Atmospheric mass loss by stellar wind from planets around main sequence M stars

We present an analytic model for the interaction between planetary atmospheres and stellar winds from main sequence M stars, with the purpose of obtaining a quick test-model that estimates the timescale for total atmospheric mass loss due to this interaction. Planets in the habitable zone of M dwarfs may be tidally locked and may have weak magnetic fields, because of this we consider the extreme case of planets with no magnetic field. The model gives the planetary atmosphere mass loss rate as a function of the stellar wind and planetary properties (mass, atmospheric pressure and orbital distance) and an entrainment efficiency coefficient $\alpha$. We use a mixing layer model to explore two different cases: a time-independent stellar mass loss and a stellar mass loss rate that decreases with time. For both cases we consider planetary masses within the range of $1\to10$ M$_{\oplus}$ and atmospheric pressures with values of 1, 5 and 10 atm. For the time dependent case, planets without magnetic field in the habitable zone of M dwarfs with initial stellar mass losses of $\leq \dot{M}_{w} < 10^{-11}$ M$_{\odot}$ yr$^{-1}$, may retain their atmospheres for at least 1 Gyr. This case may be applied to early spectral type M dwarfs (earlier than M5). Studies have shown that late type M dwarfs (later than M5) may be active for long periods of time ($\geq 4$Gyr), and because of that our model with constant stellar mass loss rate may be more accurate. For these stars most planets may have lost their atmospheres in 1 Gyr or less because most of the late type M dwarfs are expected to be active. We emphasize that our model only considers planets without magnetic fields. Clearly we must expect a higher resistance to atmospheric erosion if we include the presence of a magnetic field.Comment: Icarus, submitted. 18 pages, 6 figure

Biomarker Response to Galactic Cosmic Ray-Induced NOx and the Methane Greenhouse Effect in the Atmosphere of an Earthlike Planet Orbiting an M-Dwarf Star

Planets orbiting in the habitable zone (HZ) of M-Dwarf stars are subject to high levels of galactic cosmic rays (GCRs) which produce nitrogen oxides in earthlike atmospheres. We investigate to what extent this NOx may modify biomarker compounds such as ozone (O3) and nitrous oxide (N2O), as well as related compounds such as water (H2O) (essential for life) and methane (CH4) (which has both abiotic and biotic sources) . Our model results suggest that such signals are robust, changing in the M-star world atmospheric column by up to 20% due to the GCR NOx effects compared to an M-star run without GCR effects and can therefore survive at least the effects of galactic cosmic rays. We have not however investigated stellar cosmic rays here. CH4 levels are about 10 times higher than on the Earth related to a lowering in hydroxyl (OH) in response to changes in UV. The increase is less than reported in previous studies. This difference arose partly because we used different biogenic input. For example, we employed 23% lower CH4 fluxes compared to those studies. Unlike on the Earth, relatively modest changes in these fluxes can lead to larger changes in the concentrations of biomarker and related species on the M-star world. We calculate a CH4 greenhouse heating effect of up to 4K. O3 photochemistry in terms of the smog mechanism and the catalytic loss cycles on the M-star world differs considerably compared with the Earth

Characterizing the Habitable Zones of Exoplanetary Systems with a Large Ultraviolet/Visible/Near-IR Space Observatory

Understanding the surface and atmospheric conditions of Earth-size, rocky planets in the habitable zones (HZs) of low-mass stars is currently one of the greatest astronomical endeavors. Knowledge of the planetary effective surface temperature alone is insufficient to accurately interpret biosignature gases when they are observed in the coming decades. The UV stellar spectrum drives and regulates the upper atmospheric heating and chemistry on Earth-like planets, is critical to the definition and interpretation of biosignature gases, and may even produce false-positives in our search for biologic activity. This white paper briefly describes the scientific motivation for panchromatic observations of exoplanetary systems as a whole (star and planet), argues that a future NASA UV/Vis/near-IR space observatory is well-suited to carry out this work, and describes technology development goals that can be achieved in the next decade to support the development of a UV/Vis/near-IR flagship mission in the 2020s.Comment: Submitted in response to NASA call for white papers: "Large Astrophysics Missions to Be Studied by NASA Prior to the 2020 Decadal Survey

