About Exobiology: The Case for Dwarf K Stars
aa r X i v : . [ a s t r o - ph . S R ] J un About Exobiology: The Case for Dwarf K Stars
M. Cuntz and E. F. Guinan Department of Physics, University of Texas at Arlington,Arlington, TX 76019, USA [email protected] Department of Astrophysics and Planetary Science, Villanova University,Villanova, PA 19085, USA [email protected]
ABSTRACT
One of the most fundamental topics of exobiology concerns the identifica-tion of stars with environments consistent with life. Although it is believed thatmost types of main-sequence stars might be able to support life, particularlyextremophiles, special requirements appear to be necessary for the developmentand sustainability of advanced life forms. From our study, orange main-sequencestars, ranging from spectral type late-G to mid-K (with a maximum at early-K),are most promising. Our analysis considers a variety of aspects, including (1)the frequency of the various types of stars, (2) the speed of stellar evolution theirlifetimes, (3) the size of the stellar climatological habitable zones (CLI-HZs),(4) the strengths and persistence of their magnetic dynamo generated X-ray–UVemissions, and (5) the frequency and severity of flares, including superflares; both(4) and (5) greatly reduce the suitability of red dwarfs to host life-bearing plan-ets. The various phenomena show pronounced dependencies on the stellar keyparameters such as effective temperature and mass, permitting the assessmentof the astrobiological significance of various types of stars. Thus, we developed a“Habitable-Planetary-Real-Estate Parameter” (HabPREP) that provides a mea-sure for stars that are most suitable for planets with life. Early K stars are foundto have the highest HabPREP values, indicating they may be “Goldilocks” starsfor life-hosting planets. Red dwarfs are numerous, having long lifetimes, but theirnarrow CLI-HZs and hazards from magnetic activity make them less suitable forhosting exolife. Moreover, we provide X-ray–FUV irradiances for G0 V – M5 Vstars over a wide range of ages.
Subject headings: astrobiology – stars: activity – stars: late-type – stars: lumi-nosity function, mass function, X-ray–UV Irradiances – planetary systems 2 –
1. Introduction
Since the discovery of the first planet hosted by a Sun-like star was found more than20 years ago orbiting 51 Pegasi (Mayor & Queloz 1995), the search for life in the Universehas received unprecedented attention. It is now unequivocally the most fundamental taskof astrobiology, with significant ramifications toward other fields such as astronomy, astro-physics, microbiology, planetary and atmospheric science, and geodynamics. Generally, thesearch for life around stars, notably F–M type main-sequence stars, is mostly concentratedon two pivotal aspects. The first aspect deals with the identification of stars suitable to of-fer sustained habitable environments, whereas the second aspect focuses on planets throughelucidating which planetary properties make them (potentially) habitable. Previously, sig-nificant progress has been made on both aspects, including work by Scalo et al. (2007),Lammer et al. (2009, 2013), Horner & Jones (2010), Rugheimer et al. (2013), Kasting et al.(2014), among others.Regarding the stellar aspect, a central theme is the identification of the existence andstability of climatological habitable zones (CLI-HZs) aimed at rocky planets of differentsizes; see, e.g., Kaltenegger et al. (2012) for a updated classification scheme. Previous resultspertaining to single stars have been obtained by, e.g., Kasting et al. (1993), Kopparapu et al.(2013, 2014) and for higher-order (binary and multiple) systems by, e.g., Cuntz (2014, 2015).Furthermore, the impact of stellar evolution on the CLI-HZs has been explored as well (e.g.,Underwood et al. 2003; Rushby et al. 2013). These studies show that when stars evolve andincrease in luminosity with age, the CLI-HZs broaden and move outward. Recent work byKasting et al. (2014) considers updated CLI-HZ boundaries based on improved planetaryclimate models and discusses remote life-detection criteria. For example, for the case of theEarth, the expected increase in the luminosity of the Sun over the next ∼ ∼
100 Myr) Sun could have been higher by 30–50 times and 100–500times, respectively, than today. Generally, stellar activity is a crucial feature of all late-typestars and most pronounced in M dwarfs; it subsides as stars age. For solar-type and coolerstars (i.e., stars with outer convective zones), this process is attributable to stellar angularevolution owing to internal changes and to magnetized winds (e.g., Keppens et al. 1995; 3 –Charbonneau et al. 1997).The aim of this work is to consider detailed knowledge about stars including theirrelationships to planets to evaluate which types of stars constitute the best and most likelycandidates for the facilitation of long-term exobiology. In Sect. 2, we discuss the amounts ofhabitable planetary real estate, as determined by the sizes of the CLI-HZs and the relativefrequency of the various types of stars. In Sect. 3, we consider stellar activity, i.e., highenergetic stellar radiation, flares, superflares, and winds regarding their relevance to thecircumstellar environments. The overall assessment of habitability, i.e., “the big picture”, isconveyed in Sect. 4.
