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Register a new userSensitivity of Biosignatures on Earth-like Planets orbiting in the Habitable Zone of Cool M-Dwarf Stars to varying Stellar UV Radiation and Surface Biomass Emissions
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We find that variations in the UV emissions of cool M-dwarf stars have a potentially large impact upon atmospheric biosignatures in simulations of Earth-like exoplanets i.e. planets with Earths development, and biomass and a molecular nitrogen-oxygen dominated atmosphere. Starting with an assumed black-body stellar emission for an M7 class dwarf star, the stellar UV irradiation was increased stepwise and the resulting climate-photochemical response of the planetary atmosphere was calculated. Results suggest a Goldilocks effect with respect to the spectral detection of ozone. At weak UV levels, the ozone column was weak (due to weaker production from the Chapman mechanism) hence its spectral detection was challenging. At strong UV levels, ozone formation is stronger but its associated stratospheric heating leads to a weakening in temperature gradients between the stratosphere and troposphere, which results in weakened spectral bands. Also, increased UV levels can lead to enhanced abundances of hydrogen oxides which oppose the ozone formation effect. At intermediate UV (i.e. with x10 the stellar UV radiative flux of black body Planck curves corresponding to spectral class M7) the conditions are just right for spectral detection. Results suggest that the planetary O3 profile is sensitive to the UV output of the star from about(200-350) nm. We also investigated the effect of increasing the top-of-atmosphere incoming Lyman-alpha radiation but this had only a minimal effect on the biosignatures since it was efficiently absorbed in the uppermost planetary atmospheric layer, mainly by abundant methane. Earlier studies have suggested that the planetary methane is an important stratospheric heater which critically affects the vertical temperature gradient, hence the strength of spectral emission bands.
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We investigate atmospheric responses of modeled hypothetical Earth-like planets in the habitable zone of the M-dwarf AD Leonis to reduced oxygen (O2), removed biomass (dead Earth), varying carbon dioxide (CO2) and surface relative humidity (sRH). Results suggest large O2 differences between the reduced O2 and dead scenarios in the lower but not the upper atmosphere. Ozone (O3) and nitrous oxide (N2O) also show this behavior. Methane depends on hydroxyl (OH), its main sink. Abiotic production of N2O occurs in the upper layers. Chloromethane (CH3Cl) decreases everywhere on decreasing biomass. Changing CO2 (from x1 to x100 present atmospheric level (PAL)) and surface relative humidity (sRH) (from 0.1 percent to 100 percent) does not influence CH3Cl as much as lowering biomass. Therefore, CH3Cl can be considered a good biosignature. Changing sRH and CO2 has a greater influence on temperature than O2 and biomass alone. Changing the biomass produces ~6 kilometer (km) in effective height (H) in transmission compared with changing CO2 and sRH ( about 25km). In transmission O2 is discernible at 0.76 microns for greater than 0.1 PAL. The O3 9.6 micron band was weak for the low O2 runs and difficult to discern from dead Earth, however O3 at 0.3 microns could serve as an indicator to distinguish between reduced O2 and dead Earth. Spectral features of N2O and CH3Cl corresponded to some km H. CH4 could be detectable tens of parsecs away with ELT except for the 10-4 and 10-6 PAL O2 scenarios. O2 is barely detectable for the 1 PAL O2 case and unfeasible at lower abundances.