The MUSCLES Treasury Survey. IV. : Scaling relations for ultraviolet, Ca II K, and energetic particle fluxes from M dwarfs

Characterizing the UV spectral energy distribution (SED) of an exoplanet host star is critically important for assessing its planet's potential habitability, particularly for M dwarfs, as they are prime targets for current and near-term exoplanet characterization efforts and atmospheric models predict that their UV radiation can produce photochemistry on habitable zone planets different from that on Earth. To derive ground-based proxies for UV emission for use when Hubble Space Telescope (HST) observations are unavailable, we have assembled a sample of 15 early to mid-M dwarfs observed by HST and compared their nonsimultaneous UV and optical spectra. We find that the equivalent width of the chromospheric Ca ii K line at 3933 Å, when corrected for spectral type, can be used to estimate the stellar surface flux in ultraviolet emission lines, including H i Lyα. In addition, we address another potential driver of habitability: energetic particle fluxes associated with flares. We present a new technique for estimating soft X-ray and >10 MeV proton flux during far-UV emission line flares (Si iv and He ii) by assuming solar-like energy partitions. We analyze several flares from the M4 dwarf GJ 876 observed with HST and Chandra as part of the MUSCLES Treasury Survey and find that habitable zone planets orbiting GJ 876 are impacted by large Carrington-like flares with peak soft X-ray fluxes ≥10−3 W m−2 and possible proton fluxes ~102–103 pfu, approximately four orders of magnitude more frequently than modern-day Earth.Publisher PDFPeer reviewe

Changes in atmospheric chemistry on earthlike exoplanets across the habitable zone of main sequence stars

We have used a coupled radiative-convective photochemical column model to calculate changes in atmospheric biomarkers on a planet having Earth's composition which we situated at the inner, mid, and outer Habitable Zone (HZ) for a solar, F2V and K2V star. The HZ was defined conservatively to be suitable for humans i.e. 0°C &lt; Tsurface &gt; 30°C which led to the model calculating a narrow HZ width of (0.96-1.09) Astronomical Units (AU) for the solar-type star, (1.57-1.81) AU for the F2V star and (0.51-0.58) AU for the K2V star. Despite the narrowness of the HZ the biomarkers H2O, CH4 and CH3Cl varied by large amounts i.e. by factors 8, 11 and 10 respectively in the column on moving outwards across the solar HZ, for example. Whereas H2O decreased moving outwards across the HZ due to enhanced condensation in the troposphere, CH4 and CH3Cl increased associated with a slowing in H2O+O1D --&lt; 2OH, hence less OH, an important sink for these two compounds. Ozone changes were smaller, around a 5-10% increase across the HZ. A source-sink analysis suggested the important process was a slowing in the O3+hv sink. We also considered changes in species which impact ozone – the so-called family species (and their reservoirs) which can catalytically destroy ozone. HCl, for example is a chlorine reservoir (storage) molecule, which increased by a factor 26 in the mid-stratosphere (32km) on moving outwards over the solar HZ. For the F2V and K2V stars, similar sources and sinks dominated the chemical biomarker budget as for the solar case and columns trends were comparable across the HZ. Ratios of biomarkers are easier to detect than are absolute concentrations. Our results imply that ratios such as (O3/H2O) and (CH4/H2O) can vary by large amounts and still be consistent with habitability, even for a conservatively-defined HZ

Carbon Monoxide and the Potential for Prebiotic Chemistry on Habitable Planets Around Main Sequence M Stars

