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Home , Proxima Centauri b, Solar wind

from the TRAPPIST-1 and implications for habitability

Chuanfei Donga,b,1, Meng Jinc, Manasvi Lingamd,e, Vladimir S. Airapetianf, Yingjuan Mag, and Bart van der Holsth

aDepartment of Astrophysical Sciences, Princeton University, Princeton, NJ 08544; bPrinceton Center for , Princeton Laboratory, Princeton University, Princeton, NJ 08544; cLockheed Martin Solar and Astrophysics Laboratory, Palo Alto, CA 94304; dInstitute for Theory and Computation, Harvard–Smithsonian Center for Astrophysics, Cambridge, MA 02138; eJohn A. Paulson School of Engineering and Applied Sciences, Harvard University, Cambridge, MA 02138; fHeliophysics Division, NASA Goddard Space Flight Center, Greenbelt, MD 20771; gInstitute of and Planetary Physics, University of California, Los Angeles, CA 90095; and hCenter for Modeling, University of Michigan, Ann Arbor, MI 48109

Edited by Neta A. Bahcall, Princeton University, Princeton, NJ, and approved December 4, 2017 (received for review May 15, 2017) The presence of an over sufficiently long timescales dence from our own suggests that the erosion of the is widely perceived as one of the prominent criteria asso- atmosphere by the stellar plays a crucial role, especially for ciated with planetary surface habitability. We address the crucial -sized planets where such losses constitute the dominant question of whether the seven Earth-sized planets transiting the mechanism (13, 14), and the same could also be true for exo- recently discovered ultracool dwarf TRAPPIST-1 are capable of planets around M dwarfs (15, 16). Recent studies of atmospheric retaining their . To this effect, we carry out numerical escape rates from Proxima b (and other M-dwarf ) simulations to characterize the of TRAPPIST-1 and the also appear to indicate that the resulting ion losses are signifi- atmospheric ion escape rates for all of the seven planets. We also cant because of the extreme space conditions involved estimate the escape rates analytically and demonstrate that they (17), potentially resulting in the atmosphere being depleted are in good agreement with the numerical results. We conclude over a span ranging from tens to hundreds of millions of years that the outer planets of the TRAPPIST-1 system are capable of (15, 18–20). retaining their atmospheres over billion-year timescales. The con- Hence, in this paper we focus primarily on the atmospheric sequences arising from our results are also explored in the con- ion escape rates of the seven TRAPPIST-1 planets by adapt- text of abiogenesis, biodiversity, and searches for future exoplan- ing a sophisticated multispecies (MHD) ets. In light of the many unknowns and assumptions involved, we model which self-consistently includes ionospheric chemistry and recommend that these conclusions must be interpreted with due physics and electromagnetic forces. In this work, we do not caution. tackle the wide range of hydrodynamic escape mechanics that have been explored for terrestrial planets (16, 21) for the above exoplanets | stellar wind | atmospheric escape | reasons. The Stellar Wind of TRAPPIST-1 ith the number of detected exoplanets now exceeding W3,600 (1), exoplanetary research has witnessed many To commence our analysis of stellar wind-induced atmospheric remarkable advances recently. One of the most important areas loss, the stellar wind parameters of TRAPPIST-1 are required. in this field is the hunt for Earth-sized terrestrial planets residing Since the conditions at the TRAPPIST-1 planets in the habitable zone (HZ) of their host —the HZ repre- Significance sents the region within which a can support liquid water on its surface (2); a probabilistic version of the HZ, encompassing a wide range of planetary and stellar parameters, has also been for- The search for exoplanets has rapidly emerged as one of the mulated (3). The importance of this endeavor stems from the fact most important endeavors in . This field received that such planets can be subjected to further scrutiny to poten- a major impetus with the recent discovery of seven temper- tially resolve the question of whether they may actually harbor ate Earth-sized exoplanets orbiting the nearby ultracool dwarf life (4). star TRAPPIST-1. One of the most crucial requirements for con- Most of the recent attention has focused on exoplanets in the ventional (surface-based) is the pres- HZ of M dwarfs, i.e., low- stars that are much