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Gliese 581D Is the First Discovered Terrestrial-Mass Exoplanet in the Habitable Zone

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Gliese 581D Is the First Discovered Terrestrial-Mass Exoplanet in the Habitable Zone Gliese 581d is the first discovered terrestrial-mass exoplanet in the habitable zone The Harvard community has made this article openly available. Please share how this access benefits you. Your story matters Citation Wordsworth, Robin D., François Forget, Franck Selsis, Ehouarn Millour, Benjamin Charnay, and Jean-Baptiste Madeleine. 2011. “Gliese 581d is the first discovered terrestrial-mass exoplanet in the habitable zone.” The Astrophysical Journal Letters 733 (2) (May 12): L48. doi:10.1088/2041-8205/733/2/l48. http:// dx.doi.org/10.1088/2041-8205/733/2/L48. Published Version doi:10.1088/2041-8205/733/2/L48 Citable link http://nrs.harvard.edu/urn-3:HUL.InstRepos:27846843 Terms of Use This article was downloaded from Harvard University’s DASH repository, and is made available under the terms and conditions applicable to Open Access Policy Articles, as set forth at http:// nrs.harvard.edu/urn-3:HUL.InstRepos:dash.current.terms-of- use#OAP Gliese 581d is the first discovered terrestrial-mass exoplanet in the habitable zone Robin D. Wordsworth,1∗ Fran¸coisForget,1 Franck Selsis,2;3 Ehouarn Millour,1 Benjamin Charnay,1 Jean-Baptiste Madeleine1 1Laboratoire de M´et´eorologieDynamique, Institut Pierre Simon Laplace, Paris, France 2CNRS, UMR 5804, Laboratoire d'Astrophysique de Bordeaux, 2 rue de l'Observatoire, BP 89, F-33271 Floirac Cedex, France 3Universit´ede Bordeaux, Observatoire Aquitain des Sciences de l'Univers, 2 rue de l'Observatoire, BP 89, F-33271 Floirac Cedex, France ABSTRACT It has been suggested that the recently discovered exoplanet GJ581d might be able to support liquid water due to its relatively low mass and orbital distance. However, GJ581d receives 35% less stellar energy than Mars and is probably locked in tidal resonance, with extremely low insolation at the poles and possibly a permanent night side. Under such conditions, it is unknown whether any habitable climate on the planet would be able to withstand global glaciation and / or atmospheric collapse. Here we present three-dimensional climate simulations that demonstrate GJ581d will have a stable atmosphere and surface liquid water for a wide range of plausible cases, making it the first confirmed super-Earth (exoplanet of 2-10 Earth masses) in the habitable zone. We find that atmospheres with over 10 bar CO2 and varying amounts of background gas (e.g., ◦ N2) yield global mean temperatures above 0 C for both land and ocean-covered surfaces. Based on the emitted IR radiation calculated by the model, we propose observational tests that will allow these cases to be distinguished from other possible scenarios in the future. Subject headings: astrobiology|planet-star interactions|planets and satellites: atmospheres| techniques: spectroscopic 1. Introduction the team responsible for finding the other four planets in the system (Kerr 2010; Tuomi 2011). The local red dwarf Gliese 581 (20.3 ly from For the moment, therefore, GJ581g remains un- the Sun, M = 0:31MSun, L = 0:0135LSun, spec- confirmed. tral type M3V) (Hawley et al. 1997) has received arXiv:1105.1031v1 [astro-ph.EP] 5 May 2011 GJ581d, in constrast, which was first discov- intense interest over the last decade due to the low ered in 2007 and has a minimum mass between mass exoplanets discovered around it. As of early 5.6 and 7.1 M , has now been robustly con- 2011 it has been reported to host up to six planets Earth firmed by radial velocity (RV) observations (Udry (Udry et al. 2007; Mayor et al. 2009; Vogt et al. et al. 2007; Mayor et al. 2009; Vogt et al. 2010). 2010). One of these, GJ581g, was announced in Due to its greater distance from the host star, September 2010 and estimated to be in the hab- GJ581d was initially regarded as unlikely to have itable zone (the orbital range in which a planet's surface liquid water unless strong warming mech- atmosphere can warm the surface sufficiently to al- anisms due to e.g., CO clouds (Forget and Pier- low surface liquid water) (Kasting et al. 1993; Pier- 2 rehumbert 1997; Selsis et al. 2007) were present in rehumbert 2011). However, its discovery has been its atmosphere. Recently, simple one-dimensional strongly disputed by other researchers, including 1 radiative-convective studies (Wordsworth et al. Restricting the composition of the atmosphere to 2010b; von Paris et al. 2010; Kaltenegger et al. two species in this way allows us to determine con- 2011) have suggested that a dense atmosphere servative conditions for habitability, as it neglects could provide a significant greenhouse effect on the warming due to other greenhouse gases like GJ581d. However, the planet's tidal evolution CH4 or buffer gases like N2 or Ar (von Paris et al. poses a key problem for its habitability. 