Lifeless planets with CO2 atmospheres produce CO by CO2 photolysis. On planets around M dwarfs, CO is a long-lived atmospheric compound, as long as UV emission due to the stars chromospheric activity lasts, and the sink of CO and O2 in seawater is small compared to its atmospheric production. Atmospheres containing reduced compounds, like CO, may undergo further energetic and chemical processing to give rise to organic compounds of potential importance for the origin of life. We calculated the yield of organic compounds from CO2-rich atmospheres of planets orbiting M dwarf stars, which were previously simulated by Domagal- Goldman et al. (2014) and Harman et al. (2015), by cosmic rays and lightning using results of experiments by Miyakawaet al. (2002) and Schlesinger and Miller (1983a, 1983b). Stellar protons from active stars may be important energy sources for abiotic synthesis and increase production rates of biological compounds by at least 2 orders of magnitude compared to cosmic rays. Simple compounds such as HCN and H2CO are more readily synthesized than more complex ones, such as amino acids and uracil (considered here as an example), resulting in higher yields for the former and lower yields for the latter. Electric discharges are most efficient when a reducing atmosphere is present. Nonetheless, atmospheres with high quantities of CO2 are capable of producing higher amounts of prebiotic compounds, given that CO is constantly produced in the atmosphere. Our results further support planetary systems around M dwarf stars as candidates for supporting life or its origin

The Response of Atmospheric Chemistry on Earthlike Planets around F, G, and K stars to Small Variations in Orbital Distance

One of the prime goals of future investigations of extrasolar planets is to search for life as we know it. The Earth's biosphere is adapted to current conditions. How would the atmospheric chemistry of the Earth respond if we moved it to different orbital distances or changed its host star? This question is central to astrobiology and aids our understanding of how the atmospheres of terrestrial planets develop. To help address this question, we have performed a sensitivity study using a coupled radiative-convective photochemical column model to calculate changes in atmospheric chemistry on a planet having Earth's atmospheric composition, which we subjected to small changes in orbital position, of the order of 5-10% for a solar-type G2V, F2V, and K2V star. We then applied a chemical source-sink analysis to the biomarkers in order to understand how chemical processes affect biomarker concentrations. We start with the composition of the present Earth, since this is the only example we know for which a spectrum of biomarker molecules has been measured. We then investigate the response of the biomarkers to changes in the input stellar flux. Computing the thermal profile for atmospheres rich in H2O, CO2 and CH4 is a major challenge for current radiative schemes, due, among other things, to lacking spectroscopic data. Therefore, as a first step, we employ a more moderate approach, by investigating small shifts in planet-star distance and assuming an earthlike biosphere. To calculate this shift we assumed a criteria for complex life based on the Earth, i.e. the earthlike planetary surface temperature varied between 0 deg C‹Tsurface ‹30 deg C, which led to a narrow HZ width of (0.94-1.08) astronomical units (AU) for the solar-type G2V star (1.55-1.78) AU for the F2V star, and (0.50-0.58) AU for the K2V star. In our runs we maintained the concentration of atmospheric CO2 at its present-day level. In reality, the CO2 cycle (not presently included in our model) would likely lead to atmospheric CO2 stabilising at higher levels than considered in our runs near our quoted "outer" boundaries. The biomarkers H2O, CH4 and CH3Cl varied by factors 0.08, 17, and 16, respectively in the total column densities on moving outwards for the solar case. Whereas H2O decreased moving outwards due to cooling hence enhanced condensation in the troposphere, CH4 and CH3Cl increased associated with a slowing in H2O+O1D->2OH, hence less OH, an important sink for these two compounds. Ozone changes were smaller, around a 10% increase on moving outwards partly because cooler temperatures led to a slowing in the reaction between O3 and O1D. We also considered changes in species which impact ozone – the so-called family species (and their reservoirs), which can catalytically destroy ozone. Hydrochloric acid (HCl), for example, is a chlorine reservoir (storage) molecule, which increased by a factor 64 in the mid-stratosphere (32 km) on moving outwards for the solar case. For the F2V and K2V stars, similar sources and sinks dominated the chemical biomarker budget as for the solar case and column trends were comparable

