Exoplanet Habitability Seager, Sara
Science (American Association for the Advancement of Science),
05/2013, Volume:
340, Issue:
6132
Journal Article
Peer reviewed
The search for exoplanets includes the promise to eventually find and identify habitable worlds. The thousands of known exoplanets and planet candidates are extremely diverse in terms of their masses ...or sizes, orbits, and host star type. The diversity extends to new kinds of planets, which are very common yet have no solar system counterparts. Even with the requirement that a planet's surface temperature must be compatible with liquid water (because all life on Earth requires liquid water), a new emerging view is that planets very different from Earth may have the right conditions for life. The broadened possibilities will increase the future chances of discovering an inhabited world.
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The discovery and characterization of exoplanets have the potential to offer the world one of the most impactful findings ever in the history of astronomy—the identification of life beyond Earth. ...Life can be inferred by the presence of atmospheric biosignature gases—gases produced by life that can accumulate to detectable levels in an exoplanet atmosphere. Detection will be made by remote sensing by sophisticated space telescopes. The conviction that biosignature gases will actually be detected in the future is moderated by lessons learned from the dozens of exoplanet atmospheres studied in last decade, namely the difficulty in robustly identifying molecules, the possible interference of clouds, and the permanent limitations from a spectrum of spatially unresolved and globally mixed gases without direct surface observations. The vision for the path to assess the presence of life beyond Earth is being established.
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Some super Earths and mini Neptunes will likely have thick atmospheres that are not H sub(2)-dominated. We have developed a photochemistry-thermochemistry kinetic-transport model for exploring the ...compositions of thick atmospheres on super Earths and mini Neptunes, applicable for both H sub(2)-dominated atmospheres and non-H sub(2)- dominated atmospheres. Using this model to study thick atmospheres for wide ranges of temperatures and elemental abundances, we classify them into hydrogen-rich atmospheres, water-rich atmospheres, oxygen-rich atmospheres, and hydrocarbon-rich atmospheres. We find that carbon has to be in the form of CO sub(2) rather than CH sub(4) or CO in a H sub(2)-depleted water-dominated thick atmosphere and that the preferred loss of light elements from an oxygen-poor carbon-rich atmosphere leads to the formation of unsaturated hydrocarbons (C sub(2)H sub(2) and C sub(2)H sub(4)). We apply our selfconsistent atmosphere models to compute spectra and diagnostic features for known transiting low-mass exoplanets GJ 1214 b,HD97658 b, and 55 Cnc e. For GJ 1214 b, we find that (1) C sub(2)H sub(2) features at 1.0 and 1.5 mu min transmission and C sub(2)H sub(2) and C sub(2)H sub(4) features at 9-14 mu m in thermal emission are diagnostic for hydrocarbon-rich atmospheres; (2) a detection of water-vapor features and a confirmation of the nonexistence of methane features would provide sufficient evidence for a water-dominated atmosphere. In general, our simulations show that chemical stability has to be taken into account when interpreting the spectrum of a super Earth/mini Neptune. Water-dominated atmospheres only exist for carbon to oxygen ratios much lower than the solar ratio, suggesting that this kind of atmospheres could be rare.
Determination of an exoplanet's mass is a key to understanding its basic properties, including its potential for supporting life. To date, mass constraints for exoplanets are predominantly based on ...radial velocity (RV) measurements, which are not suited for planets with low masses, large semimajor axes, or those orbiting faint or active stars. Here, we present a method to extract an exoplanet's mass solely from its transmission spectrum. We find good agreement between the mass retrieved for the hot Jupiter HD 189733b from transmission spectroscopy with that from RV measurements. Our method will be able to retrieve the masses of Earth-sized and super-Earth planets using data from future space telescopes that were initially designed for atmospheric characterization.
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Exoplanet Atmospheres Seager, Sara; Demi, Drake
Annual review of astronomy and astrophysics,
01/2010, Volume:
48, Issue:
1
Journal Article
Peer reviewed
Open access
At the dawn of the first discovery of exoplanets orbiting Sun-like stars in the mid-1990s, few believed that observations of exoplanet atmospheres would ever be possible. After the 2002 Hubble Space ...Telescope detection of a transiting exoplanet atmosphere, many skeptics discounted it as a one-object, one-method success. Nevertheless, the field is now firmly established, with over two dozen exoplanet atmospheres observed today. Hot Jupiters are the type of exoplanet currently most amenable to study. Highlights include: detection of molecular spectral features, observation of day-night temperature gradients, and constraints on vertical atmospheric structure. Atmospheres of giant planets far from their host stars are also being studied with direct imaging. The ultimate exoplanet goal is to answer the enigmatic and ancient question, "Are we alone?" via detection of atmospheric biosignatures. Two exciting prospects are the immediate focus on transiting super Earths orbiting in the habitable zone of M-dwarfs, and ultimately the spaceborne direct imaging of true Earth analogs. PUBLICATION ABSTRACT
We present a comprehensive photochemistry model for exploration of the chemical composition of terrestrial exoplanet atmospheres. We validate the model by computing the atmospheric composition of ...current Earth and Mars and find agreement with observations of major trace gases in Earth's and Mars' atmospheres. We simulate several plausible atmospheric scenarios of terrestrial exoplanets and choose three benchmark cases for atmospheres from reducing to oxidizing. The most interesting finding is that atomic hydrogen is always a more abundant reactive radical than the hydroxyl radical in anoxic atmospheres. Whether atomic hydrogen is the most important removal path for a molecule of interest also depends on the relevant reaction rates. The atmospheric scenarios presented in this paper can serve as the benchmark atmospheres for quickly assessing the lifetime of trace gases in reducing, weakly oxidizing, and highly oxidizing atmospheres on terrestrial exoplanets for the exploration of possible biosignature gases.