longer lived ence of an atmosphere over long timescales. We determine than the , for the following reasons. First, M dwarfs are the the atmospheric escape rates numerically and analytically for most common type of stars within the Milky Way (5), implying the planets of the TRAPPIST-1 system and show that the outer that ∼1010 Earth-sized planets in the HZ of M dwarfs may exist planets are potentially likely to retain their atmospheres over in our Galaxy (6). , owing to the HZ being much closer to billion-year timescales. Our work has far-reaching and pro- such stars, it is much easier to detect exoplanets and characterize found implications for atmospheric escape and the habitabil- their atmospheres, if they do exist (7). Finally, this field has wit- ity of terrestrial exoplanets around M dwarfs. nessed two remarkable advances within the last year: the discov- ery of Proxima b (8) and the seven Earth-sized planets transiting Author contributions: C.D., M.J., M.L., Y.M., and B.v.d.H. designed research; C.D., M.J., and M.L. performed research; C.D., M.J., M.L., and V.S.A. analyzed data; and C.D., M.J., the ultracool dwarf TRAPPIST-1 (9, 10). The significance of the M.L., V.S.A., Y.M., and B.v.d.H. wrote the paper. former stems from the fact that it orbits the star closest to the The authors declare no conflict of interest. Solar System, and the latter is important because there exist as This article is a PNAS Direct Submission. many as three planets in the HZ with the possibility of life being seeded by panspermia (11). Published under the PNAS license. In light of these discoveries, the question of whether terrestrial Data deposition: The datasets reported in this paper are archived at the publicly acces- sible Pleiades Supercomputer at the NASA Advanced Supercomputing (NAS) Division, exoplanets in the HZ of M dwarfs are habitable is an important https://umich.box.com/s/1mxnjc64uvfhnxhbpi2xxipxo688183c. The Space Weather Mod- one (7). Among the many criteria identified for a planet to be eling Framework that contains the BATS-R-US code used in this study is publicly available habitable, the existence of an atmosphere has been posited as from csem.engin.umich.edu/tools/swmf. being crucial for surficial life as we know it (4, 12). It is therefore 1To whom correspondence should be addressed. Email: [email protected]. evident that the study of atmospheric losses from exoplanets con- This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10. stitutes a crucial line of enquiry. Empirical and theoretical evi- 1073/pnas.1708010115/-/DCSupplemental.

260–265 | PNAS | January 9, 2018 | vol. 115 | no. 2 www.pnas.org/cgi/doi/10.1073/pnas.1708010115 Downloaded by guest on September 29, 2021 Downloaded by guest on September 29, 2021 lss lnt iswti h rtclsrae hl l fthe stellar of supermagnetosonic all the while in (the surface, TRAPPIST-1b embedded wind. critical of are the orbit TRAPPIST- planets within the the other of of lies feature part planet) unique that closest very namely different (23). a system, at to 1 and originate leads different this have Hence, these and star winds; the on stellar regions slow of and respectively. distances speeds, fast at magnetosonic occurs fast surface and The wind stellar the ing oge al. et for Dong bar (C color location. the surface that critical Note the respectively. , shows and line pressure, dashed dynamic for The plane normalized that (AU). velocity, equatorial and unit wind The B, stellar astronomical (B) the 1 TRAPPIST-1h. depicting at to TRAPPIST-1 pressure TRAPPIST-1b near dynamic planets, wind seven solar of plane the equatorial orbits by the normalized represent pressure lines dynamic solid (z black plane The equatorial tosonic. the at wind stellar 1. Fig. surface where critical region The the distance. as stellar defined same is the stellar times) at three the evaluated (approximately (29), faster when much solution is wind TRAPPIST-1 of normal the with Compared in a Fur- found and TRAPPIST-1. (28). be of (27) can dwarfs Wind estimates Stellar approach M the latest our late concerning similar the details of on the ther typical based modeling field star for magnetic radius, the AWSoM mean period, of rotational the range the mass adapt use wide and To we a wind, 26). stellar to for 25, TRAPPIST-1 adapted (17, profiles be stars wind high-fidelity readily of can stellar reproducing and in model 24) successful self-consistently (23, conditions be corona to solar AWSoM proved The 23). been (22, wind has solar and corona for solar developed the originally simulating was that model MHD imple- global Alfv upon is data-driven the latter rely of The means must TRAPPIST-1. by of mented we wind observations, stellar the from simulating unknown presently are h taysaeselrwn ouini lutae nFg 1. Fig. in illustrated is solution wind stellar steady-state The h Dselrwn ofiuainwt eetdmgei edlns h akrudcnorshows contour background The lines. field magnetic selected with configuration wind stellar 3D The (A) TRAPPIST-1. of wind stellar steady-state The F sietclt htfor that to identical is z hwn h tla iddniynraie yteslrwn est t1A.