M stars as targets for terrestrial exoplanet searches and biosignature detection

The changing view of planets orbiting low mass stars, M stars, as potentially hospitable worlds for life and its remote detection was motivated by several factors, including the demonstration of viable atmospheres and oceans on tidally locked planets, normal incidence of dust disks, including debris disks, detection of planets with masses in the 5–20 M range, and predictions of unusually strong spectral biosignatures. We present a critical discussion of M star properties that are relevant for the long- and short-term thermal, dynamical, geological, and environmental stability of conventional liquid water habitable zone (HZ) M star planets, and the advantages and disadvantages of M stars as targets in searches for terrestrial HZ planets using various detection techniques. Biological viability seems supported by unmatched very long-term stability conferred by tidal locking, small HZ size, an apparent shortfall of gas giant planet perturbers, immunity to large astrosphere compressions, and several other factors, assuming incidence and evolutionary rate of life benefit from lack of variability. Tectonic regulation of climate and dynamo generation of a protective magnetic field, especially for a planet in synchronous rotation, are important unresolved questions that must await improved geodynamic models, though they both probably impose constraints on the planet mass. M star HZ terrestrial planets must survive a number of early trials in order to enjoy their many Gyr of stability. Their formation may be jeopardized by an insufficient initial disk supply of solids, resulting in the formation of objects too small and/or dry for habitability. The small empirical gas giant fraction for M stars reduces the risk of formation suppression or orbit disruption from either migrating or nonmigrating giant planets, but effects of perturbations from lower mass planets in these systems are uncertain. During the first 1 Gyr, atmospheric retention is at peril because of intense and frequent stellar flares and sporadic energetic particle events, and impact erosion, both enhanced, the former dramatically, for M star HZ semimajor axes. Loss of atmosphere by interactions with energetic particles is likely unless the planetary magnetic moment is sufficiently large. For the smallest stellar masses a period of high planetary surface temperature, while the parent star approaches the main sequence, must be endured. The formation and retention of a thick atmosphere and a strong magnetic field as buffers for a sufficiently massive planet emerge as prerequisites for an M star planet to enter a long period of stability with its habitability intact. However, the star will then be subjected to short-term fluctuations with consequences including frequent unpredictable variation in atmospheric chemistry and surficial radiation field. After a review of evidence concerning disks and planets associated with M stars, we evaluate M stars as targets for future HZ planet search programs. Strong advantages of M stars for most approaches to HZ detection are offset by their faintness, leading to severe constraints due to accessible sample size, stellar crowding (transits), or angular size of the HZ (direct imaging). Gravitational lensing is unlikely to detect HZ M star planets because the HZ size decreases with mass faster than the Einstein ring size to which the method is sensitive. M star Earth-twin planets are predicted to exhibit surprisingly strong bands of nitrous oxide, methyl chloride, and methane, and work on signatures for other climate categories is summarized. The rest of the paper is devoted to an examination of evidence and implications of the unusual radiation and particle environments for atmospheric chemistry and surface radiation doses, and is summarized in the Synopsis. We conclude that attempts at remote sensing of biosignatures and nonbiological markers from M star planets are important, not as tests of any quantitative theories or rational arguments, but instead because they offer an inspection of the residues from a Gyr-long biochemistry experiment in the presence of extreme environmental fluctuations. A detection or repeated nondetections could provide a unique opportunity to partially answer a fundamental and recurrent question about the relation between stability and complexity, one that is not addressed by remote detection from a planet orbiting a solar-like star, and can only be studied on Earth using restricted microbial systems in serial evolution experiments or in artificial life simulations. This proposal requires a planet that has retained its atmosphere and a water supply. The discussion given here suggests that observations of M star exoplanets can decide this latter question with only slight modifications to plans alreadyin place for direct imaging terrestrial exoplanet missions