Hot super-Earths are exoplanets with short orbital periods (<10 days), heated by their host stars to temperatures high enough for their rocky surfaces to become molten. A few hot super-Earths exhibit ...high geometric albedos (>0.4) in the Kepler band (420-900 nm). We are motivated to determine whether reflection from molten lava and quenched glasses (a product of rapidly cooled lava) on the surfaces of hot super-Earths contribute to the observationally inferred high geometric albedos. We experimentally measure reflection from rough- and smooth-textured quenched glasses of both basalt and feldspar melts. For lava reflectance values, we use specular reflectance values of molten silicates from non-crystalline solids literature. Integrating the empirical glass reflectance function and non-crystalline solids reflectance values over the dayside surface of the exoplanet at secondary eclipse yields an upper limit for the albedo of a lava-quenched glass planet surface of ∼0.1. We conclude that lava planets with solid (quenched glass) or liquid (lava) surfaces have low albedos. The high albedos of some hot super-Earths are most likely explained by atmospheres with reflective clouds (or, for a narrow range of parameter space, possibly Ca/Al oxide melt surfaces). Lava planet candidates in TESS data can be identified for follow-up observations and future characterization.
Only about 100 natural products are known to contain a nitrogen–sulfur (N–S) bond. This review thoroughly categorizes N–S bond-containing compounds by structural class. Information on biological ...source, biological activity, and biosynthesis is included, if known. We also review the role of N–S bond functional groups as post-translational modifications of amino acids in proteins and peptides, emphasizing their role in the metabolism of the cell.
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Over the past twenty years, astronomers have identified hundreds of extrasolar planets--planets orbiting stars other than the sun. Recent research in this burgeoning field has made it possible to ...observe and measure the atmospheres of these exoplanets. This is the first textbook to describe the basic physical processes--including radiative transfer, molecular absorption, and chemical processes--common to all planetary atmospheres, as well as the transit, eclipse, and thermal phase variation observations that are unique to exoplanets. In each chapter, Sara Seager offers a conceptual introduction, examples that combine the relevant physics equations with real data, and exercises. Topics range from foundational knowledge, such as the origin of atmospheric composition and planetary spectra, to more advanced concepts, such as solutions to the radiative transfer equation, polarization, and molecular and condensate opacities. Since planets vary widely in their atmospheric properties, Seager emphasizes the major physical processes that govern all planetary atmospheres. Moving from first principles to cutting-edge research, Exoplanet Atmospheres is an ideal resource for students and researchers in astronomy and earth sciences, one that will help prepare them for the next generation of planetary science. * The first textbook to describe exoplanet atmospheres * Illustrates concepts using examples grounded in real data * Provides a step-by-step guide to understanding the structure and emergent spectrum of a planetary atmosphere * Includes exercises for students
Abstract Exoplanet atmosphere transmission spectroscopy for planets orbiting M dwarf stars faces significant challenges due to contamination from stellar magnetic features, i.e., spots and faculae. ...These features make the stellar surface inhomogeneous and introduce wavelength-dependent signals in the transmission spectrum that complicate its analysis. We identify and explain why using observations at infrared wavelengths greater than a few microns partially mitigates stellar contamination. At these wavelengths the intensity sensitivity to temperature weakens, with two significant consequences. First, the contribution of spots and faculae has a diminished effect because their flux contrast to the quiet-star regions lessens. Second, the star’s spectral features compress in magnitude, an outcome of spectral features being shaped by the star’s photospheric vertical temperature gradient. Both factors are due to the Planck function moving from exponential to linear in temperature toward mid-infrared (mid-IR) wavelengths (the “Rayleigh–Jeans tail”). In contrast to stellar spectra, the depth of the transmission spectroscopy features does not fundamentally vary with wavelength as it is primarily determined by the planet’s atmospheric scale height. The magnitude of reduction in stellar contamination is a factor of a few to several at mid-IR versus near-IR wavelengths, but whether or not this is enough to bypass stellar contamination ultimately depends on the spot coverage area. Nonetheless, the flattening of thermal emission spectral features at IR wavelengths is universal.
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