2. Habitable Planetary Real Estate Parameter (HabPREP)
In the following, for the different types of main-sequence stars, we focus on the sizeof the CLI-HZs and their relative frequency to evaluate amounts of habitable planetaryreal estate. Other aspects such as the impact of magnetic-dynamo driven stellar activitywill be considered in Sect. 3. Previously, Kasting et al. (1993) utilized 1-D climate models,which were state-of-the-art at the time, to compute the CLI-HZs for a domain of main-sequence stars with spectral-types ranging from late F to early M. The basic premise of theirwork was the assumption of an Earth-type planet with a CO /H O/N atmosphere and,moreover, that habitability requires the presence of liquid water on the planetary surface.This work was significantly improved through subsequent studies, including the recent workby Kopparapu et al. (2013, 2014). Their approach encompassed numerous improvements,including the consideration of revised H O and CO absorption coefficients.For example, the work by Kopparapu et al. (2013, 2014) re-computes the recent Venus/ early Mars (RVEM) limits of the CLI-HZ introduced by Kasting et al. (1993). For a solar-like star, the range of the RVEM extends from 0.75 to 1.77 au. Other limits regarding theCLI-HZs are based on the runaway greenhouse effect (inner limit) and maximum greenhouseeffect (outer limit); in the following, these limits are used to signify the general habitablezone (GHZ). At the inner limit, the greenhouse phenomenon is enhanced by water vapor,thus promoting surface warming, which increases the atmospheric water vapor content, thusfurther raising the planet’s surface temperature. Eventually, this will lead to the rapidevaporation of all surface water. For the outer limit, it is assumed that a cloud-free CO atmosphere shall still be able to provide a surface temperature of ∼
273 K (0 ◦ C).For solar-like stars, and assuming an Earth-mass object, Kopparapu et al. (2013, 2014)identified the GHZ limits as approximately 0.95 and 1.68 au. In contrast, Kasting et al. 4 –(1993) identified them as 0.84 and 1.67 au, respectively. Another limit of habitability isgiven as the moist greenhouse limit, which for stars akin to the Sun has been updated to0.99 au. In the previous work by Kasting et al. (1993) another limit was identified given bythe first CO condensation obtained by the onset of formation of CO clouds at a temperatureof 273 K, which has not been supported by Kopparapu et al. (2013, 2014). Nevertheless, inthe present work, the moist greenhouse limit and the limit due to the first CO condensationare used to define the conservative habitable zone (CHZ), as motivated by a large array ofprevious studies. Note that in the work by Kopparapu et al. (2013, 2014) the CLI-HZ givenby the RVEM limits is referred to as GHZ, whereas the CLI-HZ between 0.95 and 1.68 au,as identified for an Earth-mass planet, is referred to as CHZ. This notation is different fromthat of the present study, which however closely follows previous conventions (see Table 1).Results are given in Table 2. An important aspect is that compared to previouslypublished versions of that figure, updates have been made for stars of T eff ∼ < Kepler field planet–hosting candidates. We also evaluated the stability ofthe CLI-HZs for stars with spectral types from ∼ F5 V to ∼ M5 V (see Table 2). In thiscase, we explored when the inner limit of the CHZ / GHZ overtakes the outer limit with thelatter recorded at the beginning of stellar main-sequence evolution. This time of t ev , usuallyreferred to as timescale of the continuous habitable zone (in reference to CHZ or GHZ),describing the region (or duration of time) when a planet can be continuously habitable(i.e., able to maintain liquid water on its surface) is relatively short for early and mid F-typestars, see also Sato et al. (2014) for further results, but very prolonged for the more slowlyevolving, cooler, low-mass main-sequence K- and M-type stars. For example, for K2 V stars, t ev is identified as ∼
22 and ∼
32 Gyr for the CHZ and GHZ, respectively, which is more thana factor of 3 longer than for G2 V stars like the Sun. For low-mass M-type stars (i.e., reddwarfs; T eff ∼ < t ev is found to exceed 100 Gyr.Next, we focus on the relative frequency of stars, as obtained by the initial mass function(IMF); see Figure 1 and 2. In this regard, it is found that low-mass stars are much morefrequent than high-mass stars given by the shape of the IMF (e.g., Kroupa 2001, 2002;Chabrier 2003; Chabrier et al. 2005). The number of stars strongly increases with decreasingstellar mass (albeit uncertainties associated with the region of formation, stellar metallicity,etc.), although the IMF shows some flattening for stellar masses below ∼ M ⊙ (i.e., mid andlate M-dwarfs), if displayed against log M with M as stellar mass. The IMF typically follows abroken power law, which is an empirical function describing the distribution of initial massesfor a stellar population. More than 75% of stars are identified as M-dwarfs. Early work 5 –establishing the IMF has been given by, e.g., Muench et al. (2000), Lucas & Roche (2000),Hillenbrand & Carpenter (2000), and Luhman et al. (2000), which focused on distinct stellarclusters including clusters located in the Orion and Taurus constellation; some of this workalso employed the