2010; Goldblatt et al. 2009; Li et al. 2009). As it is most likely either in a pseudo-synchronous The simulations were performed using a new state with a rotation period that is a function of type of GCM that we developed specifically for ex- the eccentricity, or in spin-orbit resonance like oplanet and paleoclimate studies. It uses radiative Mercury in our Solar System (Leconte et al. 2010; transfer data generated directly from high resolu- Heller et al. 2011), GJ581d should have extremely tion spectra, which allows the accurate simulation low insolation at its poles and possibly a perma- of climates for essentially any atmospheric cock- nent night side. Regions of low or zero insolation tail of gases, aerosols and clouds for which optical on a planet can act as cold traps where volatiles data exists. The dynamical core of the model was such as H2O and CO2 freeze out on the surface. adapted from the LMD Mars GCM, which uses A few previous studies (Joshi et al. 1997; Joshi an enstrophy and total angular momentum con- 2003) have examined atmospheric collapse in 3D serving finite difference scheme (Sadourny 1975; with simplified radiative transfer, but only for Forget et al. 1999). Scale-selective hyperdiffusion Earth-like atmospheric pressures or lower (0.1 to was used in the horizontal plane for stability. The 1.5 bar). For low values of stellar insolation and planetary boundary layer was parameterised us- large planetary radii, even dense CO2 atmospheres ing the method of Mellor and Yamada (1982) to will be prone to collapse, which could rule out a calculate turbulent mixing, with the latent heat of stable water cycle altogether for a super-Earth H2O also taken into account in the surface tem- like GJ581d. To conclusively evaluate whether perature calculations when necessary. A standard −2 GJ581d is in the habitable zone, therefore, three- roughness coefficient of z0 = 1 × 10 m was used dimensional simulations using accurate radiative for both rocky and ocean surfaces for simplicity, transfer are necessary. although we verified that our results were insensi- Here we present global climate model (GCM) tive to variations in this parameter. Spatial reso- simulations we performed to assess this issue. In lution of 32×32×20 in longitude, latitude and al- Section 2 we describe the model used. In Section titude was used for all simulations; we performed 3 we describe our results, and in Section 4 we dis- one comparison test at the highest rotation rate cuss implications and propose future observational with 64 × 64 × 20 resolution and found that the tests for the simulated habitable scenarios. differences were small. Our radiative transfer scheme was similar to 2. Method that we developed previously for one-dimensional simulations (Wordsworth et al. 2010a,b). For a In our simulations we made the initial hypoth- given mixture of atmospheric gases, we computed esis that GJ581d has a climate dominated by the high resolution spectra over a range of tempera- greenhouse effects of CO2 and / or H2O, as is tures, pressures and gas mixing ratios. For this the case for all rocky planets with atmospheres study we used a 6 × 9 × 7 temperature, pressure in the Solar System (Venus, Earth and Mars). To and H2O volume mixing ratio grid with values T = assess the influence of water on the climate in- f100; 150;:::; 350g K, p = f10−3; 10−2;:::; 105g dependently, we considered two classes of initial −7 −6 −1 mbar and qH2O = f10 ; 10 ;:::; 10 g, respec- condition: a rocky planet with no water, and an tively. The correlated-k method was used to pro- ocean planet, where the surface is treated as an duce a smaller database of coefficients suitable infinite water source. CO2 was taken as the pri- for fast calculation in a GCM. The model used mary constituent of the atmosphere and H2O was 38 spectral bands in the longwave and 36 in the allowed to vary freely, with surface ice / liquid shortwave, and sixteen points for the g-space in- and cloud formation (including radiative effects) tegration, where g is the cumulated distribution taken into account for either gas when necessary. function of the absorption data for each band. 2 In most simulations CO2 was assumed to be the Manabe and Wetherald (1967). Precipitation of main constituent of the atmosphere, except the H2O due to coagulation was also included using a locally habitable ice planet experiments (see Sec- simple threshold parameterisation (Emanuel and tion 3), where N2 was used. CO2 collision-induced Ivkovi-Rothman 1999). absorption was included using a parameterisation On the surface, the local albedo varied accord- based on the most recent theoretical and experi- ing to the composition (rocky, ocean, CO2 or H2O mental studies (Wordsworth et al. 2010a; Gruszka ice; see Table 1). In the wet simulations, ice for- and Borysow 1998; Baranov et al. 2004). For the mation (melting) was assumed to occur when the stellar spectrum, we used the Virtual Planet Lab- surface temperature was lower (higher) than 273 oratory AD Leo data (Segura et al.
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