(D–F AU. 1 at density wind solar the by normalized density wind stellar the showing 0 = ´ nWv oa oe ASM,a (AWSoM), Model Solar en v s = C. ∼30 v f with , ) h leioufc ersnsteciia ufc eodwihteselrwn eoe supermagne- becomes wind stellar the which beyond surface critical the represents isosurface blue The 0). = R ∗ v and s and ∼20 v f R represent- Modeling ∗ o the for h aictoso hs xrm pc ete odtoson conditions weather planet’s space the extreme of 19). these for (15, evolution of investigated ramifications the documented thoroughly The on been been effects also have its already and has (17), such pressure b of Proxima wind existence The extreme environment. about planet, an near-Earth furthermost is the the than pressure consider larger dynamic we the when Even TRAPPIST-1h, Earth. at sure much pressure wind a dynamic the to about the TRAPPIST-1b, is subjected by of orbit experienced the are the that At planets with with Earth. compared the combined pressure of dynamic When all larger density. speed, wind higher higher its is of TRAPPIST-1 lines the along and (17). conditions b to submagnetosonic Proxima wind expected between stellar is transitions be supermagnetosonic could surface frequent TRAPPIST-1b critical to pro- that the dynamo subject implying the of concomitantly, by vary distance caused also the magnetic cycle mechanism the Thus, stellar in dynamo the cess. variations a during that to observe occur rise we field context, give this even In (iii) (32). regulate and ( (i) (31), (30), perhaps dynamo could rate interaction rotational directly. star star–planet the surface, stel- host the critical its magnetized turn, with the interact strongly In within magnetically a could orbits planet with TRAPPIST-1b the As conjunction to wind). (in TRAPPIST-1b lar star of proximity host the its of because primarily System nte itnusigfaueo h tla idfrom wind stellar the of feature distinguishing Another Solar own our within exist not does scenario striking This 10 3 –10 4 ie rae hnteslrwn yai pres- dynamic wind solar the than greater times PNAS omi iwo h qaoilpaeat plane equatorial the of view zoom-in A ) | oiytepoete falocal a of properties the modify ii) aur ,2018 9, January z hwn h tla wind stellar the showing 0 = | o.115 vol. E stesm sfor as same the is 100–300 | o 2 no. times | The ) z 261 0 =

ASTRONOMY the atmospheric ion escape rates of the TRAPPIST-1 planets are us to compare and contrast their properties against those of explored in Discussion and Conclusions. exo- (43).] We note that the mass-loss rate from TRAPPIST-1 is ∼2.6 × The next two input parameters to be specified are the surface 1011 g/s, which is about 10% of the -loss rate. Although pressure and the scale height for each of the planets. The for- the density and velocity of the stellar wind are higher for mer remains unknown at this stage, and we work with the fidu- TRAPPIST-1, the smaller size of the host star is responsible cial value of 1 atm at this stage. We anticipate that the surface for yielding a value lower than that of the active young Sun pressure does not significantly alter the escape rates, at least for   extreme stellar wind conditions, as demonstrated in ref. 15. The (33). The mass-loss rate ∼0.1M˙ obtained for TRAPPIST- scale height Hx is defined as 1 is broadly consistent with the upper bound of ∼0.2M˙ for the slightly larger star, (34). kT Hx = , (1) Finally, the stellar wind parameters provided in this paper mgx are useful in determining the radio auroral emission from the TRAPPIST-1 planets, which can be used to constrain their mag- where gx is the acceleration due to for planet X . The lat- netic fields (35). In Deducing the Magnetic Fields of the Trappist-1 ter quantity can be easily computed for all of the planets since Planets, we show that the radio emission could potentially peak their and radii are known (10, 27). at