Two-Micron All Sky Survey (2MASS). Several decades ago, Miller & Scalo(1979) pointed out that the flatting of the IMF in the regime of very low mass stars maybe due to the mutual interactions between the fragments of interstellar clouds as well astheir interactions with the ambient gas rather than cloud fragmentation itself. Later on,this interpretation has been backed up by detailed numerical simulations given by, e.g.,Clark et al. (2008), which also allowed to link the variations of the initial IMF to the rateof star formation. Note that the general behavior of the IMF as obtained is also consistentwith the number count of stars in the solar neighborhood, i.e., the RECONS project (Henry2009; Henry & Jao 2015).Forming the product between the sizes of the CLI-HZs and the relative frequency of starsallows to describe the “Habitable-Planetary-Real-Estate Parameter” (HabPREP) for thedifferent types of main-sequence stars . The results reveal a maximum for stars with effectivetemperatures between about 4900 K and 5300 K (i.e., K2 V – G8 V stars) and a strongincrease for M dwarfs of effective temperatures less than about 4000 K. The overall behaviorof HabPREP is determined by two separate trends. First, the width of the CLI-HZs steadilydecreases as a function of decreasing effective temperature or mass. Second, the relativefrequency of stars according to, e.g., Kroupa (2002) and Chabrier (2003), and first increases,but then near 0.5 M ⊙ (with the exact value dependent on the set of observational data andthe particulars of the statistical approach) levels off or starts to decrease (if displayed againstlog M with M as stellar mass); both behaviors combined result in a maximum for HabPREPas said (see Figure 1). HabPREP is normalized to unity for G2 V stars, following the IMFobtained by Chabrier et al. (2005), based on the GHZ as choice for the CLI-HZ. However,the general behavior of the HabPREP function (e.g., position of its maximum) shows littledependence on the type of CLI-HZ selected; see Figure 2 for details. As discussed in Sect.3, M dwarfs (even though preferred by HabPREP) will be ruled less favorable for providinghabitable environments compared to orange dwarfs of spectral type late-G to mid-K owingto their excessively high amounts of activity. Strictly speaking, the definition of HabPREP should also consider the timescale of stability for thecontinuous habitable zone, t ev . However, the omission of this step will not affect our conclusions in a notablemanner. Stars between spectral type late-G and M have highly stable CLI-HZ (see Table 2), whereas F-typestars as well as early and mid-G stars have not. Thus considering the impact of t ev regarding HabPREPwould make the maximum between stellar types G8 V and K2 V even more pronounced.
3. Impact of High Energy X-ray–UV Radiation, (Super-)Flares and Winds
Next we discuss the impact of high energy radiation, flares, and winds on the prospect ofhabitability for HZ planets hosted by G- to M-type stars. There is a number of earlier studiesof solar-type stars on the influence of dynamo-generated energetic radiation encompassingvarious wavelength regimes, especially X-ray, extreme-UV (EUV) and far-UV (FUV) as wellas stars of different ages (e.g., G¨udel et al. 1997; Guinan & Ribas 2002; Guinan et al. 2003;Ribas et al. 2005). Previously, Ribas et al. (2005) presented results from the “Sun in Time”program indicating that the coronal–transition region X-ray–EUV ( ∼ ∼
100 (EUV) to ∼
600 (X-ray) times stronger than thoseof the present Sun. Similarly, the transition region and chromospheric FUV-UV emissionsof the young Sun were identified to be 20–60 and 10–20 times stronger, respectively, thanat present.Even though Earth is a striking counterexample as it was able to survive significantenergetic radiation and wind flows from the early Sun owing to magnetic protection (e.g.,Grießmeier et al. 2004), high levels of stellar activity are typically considered a significanthindrance to the development and sustainability of advanced exobiology. Thus, we shouldalso consider the adverse effects that the active young Sun had on the two other initialsolar-HZ planets. Because of high levels of magnetic activity of the early Sun, Venus lostits original water inventory very early and now is a very hot, dry, inhospitable planet (e.g.,Kulikov et al. 2006). Also, because of the Sun’s past high activity, coupled with the lossof its protective geomagnetic field some 3.5 Gyr ago, Mars today is too cold and dry forcomplex life at least on its surface (e.g., Fair´en et al. 2010). Thus, it could be consideredvery fortunate that the Earth’s atmosphere, water inventories and life survived, persisted,and evolved in spite of the harsh effects of the active early Sun as well as the devastatingeffects of impacts of asteroids and comets. Therefore, in the view of Fermi’s paradox, i.e.,no signs or signals of advanced life, see Chopra & Lineweaver (2016) for recent discussions,complex life could indeed be very rare.In this study we mostly focus on stellar coronal X-ray and chromospheric Ly- α fluxes ofG0 V - M5 V stars over a wide age range (see Figure 3). Ly- α serves as an excellent indicatorfor FUV emission because this emission line alone contributes 80–90% of the total stellar Based on studies by, e.g., Schr¨oder & Smith (2008) and Mowlavi et al. (2012) it has been found thatthe effective temperature of the young Sun was ∼ < α properties of M-stars over a widerange of ages has recently been carried out by Guinan et al. (2016). In this study the X-rayand FUV Ly- α fluxes, i.e., f X and f Ly α , were determined for a sample of M0-5 V stars withages from 0.1–11.5 Gyr for a nominal reference distance of 1.0 au (and also for 0.17 au, themid CLI-HZ for a ∼ M1 V star). For young M-stars (ages <