Understanding whether M-dwarf stars may host habitable planets with Earth-like atmospheres and biospheres is a major goal in exoplanet research. If such planets exist, the question remains as to whether they could be identified via spectral signatures of biomarkers. Such planets may be exposed to extreme intensities of cosmic rays that could perturb their atmospheric photochemistry. Here, we consider stellar activity of M-dwarfs ranging from quiet up to strong flaring conditions and investigate one particular effect upon biomarkers, namely, the ability of secondary electrons caused by stellar cosmic rays to break up atmospheric molecular nitrogen (N2), which leads to production of nitrogen oxides in the planetary atmosphere, hence affecting biomarkers such as ozone. We apply a stationary model, that is, without a time-dependence, hence we are calculating the limiting case where the atmospheric chemistry response time of the biomarkers is assumed to be slow and remains constant compared with rapid forcing by the impinging stellar flares. This point should be further explored in future work with time-dependent models. For the flaring case O3 is mainly destroyed via direct titration with nitrogen oxides and not via the familiar catalytic cycle photochemistry, which occurs on Earth. For scenarios with low O3, Rayleigh scattering by the main atmospheric gases became more important for shielding the planetary surface from ultra-violet radiation. A major result of this work is that the biomarker O3 survived all the stellar-activity scenarios considered except for the strong case, whereas the biomarker nitrous oxide could survive in the planetary atmosphere under all conditions of stellar activity considered here, which clearly has important implications for missions that aim to detect spectroscopic biomarkers.
We study the magnetospheric structure and the ionospheric Joule Heating of planets orbiting M-dwarf stars in the habitable zone using a set of magnetohydrodynamic (MHD) models. The stellar wind solution is used to drive a model for the planetary magnetosphere, which is coupled with a model for the planetary ionosphere. Our simulations reveal that the space environment around close-in habitable planets is extreme, and the stellar wind plasma conditions change from sub- to super-Alfvenic along the planetary orbit. As a result, the magnetospheric structure changes dramatically with a bow shock forming in the super-Alfvenic sectors, while no bow shock forms in the sub-Alfvenic sectors. The planets reside most of the time in the sub-Alfvenic sectors with poor atmospheric protection. A significant amount of Joule Heating is provided at the top of the atmosphere as a result of the planetary interaction with the stellar wind. For the steady-state solution, the heating is about 0.1-3% of the total incoming stellar irradiation, and it is enhanced by 50% for the time-dependent case. The significant Joule Heating obtained here should be considered in models for the atmospheres of habitable planets in terms of the thickness of the atmosphere, the top-side temperature and density, the boundary conditions for the atmospheric pressure, and particle radiation and transport.
Seven temperate Earth-sized exoplanets readily amenable for atmospheric studies transit the nearby ultracool dwarf star TRAPPIST-1 (refs 1,2). Their atmospheric regime is unknown and could range from extended primordial hydrogen-dominated to depleted atmospheres (refs 3-6). Hydrogen in particular is a powerful greenhouse gas that may prevent the habitability of inner planets while enabling the habitability of outer ones (refs 6-8). An atmosphere largely dominated by hydrogen, if cloud-free, should yield prominent spectroscopic signatures in the near-infrared detectable during transits. Observations of the innermost planets have ruled out such signatures (ref 9). However, the outermost planets are more likely to have sustained such a Neptune-like atmosphere (refs 10,11). Here, we report observations for the four planets within or near the systems habitable zone, the circumstellar region where liquid water could exist on a planetary surface (refs 12-14). These planets do not exhibit prominent spectroscopic signatures at near-infrared wavelengths either, which rules out cloud-free hydrogen-dominated atmospheres for TRAPPIST-1 d, e and f, with significance of 8, 6 and 4 sigma, respectively. Such an atmosphere is instead not excluded for planet g. As high-altitude clouds and hazes are not expected in hydrogen-dominated atmospheres around planets with such insolation (refs 15,16), these observations further support their terrestrial and potentially habitable nature.
The M dwarf Gliese 581 is believed to host four planets, including one (GJ 581d) near the habitable zone that could possibly support liquid water on its surface if it is a rocky planet. The detection of another habitable-zone planet--GJ 581g--is disputed, as its significance depends on the eccentricity assumed for d. Analyzing stellar activity using the H-alpha line, we measure a stellar rotation period of 130+/-2 days and a correlation for H-alpha modulation with radial velocity. Correcting for activity greatly diminishes the signal of GJ 581d (to 1.5 sigma), while significantly boosting the signals of the other known super-Earth planets. GJ 581d does not exist, but is an artifact of stellar activity which, when incompletely corrected, causes the false detection of planet g.
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