O(0.1) MHz and result in a radio flux density of O(0.1) mJy; The stellar wind parameters are obtained from the model the latter could be enhanced by two to three orders of magnitude described in The Stellar Wind of TRAPPIST-1. We must also pre- during a (CME) event. scribe the extreme UV (EUV) fluxes received at each of these planets, since the EUV flux plays an important role in regulating Ion Escape Rates for the TRAPPIST-1 Planets the extent of photoionization and the resultant stellar heating. To simulate the ion escape rates for the seven planets of the This is accomplished by using the values for the TRAPPIST-1 TRAPPIST-1 system, we use the sophisticated 3D Block Adap- planets computed in refs. 40, 41, and 44. tive Tree Solar-Wind Roe Up-Wind Scheme (BATS-R-US) mul- The planetary magnetic field is a potentially important fac- tispecies MHD (MS-MHD) model that has been extensively tor in regulating the ion escape rates. We consider the scenario tested and validated in the Solar System for and where the planets are unmagnetized because this case yields an (36–39) and was recently used to study the atmospheric losses upper limit on the allowed escape rates (15). Hence, if a planet from Proxima b (15). The reader is referred to these papers was characterized by “low” escape rates in the unmagnetized and to Atmospheric Ion Escape Rates for the TRAPPIST-1 Plan- limit, it would also typically possess low escape rates in the pres- ets for further details concerning the numerical implementation, ence of a magnetic field. It must also be borne in mind that the the model equations, and the physical and chemical processes planets orbiting TRAPPIST-1 are likely to be tidally locked, and encoded within the model. Note that the neutral atmosphere it has been argued that such planets are likely to possess weak is the source of the produced through, e.g., photoioniza- magnetic fields (45). If a planet is weakly magnetized, it is likely tion and charge exchange and that only a small fraction of them that the total ion escape rate will be comparable to (but slightly will escape into space. Hence, the atmosphere will, in addition lower than) the unmagnetized case (15). to being eroded, also undergo changes in the chemical compo- sition (15). Results from the Model. For each planet, we consider two limiting cases. The first case corresponds to the scenario with maximum The Input Parameters of the Model. For the most part, we concern dynamic (and total) pressure over one orbit of the planet. The ourselves with describing and motivating our choice of the dif- second case corresponds to the case with minimum total pres- ferent input parameters required for the BATS-R-US model. sure, but with the maximum magnetic pressure. The correspond- Before proceeding further, we caution the readers that many ing stellar wind parameters are provided in Table S2, and the of the relevant planetary and stellar wind parameters of the escape rates are in Table 1. TRAPPIST-1 system are unknown or poorly constrained. Hence, For all seven planets, the case with maximum total pressure it is important to recognize that, because of the many uncertain- yields a total atmospheric ion escape rate that is a few times ties involved, the ensuing escape rates may not necessarily be higher than that of the corresponding case with minimum total representative of the TRAPPIST-1 system. pressure. The innermost trio of planets (TRAPPIST-1b, -1c, and The BATS-R-US model relies upon an atmospheric compo- -1d) have escape rates higher than 1027 s−1, while the outermost sition akin to Venus and Mars, implying that the TRAPPIST- four have rates lower than this value when the case with max- 1 planets are also assumed to possess a similar composition. imum total pressure is considered. In comparison, the escape There are several factors that must be noted in this context. rates for Mars, Venus, and Earth are ∼1024−1025 s−1 (13), while First, as seen from our Solar System, the ion escape rates for that of Proxima b is ∼2 × 1027 (15). In our subsequent analysis, Venus, Mars, and Earth are similar despite their compositions, we focus on this case (maximum total pressure) since it leads us sizes, and magnetic-field strengths being wildly dissimilar (13, toward determining the upper bounds on the escape rates. 