500 Myr) the X-ray and f Ly α CLI-HZ irradiances are both very high and comparable in strength. As noted in this study,the FUV Ly-alpha flux dominates the FUV flux, comprising 80–90% of the total 900–1800 ˚AFUV flux and can thus be used to estimate the total FUV irradiance. In the following, weextend the study of X-ray and FUV (given by Ly- α ) irradiances of M-stars as a function ofage, to more massive and luminous G and K stars.Note that the H I Ly- α ∼ α emission is the main contributor to the heating, ionization andphotochemistry of the upper atmosphere of the Earth as well as of many solar-system plan-ets and moons (Holland 1984); recent results have also been obtained for planets in exosolarsystems (e.g., Miguel et al. 2015). For Earth, the Ly- α flux plays a major role in the pho-todissociation of important molecules such as H O, CO , CH , O , and O in planetaryatmospheres. Fortunately, reliable measures of Ly- α emission fluxes have meanwhile becomeavailable, obtained with HST (see, e.g., France et al. 2013; Linsky et al. 2013). These stel-lar Ly- α integrated fluxes have been reconstructed from HST-STIS and COS spectra for asample ( ∼
50) of main-sequence F5 – M5 stars (Linsky et al. 2013; Guinan et al. 2016).Similar to the M-star study of Guinan et al. (2016), we utilize the Ly- α and X-rayflux measures of G and K stars obtained by Linsky et al. (2013). (Note that these authorsreport those fluxes for a reference distance of 1.0 au from the star but they can easily betransformed to any other distance following the inverse square law.) These X-ray fluxes weresupplemented by additional X-ray measures found in the literature, previously obtained withROSAT, XMM-Newton or Chandra. Ages were estimated from open cluster (such as Pleiadesand Hyades) or moving group memberships (such as the Ursae Majoris Moving Group),stellar rotation rates (through employing rotation–age relations) or from memberships inwide binary systems (such as Proxima Centauri) in which one component has an isochronalor astereoseismic age measures (e.g., α Cen). X-ray fluxes are available for a large number ofstars from all-sky X-ray surveys like ROSAT as well as from more recent X-ray observationsby Chandra and XMM-Newton. Mean milestone age baseline X-ray flux–age calibrationsof G and K stars were obtained from ROSAT studies of Pleiades cluster (age ∼ ∼ , also referred to as homeothermic distance (HTD), which for M0-6 dwarfs rangesbetween ∼ ∼ L X – age relations, which for G and M dwarfshave been obtained by G¨udel (2007) and Guinan et al. (2016), respectively. In addition,X-ray and FUV (Ly- α ) flux calibrations were made for K-stars in similar manner as wasdone for M-stars. For intermediate stellar types adequate interpolation is used. Data forstellar activity at ages of ∼ α Cen A & B, Proxima Cen,and 40 Eri A & C. As shown in Figure 3 and 4, our analysis shows that for low-mass stars(especially stars cooler than ∼ M3 V), the amounts of planetary X-ray and FUV irradianceat HTDs are drastically increased due higher X-ray–FUV emissions of young stars and thecloseness of the HTDs (i.e., < . α ) fluxes at the stellar HTDs, they can also be applied to any otherdistance following the inverse square law. Since the Ly- α emission flux contributes 80–90%of the total FUV flux, multiplying the Ly- α flux by ∼ α HTD fluxes are com-parable for young stars (age < .
65 Gyr) of all spectral types. At an age of 0.1 Gyr, theX-ray HTD fluxes are ∼ × higher than the corresponding HTD Ly- α fluxes. However,by an age of about 5 Gyr, the Ly- α HTD fluxes are approximately 10–30 times larger thanthe corresponding X-ray HTD fluxes across all spectral types. Thus, we find that for olderG–M stars, the Ly- α HTD fluxes dominate the X-ray–FUV spectral region. It is also seenthat the largest changes with stellar age occur for the X-ray HTD fluxes. For example, forsolar-type G2–8 V stars, the HTD X-ray flux is ∼ × higher for very young (0.1 Gyr) starsrelative to older stars at ages of 5 Gyr.The HTD X-ray and Ly- α fluxes are very high for stars cooler than about M3 V com-pared to G and K stars of corresponding ages (see Figure 3). For example, compared to a Earth-equivalent distances mostly depend on the stellar luminosity but the stellar effective temperatureneeds to be considered as well. For example, K and M-dwarfs have more emission in the near-IR comparedto G-dwarfs, and a planet can absorb more near-IR radiation from water-vapor around K and M-dwarfsmaking them warmer at an Earth-equivalent distance if no effective temperature correction for that distanceis applied. Previously, corrective formulae were given by Underwood et al. (2003) and Selsis et al. (2007)that are used by us, including recasting in response to the studies of Kopparapu et al. (2013, 2014). T eff ∼ × higher. This is primarily due to the very small values of HTDs for these low-luminositystars with HTD < .
