Astronomy & Astrophysics manuscript no. Wunderlich_etal_2019_arxiv c ESO 2019 May 8, 2019 Detectability of atmospheric features of Earth-like planets in the habitable zone around M dwarfs Fabian Wunderlich1, Mareike Godolt1, John Lee Grenfell2, Steffen Städt3, Alexis M. S. Smith2, Stefanie Gebauer2, Franz Schreier3, Pascal Hedelt3, and Heike Rauer1; 2; 4 1 Zentrum für Astronomie und Astrophysik, Technische Universität Berlin, Hardenbergstraße 36, 10623 Berlin, Germany e-mail: [email protected] 2 Institut für Planetenforschung, Deutsches Zentrum für Luft- und Raumfahrt, Rutherfordstraße 2, 12489 Berlin, Germany 3 Institut für Methodik der Fernerkundung, Deutsches Zentrum für Luft- und Raumfahrt, 82234 Oberpfaffenhofen, Germany 4 Institut für Geologische Wissenschaften, Freie Universität Berlin, Malteserstr. 74-100, 12249 Berlin, Germany ABSTRACT Context. The characterisation of the atmosphere of exoplanets is one of the main goals of exoplanet science in the coming decades. Aims. We investigate the detectability of atmospheric spectral features of Earth-like planets in the habitable zone (HZ) around M dwarfs with the future James Webb Space Telescope (JWST). Methods. We used a coupled 1D climate-chemistry-model to simulate the influence of a range of observed and modelled M-dwarf spectra on Earth-like planets. The simulated atmospheres served as input for the calculation of the transmission spectra of the hy- pothetical planets, using a line-by-line spectral radiative transfer model. To investigate the spectroscopic detectability of absorption bands with JWST we further developed a signal-to-noise ratio (S/N) model and applied it to our transmission spectra. Results. High abundances of methane (CH4) and water (H2O) in the atmosphere of Earth-like planets around mid to late M dwarfs increase the detectability of the corresponding spectral features compared to early M-dwarf planets. Increased temperatures in the middle atmosphere of mid- to late-type M-dwarf planets expand the atmosphere and further increase the detectability of absorption bands. To detect CH4,H2O, and carbon dioxide (CO2) in the atmosphere of an Earth-like planet around a mid to late M dwarf observing only one transit with JWST could be enough up to a distance of 4 pc and less than ten transits up to a distance of 10 pc. As a consequence of saturation limits of JWST and less pronounced absorption bands, the detection of spectral features of hypothetical Earth-like planets around most early M dwarfs would require more than ten transits. We identify 276 existing M dwarfs (including GJ 1132, TRAPPIST-1, GJ 1214, and LHS 1140) around which atmospheric absorption features of hypothetical Earth-like planets could be detected by co-adding just a few transits. Conclusions. The TESS satellite will likely find new transiting terrestrial planets within 15 pc from the Earth. We show that using transmission spectroscopy, JWST could provide enough precision to be able to partly characterise the atmosphere of TESS findings with an Earth-like composition around mid to late M dwarfs. 1. Introduction trast ratio and transit depth of an Earth-like planet around cool host stars favours a detection of spectral atmospheric features. Dozens of terrestrial planets in the habitable zone (HZ) have Hence, planets transiting M dwarfs are prime targets for future been found so far. Most of these planets orbit cool host stars, the 1 telescopes such as the James Webb Space Telescope (JWST; so-called M dwarfs . The Transiting Exoplanet Survey Satellite Gardner et al. 2006; Deming et al. 2009) and the European Ex- (TESS) is expected to find many more of these systems in our tremely Large Telescope (E-ELT; Gilmozzi & Spyromilio 2007; solar neighbourhood in the near future (Ricker et al. 2014; Sul- Marconi et al. 2016). livan et al. 2015; Barclay et al. 2018). Whether these planets Additionally model simulations show that planets with an Earth- can have surface conditions to sustain (complex) life is still de- like composition can build up increased amounts of biosignature bated (e.g. Tarter et al. 2007; Shields et al. 2016). Because of gases like ozone (O3) and related compounds like water (H2O) the close-in HZ, M-dwarf planets are likely tidally locked (e.g. and methane (CH4), which further increases their detectability Kasting et al. 1993; Selsis et al. 2008). To allow for habitable sur- (Segura et al. 2005; Rauer et al. 2011; Grenfell et al. 2013; face conditions they require