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.
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.
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.
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.
Low-albedo Surfaces of Lava Worlds Essack, Zahra; Seager, Sara; Pajusalu, Mihkel
The Astrophysical journal,
08/2020, Volume:
898, Issue:
2
Journal Article
Peer reviewed
Open access
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.
We explore the minimum distance from a host star where an exoplanet could potentially be habitable in order not to discard close-in rocky exoplanets for follow-up observations. We find that the inner ...edge of the Habitable Zone for hot desert worlds can be as close as 0.38 AU around a solar-like star, if the greenhouse effect is reduced (~1% relative humidity) and the surface albedo is increased. We consider a wide range of atmospheric and planetary parameters such as the mixing ratios of greenhouse gases (water vapor and CO sub(2)), surface albedo, pressure, and gravity. Intermediate surface pressure (~1-10 bars) is necessary to limit water loss and to simultaneously sustain an active water cycle. We additionally find that the water loss timescale is influenced by the atmospheric CO sub(2) level, because it indirectly influences the stratospheric water mixing ratio. If the CO sub(2) mixing ratio of dry planets at the inner edge is smaller than 10 super(-4), the water loss timescale is ~1 billion years, which is considered here too short for life to evolve. We also show that the expected transmission spectra of hot desert worlds are similar to an Earth-like planet. Therefore, an instrument designed to identify biosignature gases in an Earth-like atmosphere can also identify similarly abundant gases in the atmospheres of dry planets. Our inner edge limit is closer to the host star than previous estimates. As a consequence, the occurrence rate of potentially habitable planets is larger than previously thought.
The atmosphere of Venus remains mysterious, with many outstanding chemical connundra. These include the unexpected presence of ∼10 ppm O
in the cloud layers, an unknown composition of large particles ...in the lower cloud layers, and hard to explain measured vertical abundance profiles of SO
and H
O. We propose a hypothesis for the chemistry in the clouds that largely addresses all of the above anomalies. We include ammonia (NH
), a key component that has been tentatively detected both by the Venera 8 and Pioneer Venus probes. NH
dissolves in some of the sulfuric acid cloud droplets, effectively neutralizing the acid and trapping dissolved SO
as ammonium sulfite salts. This trapping of SO
in the clouds, together with the release of SO
below the clouds as the droplets settle out to higher temperatures, explains the vertical SO
abundance anomaly. A consequence of the presence of NH
is that some Venus cloud droplets must be semisolid ammonium salt slurries, with a pH of ∼1, which matches Earth acidophile environments, rather than concentrated sulfuric acid. The source of NH
is unknown but could involve biological production; if so, then the most energy-efficient NH
-producing reaction also creates O
explaining the detection of O
in the cloud layers. Our model therefore predicts that the clouds are more habitable than previously thought, and may be inhabited. Unlike prior atmospheric models, ours does not require forced chemical constraints to match the data. Our hypothesis, guided by existing observations, can be tested by new Venus in situ measurements.
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