15 au as well as their more efficient magnetic dynamos due to their deepconvective zones. To illustrate this, a HTD planet hosted by a solar-age ( ∼ ∼ × and ∼ × more X-ray andFUV radiation, respectively, than the Earth presently receives from the Sun. As shown inTable 2, at younger ages ( ∼ ∼ × and ∼ × higher, respec-tively, at 0.1 Gyr compared to ages of ∼ ∼
350 and ∼ E > ergs (see Maehara et al. 2012). Generally, ithas been found that flares are most intense and frequent in young stars and stars of latespectral types, notably M-dwarfs. Aside from the flare properties (i.e., strength, spectralenergy distribution, frequency, stochasticity) effects on Earth-type planets in CLI-HZs areheavily determined by the CLI-HZ’s proximity, i.e., the very small values for the HTDsfor those stars. Thus, detailed studies about the impact of flares on possible exolife entailoutcomes that are unfavorable or mixed at best (e.g., Segura et al. 2010; Kasting et al. 2014).Superflares as given by Kepler data are found for stars of spectral types of G to M. Forexample, Candelaresi et al. (2014) reported that from the more than 100,000 stars includedthe study, 380 show superflares with a total of 1690 such events. With decreasing effectivetemperature, an increase in the superflare rate is observed, which is consistent to previousfindings, and in alignment to dynamo theory. They also conclude that the resulting statisticsof the dissipation energy is similar to the observed flare statistics as a function of the inverseRossby number. From the perspective of this study, superflares are expected to be mostsignificant for Earth-equivalent planets in the consideration of their close proximity to thestars, i.e., small HTDs. The effects of flares on habitability of planets is complex and beyondthe scope of this paper; it will be considered by us in a subsequent paper.Dense stellar winds (as present in active young stars) are also expected of having anadverse impact on circumstellar habitability (Johnstone et al. 2015). Lammer et al. (2003)showed that the combination of high X-ray–UV irradiance and strong (more dense) stellarwinds expected from young G, K, and M dwarfs can, via ion-pickup mechanisms, act synergi-cally to strip away atmospheres of close-in planets (e.g., Vidal-Madjar et al. 2003). Unless ahosted CLI-HZ planet has a strong and persistent magnetic field, amounting to a protectivemagnetosphere, there is a significant possibility that the planet will lose most, if not all, 10 –of its atmosphere including its water inventories (e.g., Grießmeier et al. 2004). In our solarsystem, the early loss of water on Venus (Kulikov et al. 2006) and ∼ κ Ceti (G5 V), a young ( ∼ ∼
50 times the present Sun. This result agrees very wellwith recent wind density estimates of solar-type stars with age by Airapetian & Usmanov(2016). Since stellar winds scale to magnetic fields and activity (e.g., Schrijver et al. 2003;Preusse et al. 2005; Cranmer & Saar 2011), it can be assumed that winds are most dense inyoung, magnetically active stars and planets at close-in HTDs (as given for late-K and Mdwarfs) are most affected.
4. The Big Picture
The aim of this work is a multi-facet attempt to identify types of stars consistent with thedurable existence of life, notwithstanding extremophiles, for which according to terrestrialanalyses (e.g., Rothschild & Mancinelli 2001) large windows of opportunities might exist.General stellar aspects motivate us to focus on the main-sequence rather than pre- or post-main-sequence scenarios, which are typically of highly transitory nature. Aspects of primaryimportance include the frequency of the various types of stars, the size of the stellar CLI-HZs,and the rapidness of stellar evolution for various types of main-sequence stars. Followingprevious work by, e.g., Kopparapu et al. (2013, 2014) that indicates the decrease in thesize of the CLI-HZs with decreasing stellar mass, luminosity and effective temperature –irrespectively of the planetary climatological criteria for defining the CLI-HZ limits – tendto favor higher-mass stars. That is F–G-type stars have wide CLI-HZ limits whereas lowermass mid-K and M-type stars have narrow CLI-HZs located close to the star. Low-mass stars,however, are much more frequent than high-mass stars as given by the shape of the IMF(e.g., Kroupa 2001, 2002; Chabrier et al. 2005). The total number of stars greatly increaseswith decreasing stellar mass, although there is an onset of flattening or modest decrease formasses below ∼ M ⊙ . Hence, more than 75% of stars are identified as M-type dwarfs.This result is also consistent with the number count of stars in the solar neighborhood, i.e.,the RECONS project (Henry 2009; Henry & Jao 2015).Based on the combined behaviors of the CLI-HZs and the IMF, we are able to con-clude that, from a statistical perspective, orange dwarf stars, ranging from spectral type 11 –late-G to mid-K are most promising regarding long-term exobiology. More massive stars( M ∼ > . M ⊙ ), such as F-type stars, are known to rapidly evolve away from the main-sequence (e.g., Meynet et al. 1993; Mowlavi et al. 2012), a significant disadvantage for theevolution and sustainment of exolife, though limited opportunities for circumstellar hab-itability may still exist (e.g., Cockell 1999; Sato et al. 2014). Regarding M-type dwarfs,numerous studies have been pursued about the prospects of providing habitable environ-ments (e.g., Segura et al. 2005; Lammer 2007; Tarter et al. 2007; Scalo et al. 2007; Lissauer2007; Kasting et al. 2014). Generally, these studies convey multiple adverse aspects regard-ing supporting habitable environments, including (but not limited to) intense high-energyradiative emissions, strong stellar flares, the narrowness of the CLI-HZ, and possible geody-namic planetary insufficiencies (e.g., lack of volatiles).To date, there has been a large array of research targeting both flares and energeticradiation for stars of different ages and spectral types. Examples of flare studies includework by, e.g., Hawley et al. (2003), Robinson et al. (2005), and Davenport et al. (2012).Generally, it has been found that flares are most intense and frequent in young stars andstars of late spectral types, notably M-dwarfs (e.g., Feigelson et al. 2007). Aside from theflare properties (i.e., strength, spectral energy distribution, frequency, stochasticity) effectson Earth-type planets in CLI-HZs are heavily determined by the CLI-HZ’s proximity, i.e.,the small values ( < . Kepler data arefound for stars of spectral types of G to M, but again are expected to have most adverseimpacts for planets of M-dwarfs. A large number of superflares have also been detected forsolar-type stars (e.g., Maehara et al. 2012, 2015; Shibayama et al. 2013; Katsova & Livshits2015). Surprisingly, superflares may have favorable ramifications for possible exolife aroundG-type (and perhaps early K-type) stars (Airapetian et al. 2016), as they might trigger theproduction of hydrogen cyanide (HCN), an essential molecule of prebiotic chemistry.In the present work, we focus on the X-ray and FUV Ly- α irradiances for planets inthe CLI-HZ, notably located at the HTD for stars of different effective temperatures, lu-minosities, and ages. For most stars the X-ray fluxes dominate the EUV fluxes and theLy- α emission fluxes dominate in the FUV region and thus are both critical (see Table 3and Figure 3). Ultimately, high levels of energetic radiation lead to significant planetaryatmospheric evaporation, as discussed by, e.g., Lammer et al. (2003), Vidal-Madjar et al.