14), thereby indicating that the ion escape rates may be rela- Using the mixing-length formalism of ref. 46 in conjunction tively sensitive to stellar wind parameters compared with plan- with the definitions of the stellar and planetary mass-loss rates, it etary properties (the difference in stellar wind parameters at can be shown that O(1)  2 Venus, Earth, and Mars is only up to a factor); this is also Rx Nx M˙ ?, (2) partly borne out by the atmospheric ion escape rate calculations ∝ a for Proxima b (15, 19). In addition, it has been shown recently that the ion escape rates are only weakly dependent on the sur- where Nx is the atmospheric escape rate arising from stellar wind face pressure (15). We observe that the inner planets of the stripping, M˙ ? is the -loss rate, and Rx and a are the TRAPPIST-1 system could have experienced significant losses of radius and semimajor axis of the planet X , respectively (47, 48). H2 and water over fast timescales (40, 41), leaving behind other We have normalized the escape rates for the TRAPPIST-1 plan- atmospheric components. Finally, a Venus-like atmosphere for ets in terms of the escape rate for TRAPPIST-1b and compare the inner planets cannot be ruled out empirically, as noted in the numerical and analytical predictions in Fig. 2. ref. 42. [In broader terms, gaining a thorough understanding An inspection of Fig. 2 reveals that the analytical formula is in of Venus-type exoplanets is highly relevant, because it allows excellent agreement with the numerical simulations, especially

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J is by ions experienced of that higher much with escape is compared pressure planets total these the that at fact the of because pressed (P pressure total minimum with case the of for that number Mach than TRAPPIST-1b magnetosonic larger wind is solar TRAPPIST-1b The at TRAPPIST-1g. rate at loss is ion it the contours, that color evident the From the lines. with magnetic-field along interplanetary -1g) and (TRAPPIST-1b system TRAPPIST-1 the the in constrained. poorly higher are time- were rates the mass-loss since rates stellar difficult dependent escape are important estimates atmospheric is quantitative although it the past, results, that analytic note the and to simulations the However, fac- both planets. a for unmagnetized from by rates values escape numerical atmospheric the within of that fall tor find we results planets, analytical inner atmospheres the the for retaining Even of timescales. capable gigayear are over potentially which being -1h) to as (TRAPPIST-1e regarded planets four outermost the for oge al. et Dong of inspection an through verified S2 be Table also TRAPPIST-1b. at can velocity that conclusions wind than These stellar larger the obviously star; is the TRAPPIST-1g at from stel- away high accelerates the wind 3, of lar Fig. consequence density. a mostly wind is lar the wind with high stellar consistent The “slow” fully the picture. is wind which solar/stellar wind, classical stellar slow the signifies other mum all in present system. TRAPPIST-1 is the shock of bow planets the and case, cases hand, above In the ). other to and the the contrast inside On located (e.g., magnetosphere planets. for global observed eight planetary been the has our condition of in a prevalent any such not for is System par- condition is Solar this This since ). submagnetosonic noteworthy its ticularly of (because planet the equals × rmFg ,w bev httepam onaisaecom- are boundaries plasma the that observe we 3, Fig. From of planets two from escape ions the how depicts 3 Fig. ae ntebcgon oosi i.3, Fig. in colors background the on Based B P ec,Eq. Hence, .2 ocsi h oetmeuto 3) ial,w refer we Finally, (38). equation momentum the in forces tot 0. . n hrfr hr sn hc omdi rn of front in formed shock no is there therefore and 94, orsod otefs tla idadmaximum and wind stellar fast the to corresponds i.S1 Fig. hc eit h oopei rfie of profiles ionospheric the depicts which , 2 Bottom a aiiaeaqiketmto fthe of estimation quick a facilitate may iiu oa pressure total Minimum pressure total Maximum s in pressure rates Total escape Ion 1. Table Trappist-1c Trappist-1b Trappist-1h Trappist-1g Trappist-1f Trappist-1e Trappist-1d Trappist-1c Trappist-1b Trappist-1h Trappist-1g Trappist-1f Trappist-1e Trappist-1d o loidctsta h stel- the that indicates also row P Bottom tot aietdin manifested o,mini- row, 4.23 9.33 1.06 2.17 5.23 7.01 1.29 1.54 5.56 4.52 9.33 1.88 2.20 2.81 O × × × × × × × × × × × × × × −1 + P 10 10 10 10 10 10 10 10 10 10 10 10 10 10 tot tot 26 26 26 26 26 27 27 27 25 25 26 26 26 26 ) 3.37 2.83 3.80 1.38 2.09 4.99 1.65 2.71 2.69 5.85 4.30 4.19 