SPECULOOS exoplanet search and its prototype on TRAPPIST

SPECULOOS exoplanet search and its prototype on TRAPPIST

Authors:

Burdanov et al

Abstract:

One of the most significant goals of modern science is establishing whether life exists around other suns. The most direct path towards its achievement is the detection and atmospheric characterization of terrestrial exoplanets with potentially habitable surface conditions. The nearest ultracool dwarfs (UCDs), i.e. very-low-mass stars and brown dwarfs with effective temperatures lower than 2700 K, represent a unique opportunity to reach this goal within the next decade. The potential of the transit method for detecting potentially habitable Earth-sized planets around these objects is drastically increased compared to Earth-Sun analogs. Furthermore, only a terrestrial planet transiting a nearby UCD would be amenable for a thorough atmospheric characterization, including the search for possible biosignatures, with near-future facilities such as the James Webb Space Telescope. In this chapter, we first describe the physical properties of UCDs as well as the unique potential they offer for the detection of potentially habitable Earth-sized planets suitable for atmospheric characterization. Then, we present the SPECULOOS ground-based transit survey, that will search for Earth-sized planets transiting the nearest UCDs, as well as its prototype survey on the TRAPPIST telescopes. We conclude by discussing the prospects offered by the recent detection by this prototype survey of a system of seven temperate Earth-sized planets transiting a nearby UCD, TRAPPIST-1.

Space Technology for Directly Imaging and Characterizing Exo-Earths

Space Technology for Directly Imaging and Characterizing Exo-Earths

Authors:

Crill et al

Abstract:

The detection of Earth-like exoplanets in the habitable zone of their stars, and their spectroscopic characterization in a search for biosignatures, requires starlight suppression that exceeds the current best ground-based performance by orders of magnitude. The required planet/star brightness ratio of order 1e-10 at visible wavelengths can be obtained by blocking stellar photons with an occulter, either externally (a starshade) or internally (a coronagraph) to the telescope system, and managing diffracted starlight, so as to directly image the exoplanet in reected starlight. Coronagraph instruments require advancement in telescope aperture (either monolithic or segmented), aperture obscurations (obscured by secondary mirror and its support struts), and wavefront error sensitivity (e.g. line-of-sight jitter, telescope vibration, polarization). The starshade, which has never been used in a science application, benefits a mission by being decoupled from the telescope, allowing a loosening of telescope stability requirements. In doing so, it transfers the difficult technology from the telescope system to a large deployable structure (tens of meters to greater than 100 m in diameter) that must be positioned precisely at a distance of tens of thousands of kilometers from the telescope. We describe in this paper a roadmap to achieving the technological capability to search for biosignatures on an Earth-like exoplanet from a future space telescope. Two of these studies, HabEx and LUVOIR, include the direct imaging of Earth-sized habitable exoplanets as a central science theme.

The O2 A-Band in the Fluxes and Polarization of Starlight Reflected by Earth-Like Exoplanets

The O2 A-Band in the Fluxes and Polarization of Starlight Reflected by Earth-Like Exoplanets


Authors:

Fauchez et al

Abstract:

Earth-like, potentially habitable exoplanets are prime targets in the search for extraterrestrial life. Information about their atmospheres and surfaces can be derived by analyzing the light of the parent star reflected by the planet. We investigate the influence of the surface albedo A s, the optical thickness b cloud, the altitude of water clouds, and the mixing ratio of biosignature O2 on the strength of the O2 A-band (around 760 nm) in the flux and polarization spectra of starlight reflected by Earth-like exoplanets. Our computations for horizontally homogeneous planets show that small mixing ratios (η < 0.4) will yield moderately deep bands in flux and moderate-to-small band strengths in polarization, and that clouds will usually decrease the band depth in flux and the band strength in polarization. However, cloud influence will be strongly dependent on properties such as optical thickness, top altitude, particle phase, coverage fraction, and horizontal distribution. Depending on the surface albedo and cloud properties, different O2 mixing ratios η can give similar absorption-band depths in flux and band strengths in polarization, especially if the clouds have moderate-to-high optical thicknesses. Measuring both the flux and the polarization is essential to reduce the degeneracies, although it will not solve them, especially not for horizontally inhomogeneous planets. Observations at a wide range of phase angles and with a high temporal resolution could help to derive cloud properties and, once those are known, the mixing ratio of O2 or any other absorbing gas.

Exoplanet Biosignatures: Observational Prospects

Exoplanet Biosignatures: Observational Prospects

Authors:

Fujii et al

Abstract:

We provide an overview of the prospects for biosignature detection and general characterization of temperate Earth-sized planets. We review planned space-based missions and ground-based projects as well as the basic methods they will employ, and summarize which exoplanet properties will become observable as these new facilities come on line. The observational strategies depend on whether the planets are transiting or not as well as on the spectral type of the host star. There is a reasonable expectation that the first constraints on spectroscopic features of atmospheres will be obtained before 2030. Successful initial characterization of a few nearby targets will be an important touchstone toward a more detailed scrutiny and/or a larger survey to address statistical questions such as the occurrence rate of habitable environments. The broad outlook which this paper presents may help develop a framework to evaluate the possibility of biospheres based on the observables, and consider new methodologies to characterize exoplanets of astrobiological interest.