(2003), and Penz & Micela (2008). This behavior entails adverse consequences for the hab-itable environments of late-K and M dwarfs (see Figure 4), which is in part also caused thevery close proximity of the CLI-HZs. Additionally, as argued by Raymond et al. (2007),there is a decreased probability of habitable planet formation around those stars. It has 12 –also been pointed out that accreting planets, if formed, subsequently located in the CLI-HZaround stars cooler than K5 (including the full range of M-dwarfs) are most likely subjectedto runaway greenhouse processes, and thus may lose substantial amounts of water initiallydelivered to them (Ramirez & Kaltenegger 2014; Luger & Barnes 2015; Tian & Ida 2015),thus emerging as (almost certainly) uninhabitable dry planets (“desert worlds”). Partic-ularly, Tian & Ida (2015) argued that Earth-mass planets with Earth-like water contentsorbiting M-dwarfs have a 10–100 times reduced likelihood than around G dwarfs.The results for red dwarfs, as discussed, add further exobiological relevance in supportof late-G to mid-K orange dwarfs (i.e., T eff between approximately 4900 K and 5300 K)as more likely hosts for planets supporting complex life. Another intriguing aspect thatalso tends to support our main conclusion is the onset of tidal locking, which for planetslocated in the CLI-HZ (both CHZ and GHZ), pertaining to a timescale of ∼ ± ∼ K3 V stars). Thisindicates that for planets hosted by stars hotter and more luminous, due to their moredistant CLI-HZs, tidal locking is relatively unlikely (although it may still occur at a laterevolutionary time), whereas for planets around cooler stars, tidal locking is expected tohave happened – even though it should be pointed out that tidal locking by itself does notnecessarily exclude habitability (e.g., Barnes et al. 2008), although its absence leads to morethermally balanced planetary climates. However, tidal-locking reduces the rotation periodof the planet and thus possibly reduces the planet’s protective geomagnetic field. Without arobust magnetic field, ion-pick up mechanisms, combined with strong X-ray–UV radiation,strong winds, and flares, could strip the planet of its atmosphere (e.g., Lammer et al. 2008).On the other hand, Driscoll & Barnes (2015) showed that tidally-locked terrestrial planetscould have their geomagnetic fields sustained (or even enhanced) from the tidal heating oftheir iron-nickel cores.Another reason why K-dwarfs are expected to provide habitable environments stemsfrom recent planetary geodynamic studies. Haqq-Misra et al. (2016) investigated the im-pact of limit cycles on the width of stellar habitable zones. Limit cycles mean that plan-ets positioned in the CLI-HZs cannot maintain stable, warm climates, but rather shouldoscillate between long, globally glaciated states and shorter periods of climatic warmth.Haqq-Misra et al. (2016) argue that such conditions, similar to those experienced on Earth(“Snowball Earth”) would be disadvantageous to the development of complex life, includingintelligent life. Thus, limit cycles reduce the usable extent of CLI-HZs as they compromisehabitability for planets near the CLI-HZs’ outer rim. The authors point out that for planetsaround K and M dwarfs, limit cycles should not occur, thus allowing to foster habitableenvironments based on this criterion. However, Haqq-Misra et al. (2016) view M-dwarfs asless suitable for exolife in part based on the same reasoning as conveyed in this study. 13 –Clearly, the search for planets around different types of stars continues at an unprece-dented pace. A fairly recent example includes the detection of a possible Earth-mass planetby Dumusque et al. (2012) orbiting α Cen B (K1 V), a member of the closest stellar systemto the Sun, albeit this planet orbits very close to the star is not located within the CLI-HZ. Furthermore, five possible super-Earth planets have been reported to orbit the nearbyG8.5 V star τ Ceti (Tuomi et al. 2013). Two of these large Earth-size planet candidates (i.e., τ Ceti e and f) orbit near the inner and outer boundaries of this old star’s continuously hab-itable zone (Pagano et al. 2015) and are thus potentially habitable. Terrestrial-size planetshave also been reported for several other nearby orange dwarfs as well as from the
Kepler
Mission . Finding habitable planets around low-mass K–M stars continues to be a challeng-ing task due to the impacts of the stellar radiative environments, intense plasma fluxes, andthe narrowness of the CLI-HZs. Additional planets are expected from the extended Kepler
Mission (K2), continuing and new high precision spectroscopic radial velocity studies, as wellas from the
Transiting Exoplanet Survey Satellite (TESS) and Gaia.However, it is noteworthy that recent statistical analyses of
Kepler
Mission exoplanetdata indicate that a significant fraction (i.e., 15% – 25%) of red dwarfs is expected to hostterrestrial-size planets within their CLI-HZs (e.g., Bonfils et al. 2013; Dressing & Charbonneau2013, 2015). This implies that tens of billions Earth-size planets in the Milky Way alonehosted by M-dwarfs could have a chance of being potentially habitable, even though poten-tial life forms would most likely constitute extremophiles by terrestrial standards. Possiblescenarios include that these potentially habitable planets could have escaped the strong X-ray–UV radiation of their host stars by forming beyond the stellar CLI-HZs. Thereafter,they could have migrated into the CLI-HZs when their host stars had become older and lessactive. Another possibility is that the planets could have lost their original atmospheres andwater inventories at a much later time (ages > < . For further information, see Planetary Habitability Laboratory, http://phl.upr.edu.