3.07 9.22 hc r xetdt ecnieal 2) eenticue in included heating, not tidal were as of (27), model.] effects considerable life the be the surficial to but expected to (49), are state amenable which snowball be a not enter plan- may they TRAPPIST- periods. this TRAPPIST-1g that long suggest of and to over HZ 1f appear atmosphere simulations the will stable climate in 3D a TRAPPIST-1g planet [The support that a to likely for system chance etary seems best it the lines, represent same loss. ion the atmospheric of sug- Along perspective them, the may among from planet purely this “stable” viewed most when collectively, the is taken TRAPPIST-1h that Hence, gest TRAPPIST-1h. to TRAPPIST- rate from escape 1b outward, overall moves the one that as 2 monotonically Fig. declines from see also we Moreover, 1h. hs lnt a eanteramshrs h ausrange values The atmospheres. their retain from can planets these sur- a have of to pressure assumed face and sized Earth approximately are planets was of upper It an pressure. has bound TRAPPIST-1b dynamic) planet (or innermost the total that maximum concluded the to subject and o xsfrtecs ihmxmmttlpesr.Tesvndsic points distinct system. TRAPPIST-1 seven the The of pressure. planets total seven maximum the with represent case the for axis jor 2. Fig. is TRAPPIST-1h planet outermost the O × × × × × × × × × × × × × × 2 + O 10 10 10 10 10 10 10 10 10 10 10 10 10 10 h lto h omlzdecp aea ucino h semima- the of function a as rate escape normalized the of plot The 25 25 25 25 25 25 26 26 25 25 25 25 25 25 10 5. 8 1.10 1.14 1.32 1.52 2.92 6.98 1.32 1.19 4.39 1.38 1.10 1.25 1.04 2.76  92 o RPIT1 to TRAPPIST-1b for y CO × 1 × × × × × × × × × × × × × × 2 + t,w a siaetetmsae vrwhich over timescales the estimate can we atm, 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 PNAS 25 24 25 25 25 25 26 26 24 25 25 25 25 25 27 s 1.34 1.81 5.92 1.01 1.29 2.58 5.68 7.40 7.66 1.66 2.42 2.74 3.23 5.42 −1 | Total aur ,2018 9, January hl h orsodn au for value corresponding the while × × × × × × × × × × × × × × 10 10 10 10 10 10 10 10 10 10 10 10 10 10 27 26 26 26 26 27 27 27 25 26 26 26 26 26 O 1.29 10 | 10 o.115 vol. ×  o TRAPPIST- for y 10 26 | s −1 o 2 no. sthe As . | 263

ASTRONOMY Fig. 3. The logarithmic-scale contour plots of the O+ ion density (Top row) with magnetic field lines (in white) and stellar wind speed (Bottom row) with stellar wind velocity vectors (in white) in the meridional plane. Left two columns correspond to TRAPPIST-1b (minimum and maximum Ptot ) while Right two columns represent TRAPPIST-1g (minimum and maximum Ptot ). The X and Z coordinates have been normalized in terms of the corresponding planetary radius.

At this stage, we must reiterate the caveats discussed earlier. We have also shown that the ionospheric profiles for the First, most of the planetary and stellar parameters are partly TRAPPIST-1 planets in the HZ are not sensitive to the stellar or wholly unknown since the appropriate observations are not wind conditions at altitudes .200 km (Fig. S1). This is an impor- currently existent. The presence of (i) a more massive atmo- tant result in light of the considerable variability and intensity of sphere, (ii) a different atmospheric composition, and (iii) the the stellar wind, since it suggests that the lower regions (such as the planet’s magnetic field is likely to alter the extent of atmospheric planetary surface) may remain mostly unaffected under normal loss to some degree. Nonetheless, it can be surmised that the space weather conditions. [Stellar flares may have either a dele- unmagnetized cases (with maximum total pressure) considered terious or a beneficial effect on prebiotic chemistry that is depen- herein do yield robust upper bounds on the atmospheric ion dent on a complex and interconnected set of factors (55, 56).] escape rates. Apart from atmospheric loss, it is possible that out- Let us turn our attention to Table 1 for the seven Earth-sized gassing processes could very well replenish the atmosphere (50). exoplanets of the TRAPPIST-1 system. It is seen that the ion Hence, resolving the existence of an atmosphere over gigayear escape rate reduces as one moves outward. Hence, for similar timescales necessitates an in-depth understanding of the inter- multiplanetary systems around low-mass stars, it may be more play between source and loss mechanisms. Finally, it is impor- prudent to focus on the outward planet(s) in the HZ for detect- tant to note that stellar properties evolve