Exoplanet Biosignatures: A Framework for Their Assessment

Exoplanet Biosignatures: A Framework for Their Assessment

Authors:

Catling et al

Abstract:

Finding life on exoplanets from telescopic observations is the ultimate goal of exoplanet science. Life produces gases and other substances, such as pigments, which can have distinct spectral or photometric signatures. Whether or not life is found in future data must be expressed with probabilities, requiring a framework for biosignature assessment. We present such a framework, which advocates using biogeochemical "Exo-Earth System" models to simulate potentially biogenic spectral or photometric data. Given actual observations, these simulations are then used to find the Bayesian likelihoods of those data occurring for scenarios with and without life. The latter includes "false positives" where abiotic sources mimic biosignatures. Prior knowledge of factors influencing inhabitance, including previous observations, is combined with the likelihoods to give the probability of life existing on a given exoplanet. Four components of observation and analysis are used. 1) Characterization of stellar (e.g., age and spectrum) and exoplanetary system properties, including "external" exoplanet parameters (e.g., mass and radius) to determine its suitability for life. 2) Characterization of "internal" exoplanet parameters (e.g., climate) to evaluate whether an exoplanet surface can host life. 3) Assessment of potential biosignatures through environmental context (components 1-2) and corroborating evidence. 4) Exclusion of false positives. The resulting Bayesian probabilities of life detection map to five confidence levels, ranging from "very likely" to "very unlikely" inhabited.

Understanding Oxygen as a Biosignature in the Context of Its Environment

Exoplanet Biosignatures: Understanding Oxygen as a Biosignature in the Context of Its Environment

Authors:

Meadows et al

Abstract:
Here we review how environmental context can be used to interpret whether O2 is a biosignature in extrasolar planetary observations. This paper builds on the overview of current biosignature research discussed in Schwieterman et al. (2017), and provides an in-depth, interdisciplinary example of biosignature identification and observation that serves as a basis for the development of the general framework for biosignature assessment described in Catling et al., (2017). O2 is a potentially strong biosignature that was originally thought to be an unambiguous indicator for life at high-abundance. We describe the coevolution of life with the early Earth's environment, and how the interplay of sources and sinks in the planetary environment may have resulted in suppression of O2 release into the atmosphere for several billion years, a false negative for biologically generated O2. False positives may also be possible, with recent research showing potential mechanisms in exoplanet environments that may generate relatively high abundances of atmospheric O2 without a biosphere being present. These studies suggest that planetary characteristics that may enhance false negatives should be considered when selecting targets for biosignature searches. Similarly our ability to interpret O2 observed in an exoplanetary atmosphere is also crucially dependent on environmental context to rule out false positive mechanisms. We describe future photometric, spectroscopic and time-dependent observations of O2 and the planetary environment that could increase our confidence that any observed O2 is a biosignature, and help discriminate it from potential false positives. By observing and understanding O2 in its planetary context we can increase our confidence in the remote detection of life, and provide a model for biosignature development for other proposed biosignatures.

Is Kepler-452b Habitable?

Quantitative estimates of the surface habitability of Kepler-452b

Authors:

Silva et al

Abstract:
Kepler-452b is currently the best example of an Earth-size planet in the habitable zone of a sun-like star, a type of planet whose number of detections is expected to increase in the future. Searching for biosignatures in the supposedly thin atmospheres of these planets is a challenging goal that requires a careful selection of the targets. Under the assumption of a rocky-dominated nature for Kepler-452b, we considered it as a test case to calculate a temperature-dependent habitability index, h050, designed to maximize the potential presence of biosignature-producing activity (Silva et al.\ 2016). The surface temperature has been computed for a broad range of climate factors using a climate model designed for terrestrial-type exoplanets (Vladilo et al.\ 2015). After fixing the planetary data according to the experimental results (Jenkins et al.\ 2015), we changed the surface gravity, CO2 abundance, surface pressure, orbital eccentricity, rotation period, axis obliquity and ocean fraction within the range of validity of our model. For most choices of parameters we find habitable solutions with h050>0.2 only for CO2 partial pressure pCO2≲0.04\,bar. At this limiting value of CO2 abundance the planet is still habitable if the total pressure is p≲2\,bar. In all cases the habitability drops for eccentricity e≳0.3. Changes of rotation period and obliquity affect the habitability through their impact on the equator-pole temperature difference rather than on the mean global temperature. We calculated the variation of h050 resulting from the luminosity evolution of the host star for a wide range of input parameters. Only a small combination of parameters yield habitability-weighted lifetimes ≳2\,Gyr, sufficiently long to develop atmospheric biosignatures still detectable at the present time.