14 –per, the close-in GHZ planets hosted by late-K and M dwarfs experience at least an order ofmagnitude higher levels ionizing radiation and far more extended periods than the Earth. Tomake matters worse, there is a greater likelihood of large flares from young stars. The levelsof the resulting ionizing X-ray–UV radiation and energetic plasmas from flares could erode oreliminate the planet’s atmosphere and water inventories, thus greatly reducing the suitabilityof the planet for sustained life. These effects as well as other factors (including biologicaland chemical bottlenecks, asteroid bombardments, among others), as recently discussed byChopra & Lineweaver (2016) and references therein, could greatly diminish the likelihood ofdevelopment of multicellular complex life. The lack of evidence of technologically advancedcivilizations such as radio signals and other modes of one-way communication are referredto as the Fermi’s Paradox, which continues to remain unresolved (see Chopra & Lineweaver2016). As discussed in this paper, decisive impediments (roadblocks) to life could be at-tributable to high energy radiation, strong stellar winds and flares experienced by planetsduring the first billion years after formation.This research is supported by the NSF and NASA through grants NSF/RUI-1009903,HST-GO-13020.001-A and Chandra Award GO2-13020X to Villanova University (E. F. G.).We are very grateful for this support. Furthermore, it has been supported in part by NASAthrough the American Astronomical Society’s Small Research Grant Program as well as theSETI Institute (M. C.). We also wish to acknowledge the availability of HST data throughthe MAST website hosted by the Space Telescope Science Institute and the Chandra X-ray data through the HEASARC website hosted by the Astrophysics Science Division atNASA/GSFC and the High Energy Astrophysics Division of the Smithsonian AstrophysicsObservatory (SAO). Furthermore, we would like to thank an anonymous referee for her/hisuseful suggestions allowing us to improve the manuscript. This research made use of publicdatabases hosted by SIMBAD, and maintained by CDS, Strasbourg, France. Moreover, wewish to acknowledge assistance by Zhaopeng Wang with computer graphics.Facilities: ROSAT, XMM-Newton, Chandra, HST (COS),
Kepler
15 –
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20 –
CLI−HZ and IMF
Temperature (K) H ab i t ab l e Z one L i m i t s ( A U ) I M F Kro01Cha05
Fig. 1.— Inner and outer limits of the CLI-HZ, represented by the GHZ (black lines) with itsextent depicted as grayish area. The HTDs are depicted as well (dashed line). Additionally,we show the behavior of the IMF (normalized to unity for 1 M ⊙ ) as given by Kroupa (2001)(red line) and Chabrier et al. (2005) (blue line). Also note that IMF does not flatten near0.5 M ⊙ if displayed against the stellar effective temperature, though it does if displayedagainst log M (with M as stellar mass), the most customary approach. 21 – Temperature (K) H ab P R EP Habitable Domains
Sal55Kro01 Cha05
Fig. 2.— HabPREP values depicted as function of the stellar effective temperature withthe IMF chosen after Kroupa (2001) (red) and Chabrier et al. (2005) (blue). Results aregiven regarding the GHZ (solid lines), RVEM (dashed lines, top), and the CHZ (dashedlines, bottom). For comparison and historic reasons, the HabPREP values for the GHZ havealso been given regarding the IMF of Salpeter (1955) (dotted line). Note that the overallbehavior of HabPREP shows little dependence on the type of the CLI-HZ, i.e., RVEM, GHZ,or CHZ. 22 –
G2−8 K0−8 M0−3 M4−6
Stellar Spectral Types l og f L y α , l og f x Lyman− α and X−ray Irradiance −0.50.00.51.01.52.02.53.03.54.04.5 Fig. 3.— Lyman- α and X-ray irradiances (in ergs s − cm − ) for different sets of main-sequencestars regarding planets located at HTDs. Stellar ages of about 0.1, 0.65, 1.0, 5.0, and 10 Gyrare represented by the colors red , purple , green , blue , and brown , respectively. Large-widthbars indicate the results for Lyman- α , whereas small-width bars (with slightly lesser brightcolors) convey the results for the X-ray irradiances. Note that the y -axis uses logarithmicunits, allowing us to display the drastic increase of both f Ly α and f X between G dwarfs andlate M dwarfs. For the various G, K and M spectral type bins, the X-ray and FUV Ly- α HTD fluxes all increase with decreasing stellar age. The increase of the HTD X-ray fluxfrom 0.1 to 5.0 Gyr is up to 500 times, whereas for the same age range, the increase in FUVLy- α HTD fluxes is much less (see also Table 3). The latter is typically ∼ α HTD fluxes for starsof similar ages from G2–8 dwarfs to M4–6 dwarfs increase over 100–500 times. Thus HTDplanets hosted by an M4–6 dwarfs (due to the very small HTDs) have extremely high levelsof X-ray and FUV irradiances, particularly when young. 23 – ev (CHZ)3 5 10 20 30t ev (GHZ) Temperature (K) H ab P R EP Overview