over time, imply- ing atmospheres since their escape rates could be lower. Simi- ing that the escape rates are also likely to change accordingly. larly, when confronted with two planets with similar values of Rx In the case of M dwarfs such as TRAPPIST-1, the pre–main- and a (Eq. 2), we propose that searches should focus on stars sequence phase is particularly long and intense and expected to with lower mass-loss rates and stellar magnetic activity. Finally, have an adverse impact on atmospheric losses (7, 51). The ensu- our results and implications are also broadly applicable to future ing effect of extreme ultraviolet radiation on hydrodynamic and planetary systems detected around M and K dwarfs endowed ion escape rates during this phase has not been investigated in with similar features (11). this paper. To summarize, we have studied the atmospheric ion escape Bearing these limitations in mind, we now turn to a discus- rates from the seven planets of the TRAPPIST-1 system by sion of the implications. We have argued that TRAPPIST-1h and assuming a Venus-like composition. This was done by using TRAPPIST-1g represent the most promising candidates in terms numerical models to compute the properties of the stellar wind of retaining atmospheres over gigayear timescales. Instead, if and the escape rates, and the latter were shown to match the the atmosphere were to be depleted over O 108 y, this could analytical predictions. We demonstrated that the outer plan- prove to be problematic for the origin of life (abiogenesis) on ets of the TRAPPIST-1 system (most notably TRAPPIST-1h the planet although it must be acknowledged that the actual and TRAPPIST-1g) are capable of retaining their atmospheres timescale for abiogenesis on Earth and other planets remains over gigayear timescales. However, as many factors remain unre- unknown (52). Abiogenesis has been argued to be accompanied solved at this stage, future missions such as the James Webb by an increase in biological (e.g., genomic) complexity over time Space will play a crucial role in constraining the (53, 54), although this growth is not uniform and may be con- atmospheres of the TRAPPIST-1 planets (57). In particular, a tingent on environmental fluctuations. Hence, ceteris paribus, a recent study concluded that spectral features for six of the seven planet capable of sustaining a stable atmosphere over long time TRAPPIST-1 planets could be detected with <20 transits with periods (along with retaining a stable climate) might have a 5 σ accuracy (58). Such observations would help constrain the- greater chance of hosting complex surficial organisms. The outer oretical predictions, pave the way toward looking for biosigna- planets of the TRAPPIST-1 system may therefore lead to more tures, and empirically estimate the putative habitability of these diverse biospheres eventually. planets.

264 | www.pnas.org/cgi/doi/10.1073/pnas.1708010115 Dong et al. Downloaded by guest on September 29, 2021 Downloaded by guest on September 29, 2021 9 iaeinV,e l 21)Hwhsial r pc ete fetdhabitable affected weather space are hospitable How (2017) al. losses. et atmospheric VS, via worlds Airapetian water of 19. dehydration The (2017) al. et C, Dong 18. deter- escape that evidence The shoreline: cosmic The (2017) of DC study A Catling habitable? KJ, b Centauri Zahnle Proxima Is 16. (2017) O Cohen Y, Ma escape M, Atmospheric Lingam (2016) C, G Dong Wieser Stenberg 15. H, Nilsson for YJ, Implications Ma F, atmospheres: Bagenal DA, planetary Brain of evolution 14. habitable? and planet Origin a (2013) makes H Lammer What (2009) 13. al. et H, sys- TRAPPIST-1 Lammer the in 12. panspermia interplanetary Enhanced (2017) A Loeb M, Lingam ultracool nearby the 11. around planets terrestrial temperate Seven (2017) al. et M, Gillon 10. 0 acaSg ,Goe ,DaeJ,GoofG oe 21)O h antcpro- magnetic the On (2017) O Cohen G, Gronoff JJ, Drake A, Glocer K, Garcia-Sage 20. b. Centauri Proxima of weather space The (2016) O Cohen JJ, Drake C, Garraffo 17. 1 wnJ,AvrzM 21)U rvneaoaino ls-npaes Energy- planets: close-in of evaporation driven UV (2016) MA Alvarez JE, Owen 21. 8 oi ,e l 21)Lresaemgei oooiso aeMdwarfs. M late of topologies magnetic Large-scale (2010) TRAPPIST-1. al. in et chain resonant J, seven-planet of Morin A heating (2017) 28. al. et atmospheric R, Luger and structure 27. Magnetospheric (2014) al. habitable potentially et on O, fields Cohen magnetic dwarf 26. 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