmospheric compositions for chemical final signal-to-noise. disequilibria of biological origins (Schwieterman et al., Such considerations motivated the emergence of sev- 2018), and, maybe, to reveal the existence of life be- eral projects aiming to search the nearest M-dwarfs yond our solar system. This extension is not possible for transits before the launch of JWST, e.g. MEarth with the astronomical facilities currently in operation, (Nutzman and Charbonneau, 2008) or TESS (Deming including with the Hubble Space Telescope (HST), but et al., 2009). Among them, the ground-based survey it could be initiated from 2021 with the launch of JWST. SPECULOOS (Gillon, 2018; Burdanov et al., 2018; Del- Thanks to its much improved infrared spectral cover- rez et al., 2018) chose to gamble on ‘ultracool’ (i.e. age, sensitivity, and resolution over HST, JWST should later than M6) dwarf stars, at the bottom of the main- in theory be able to probe by eclipse (i.e. transit or oc- sequence, despite that most theoretical expectations cultation) spectroscopy the atmospheric composition presented such super-low-mass (≤ 0:1M ) stars as un- of a transiting temperate planet similar in size and likely to form short-period rocky planets similar in size mass to Earth. Nevertheless, signal-to-noise considera- to Earth (e.g. Raymond, Scalo, and Meadows, 2007). tions result in the possibility of such an achievement While SPECULOOS started officially its operation in only for a planet transiting a very nearby (up to 10- 2019 (Jehin et al., 2018; Ducrot and Gillon, 2019), 20 pc) and very-late-type (∼M5 and later) M-dwarf it was in fact initiated in 2011 as a prototype survey (Charbonneau, 2009; Kaltenegger and Traub, 2009; targeting a limited sample of fifty nearby ultracool de Wit and Seager, 2013). Indeed, the same planet dwarfs with the TRAPPIST 60cm robotic telescope in orbiting a bigger star will produce smaller transmission Chile (Gillon et al., 2013). This prototype aimed only and emission signals, and this drop is very sharp when to assess the feasibility of SPECULOOS, but it did much one gets away from the bottom of the main-sequence. better than that. Indeed, in 2015, it detected three For instance, Figure1 shows transmission spectra for transiting Earth-sized planets in orbit around a Jupiter- three photochemically self-consistent Earth-like plan- sized M8-type star 12 parsec away (Gillon et al., 2016). ets around three different M dwarfs with different radii. In 2016, the intensive photometric follow-up of the star The relative transit depth of the prominent near-IR (renamed TRAPPIST-1) - with a critical contribution CH4 bands of an Earth-like planet orbiting an M8-type of the Spitzer Space Telescope - revealed that it was dwarf star is expected to be ∼50ppm, but only ∼5 ppm in fact orbited by seven transiting Earth-sized planets, for a M3.5V host star. The floor noise of JWST instru- all of them close or within the HZ of the star (Gillon ments is expected to be of at least 10 ppm (Fauchez et al., 2017). With semi-major axes ranging from 1 et al., 2019a), this ‘JWST opportunity’ is thus clearly to 6% of an astronomical unit, these planets form a limited to the latest-type red dwarfs. This conclusion super compact planetary system whose long-term sta- is strengthened by the habitable zone moving closer to bility is ensured by its resonant architecture (Luger Page 2 of 23 The TRAPPIST-1 JWST Community Initiative et al., 2017). While transit photometry brought pre- et al., 2020). To assess this ‘better target to come’ cise measurements of the planets’ sizes (Delrez et al., hypothesis, we cross-matched the Gaia DR2 catalog 2018), their strong mutual interactions made possible (Gaia Collaboration et al., 2018) with the 2MASS point- to infer initial estimates of their masses using the tran- source catalog (Skrutskie et al., 2006) to build a list sit timing variations method (TTVs; Agol et al., 2005; of M-dwarfs and L-dwarfs within 40pc (see AnnexA). Holman and Murray, 2005), the resulting densities sug- For each of them, we estimated the basic physical pa- gesting rocky compositions maybe more volatile-rich rameters (mass, size, effective temperature), and then than Earth’s (Grimm et al., 2018, Agol et al. in prep.). computed the value of a metric for JWST transmission The discovery of TRAPPIST-1 was followed by sev- spectroscopy in the near-infrared (with NIRSPEC in eral theoretical studies aiming to assess the potential prism mode) of a putative transiting ‘Earth-like’ planet, of JWST to probe the atmospheric composition of its i.e. with the same mass, size, and equilibrium tem- planets (Barstow and Irwin, 2016; Morley et al., 2017; perature (Teq) than our planet. We also performed Batalha et al., 2018; Krissansen-Totton et al., 2018; the same exercise for occultation photometry in the Wunderlich et al., 2019; Fauchez et al., 2019a; Lustig- mid-infrared (with MIRI in imaging mode) for an Earth- Yaeger, Meadows, and Lincowski, 2019).