Kro01Cha05 RECONSRapidRotators TidallyLocked X−RAYS X − r a y I rr ad i an c e Fig. 4.— Depiction of various functions and quantities, indicating the significance (suit-ability) of late-G ( ∼ G8 V) to mid-K ( ∼ K5 V) type stars (i.e., “dwarf orange stars”) forexobiology. The normalized habitable real estate, defined on the left y -axis of the plot bythe product of the initial mass function and the HZ extent (IMF × HZ) is displayed versusthe stellar effective temperature. As shown, the amount of habitable planetary real estate,HabPREP, with the IMF given by Kroupa (2001) and Chabrier et al. (2005) as well as basedon the results from the RECONS project (see text), forms an aggregate maximum at thetemperature range for orange dwarfs. We also convey the values of t ev (in Gyr) for the CHZand GHZ, describing the timescales of stability for the continuous habitable zone (with databeyond 30 Gyr disregarded). For a tidal locking timescale of 4.5 Gyr, we show the domainfor the onset of tidal locking for planets located in the GHZs (grayish area). The dividingline (green) separates tidal locking and fast rotations for planets at HTDs. Moreover, in theform of a histogram (magenta) defined on the right y -axis of the plot, we show domains ofpossible exobiological exclusion owing to high levels of X-ray irradiance. We report distinctfactors of enhancements in f X (650 Myr) for planets at HTDs relative to a solar-type G2 Vstar of that age. For example, the values for X-ray irradiance are found to be about 2 × , 5 × ,and 10 × higher for stars of T eff ≃ ∼ K8 V), 3570 K ( ∼ M2 V), and 3390 K ( ∼ M3 V),respectively. Factors larger than 100 are identified for stars of T eff < ∼ M5 V). Forthe latter, the CLI-HZ is very close ( < .
05 au) to the host star. 24 –Table 1. Habitable Zone Limits and DefinitionsDescription Models Definitions... Kas93 Kop1314 Kas93 Kop1314 This workRecent Venus 0.75 0.75 RVEM optimistic RVEMRunaway greenhouse effect 0.84 0.95 GHZ conservative GHZMoist greenhouse effect 0.95 0.99 CHZ ... CHZ ∗ First CO condensation 1.37 ... CHZ ... CHZ ∗ Maximum greenhouse effect 1.67 1.68 GHZ conservative GHZEarly Mars 1.77 1.77 RVEM optimistic RVEMNote. — Considering that the “first CO condensation” limit is not supported bythe work of Kopparapu et al. (2013, 2014) reduces the relevance of the CHZ, hencelabelled as ( ∗ ). Nevertheless, we still convey this limit to allow comparisons withprevious work. Kopparapu et al. (2013) use the terms “optimistic” and “conservative”limits; however, in several previous studies those limits were identified as the GHZ andCHZ limits, respectively, rather than the RVEM and GHZ limits. 25 –Table 2. CLI-HZ Evolutionary Time Scales and LimitsSp. Type T eff t ev CLI-HZ Limits... ... CHZ GHZ CHZ GHZ RVEM... (K) (Gyr) (Gyr) (au) (au) (au)F0 7100 1.4 2.1 2.32 – 2.87 2.16 – 3.72 1.70 – 3.93F2 6830 1.8 2.4 1.97 – 2.51 1.85 – 3.20 1.46 – 3.37F5 6530 2.3 3.1 1.72 – 2.24 1.62 – 2.82 1.28 – 2.97F8 6150 2.8 5.0 1.33 – 1.78 1.26 – 2.21 1.00 – 2.33G0 5940 3.7 7.5 1.17 – 1.58 1.11 – 1.96 0.88 – 2.06G2 5780 4.8 9.9 0.99 – 1.36 0.95 – 1.68 0.75 – 1.77G5 5670 7.8 13.9 0.91 – 1.26 0.87 – 1.55 0.69 – 1.63G8 5460 11.8 19.2 0.82 – 1.14 0.79 – 1.40 0.62 – 1.48K0 5250 16.0 24.8 0.69 – 0.98 0.67 – 1.20 0.53 – 1.27K2 5050 21.9 32.0 0.59 – 0.84 0.57 – 1.02 0.45 – 1.08K5 4410 30.4 41.4 0.39 – 0.58 0.38 – 0.70 0.30 – 0.74K8 4000 > >
50 0.27 – 0.41 0.26 – 0.49 0.21 – 0.52M0 3800 > >
100 0.22 – 0.34 0.21 – 0.41 0.17 – 0.43 26 –Table 3. Planetary Lyman- α and X-ray Irradiance for G, K and M Dwarfs T eff HTD 0.1 Gyr 0.65 Gyr 1.0 Gyr 5.0 Gyr 10 Gyr(K) (au) (ergs s − cm − ) (ergs s − cm − ) (ergs s − cm − ) (ergs s − cm − ) (ergs s − cm − )... ... f Ly α f X f Ly α f X f Ly α f X f Ly α f X f Ly α f X ≡≡