a mechanism to redistribute the heat Rugheimer et al. 2015). Knowing the ultraviolet radiation (UV) arXiv:1905.02560v1 [astro-ph.EP] 7 May 2019 from the dayside to the nightside (e.g. atmospheric or ocean heat of the host star is crucial to understand the photochemical pro- transport; Joshi et al. 1997; Joshi 2003; Selsis et al. 2008; Yang cesses in the planetary atmosphere (see e.g. Grenfell et al. 2014; et al. 2013; Hu & Yang 2014). Another drawback of a planet Rugheimer et al. 2015). M dwarfs vary by several orders in the lying close to its host star is the high luminosity during the pre- UV flux from inactive to active stars (Buccino et al. 2007; France main-sequence phase (e.g. Ramirez & Kaltenegger 2014; Luger et al. 2016). & Barnes 2015; Tian & Ida 2015) or that it might be subject to The approach of this study is to investigate the influence of a strong stellar cosmic rays (see e.g. Grießmeier et al. 2005; Se- range of spectra of active and inactive stars with observed and gura et al. 2010; Tabataba-Vakili et al. 2016; Scheucher et al. modelled UV radiation on Earth-like planets in the HZ around 2018). On the other hand M-dwarf planets are favourable tar- M dwarfs. We furthermore aim to determine whether the result- gets for the characterisation of their atmosphere. The high con- ing planetary spectral features could be detectable with JWST. Previous studies showed that multiple transits are needed to de- 1 phl.upr.edu/projects/habitable-exoplanets-catalog Article number, page 1 of 18 A&A proofs: manuscript no. Wunderlich_etal_2019_arxiv Table 1. Stellar parameters for the Sun and all M dwarfs used in this study. Stars labelled with an asterisk are active M dwarfs. Effective tempera- tures of the stars which are included in the MUSCLES database (Teff[K] MUSC.) are taken from Loyd et al. (2016). −3 Star Type Teff[K] Lit. Teff[K] MUSC. R=R M=M L=L [10 ] d [pc] Sun G2 5772 - 1.000 1.000 1000.00 0.00 GJ 832 M1.5a 3657b 3816±250c 0.480a 0.450±0.050a 26.00d 4.93a GJ 176 M2e 3679±77e 3416±100c 0.453±0.022e 0.450e 33.70±1.80e 9.27e GJ 581 M2.5 f 3498±56 f 3295±140c 0.299±0.010 f 0.300 f 12.05±0.24 f 6.00 f g g c g +0:071 g g h GJ 436 M3 3416±56 3281±110 0.455±0.018 0.507−0:062 25.30±1.20 10.23 GJ 644 M3*i 3350 j - 0.678k 0.416±0.006i 26.06 j 6.50k AD Leo M3.5*b 3380b - 0.390l 0.420l 24.00m 4.89b GJ 667C M3.5n 3350o 3327±120c 0.460p 0.330±0.019o 13.70q 6.80q e e +120 c e e e e GJ 876 M4 3129±19 3062−130 0.376±0.006 0.370 12.20±0.20 4.69 GJ 1214 M4.5r 3252±20s 2935±100c 0.211±0.011s 0.176±0.009s 4.05±0.19s 14.55±0.13s Proxima Cen. M5.5t 3054±79t - 0.141±0.007t 0.118t 1.55±0.02t 1.30r TRAPPIST-1 M8u 2559±50u - 0.117±0.004u 0.080±0.007u 0.52±0.03u 12.10±0.40u References. (a) Bailey et al. (2008); (b) Gautier III et al. (2007); (c) Loyd et al. (2016); (d) Bonfils et al. (2013); (e) von Braun et al. (2014); (f) von Braun et al. (2011); (g) von Braun et al. (2012); (h) Butler et al. (2004); (i) Segransan et al. (2000); (j) Reid & Gilmore (1984); (k) Giampapa et al. (1996); (l) Reiners et al. (2009); (m) Pettersen & Coleman (1981); (n) Neves et al. (2014); (o) Anglada-Escudé et al. (2013a); (p) Kraus et al. (2011); (q) van Leeuwen (2008); (r) Lurie et al. (2014); (s) Anglada-Escudé et al. (2013b); (t) Boyajian et al. (2012); (u) Gillon et al. (2017) tect any spectral feature of an Earth-like planet with transmis- (2010), Rauer et al. (2011), von Paris et al. (2015), Gebauer et al. sion or emission spectroscopy (Rauer et al. 2011; von Paris et al. (2017), Gebauer et al. (2018a), and Gebauer et al. (2018b). The 2011; Barstow et al. 2016; Barstow & Irwin 2016). Rauer et al. atmosphere in the climate module is divided into 52 pressure (2011) found that CH4 of an Earth-like planet around AD Leo layers and the chemistry model into 64 equidistant altitude (at 4.9 pc) would be detectable by co-adding at least three tran- levels. sits using transmission spectroscopy with JWST. To detect O3 at The radiative transfer of the climate module is separated into least ten transits would be required. For the TRAPPIST-1 planets a short wavelength region from 237.6 nm to 4.545 µm with c and d at 12.1 pc, 30 transits are needed to detect an Earth-like 38 wavelength bands for incoming stellar radiation and a long concentration of O3 with JWST (Barstow & Irwin 2016). wavelength region from 1 µm to 500 µm in 25 bands for In this study we first apply a 1D atmosphere model to calculate planetary and atmospheric thermal radiation (von Paris et al.