A Review of Potential Biosignature Detection Methods

Exoplanet Biosignatures: A Review of Remotely Detectable Signs of Life 

Authors: 

Schwieterman et al 

Abstract: 

In the coming years and decades, advanced space- and ground-based observatories will allow an unprecedented opportunity to probe the atmospheres and surfaces of potentially habitable exoplanets for signatures of life. Life on Earth, through its gaseous products and reflectance and scattering properties, has imprinted evidence of its presence upon the spectrum of our planet. Aided by the universality of the laws of physics and chemistry, we turn to Earths biosphere, both in the present and through geologic time, for informative analogies of what signatures to search for elsewhere. Here we have compiled an overview of our current understanding of potential exoplanet biosignatures including gaseous, surface, and temporal biosignatures. We additionally survey biogenic spectral features that are well-known in the specialist literature but have not yet been robustly vetted in the context of exoplanet biosignatures. We briefly review advances in assessing biosignature plausibility, including novel methods for determining chemical disequilibrium from remotely obtainable data and assessment tools for determining the minimum biomass required for a given atmospheric signature. We focus particularly on advances made since the seminal review by Des Marais et al. (2002). The purpose of this work is not to propose new biosignatures strategies, a goal left to companion papers in this series, but to review the current literature and provide a foundation for a path forward.

Nitrogen & Oxygen in the Exoplanet Atmosphere is a Biosignature

Modeling pN2 through Geological Time: Implications for Planetary Climates and Atmospheric Biosignatures

Authors:

Stüeken et al

Abstract:

Nitrogen is a major nutrient for all life on Earth and could plausibly play a similar role in extraterrestrial biospheres. The major reservoir of nitrogen at Earth's surface is atmospheric N2, but recent studies have proposed that the size of this reservoir may have fluctuated significantly over the course of Earth's history with particularly low levels in the Neoarchean—presumably as a result of biological activity. We used a biogeochemical box model to test which conditions are necessary to cause large swings in atmospheric N2 pressure. Parameters for our model are constrained by observations of modern Earth and reconstructions of biomass burial and oxidative weathering in deep time. A 1-D climate model was used to model potential effects on atmospheric climate. In a second set of tests, we perturbed our box model to investigate which parameters have the greatest impact on the evolution of atmospheric pN2 and consider possible implications for nitrogen cycling on other planets. Our results suggest that (a) a high rate of biomass burial would have been needed in the Archean to draw down atmospheric pN2 to less than half modern levels, (b) the resulting effect on temperature could probably have been compensated by increasing solar luminosity and a mild increase in pCO2, and (c) atmospheric oxygenation could have initiated a stepwise pN2 rebound through oxidative weathering. In general, life appears to be necessary for significant atmospheric pN2 swings on Earth-like planets. Our results further support the idea that an exoplanetary atmosphere rich in both N2 and O2 is a signature of an oxygen-producing biosphere.

Is it Possible to Detect 3d Structure to Exoplanet Vegetation?

Detecting 3D vegetation structure with the Galileo space probe: Can a distant probe detect vegetation structure on Earth?

Authors:

Doughty et al

Abstract:

Sagan et al. (1993) used the Galileo space probe data and first principles to find evidence of life on Earth. Here we ask whether Sagan et al. (1993) could also have detected whether life on Earth had three-dimensional structure, based on the Galileo space probe data. We reanalyse the data from this probe to see if structured vegetation could have been detected in regions with abundant photosynthetic pigments through the anisotropy of reflected shortwave radiation. We compare changing brightness of the Amazon forest (a region where Sagan et al. (1993) noted a red edge in the reflectance spectrum, indicative of photosynthesis) as the planet rotates to a common model of reflectance anisotropy and found measured increase of surface reflectance of 0.019 versus a 0.007 predicted from only anisotropic effects. We hypothesize the difference was due to minor cloud contamination. However, the Galileo dataset had only a small change in phase angle (sun-satellite position) which reduced the observed anisotropy signal and we demonstrate that theoretically if the probe had a variable phase angle between 0-20, there would have been a much larger predicted change in surface reflectance of 0.06 and under such a scenario three-dimensional vegetation structure on Earth could possibly have been detected. These results suggest that anisotropic effects may be useful to help determine whether exoplanets have three-dimensional vegetation structure in the future but that further comparisons between empirical and theoretical results are first necessary.

Polarized Scattering and Biosignatures in Exoplanetary Atmospheres

Polarized Scattering and Biosignatures in Exoplanetary Atmospheres

Author:

Berdyugina

Abstract:

Polarized scattering in planetary atmospheres is computed in the context of exoplanets. The problem of polarized radiative transfer is solved for a general case of absorption and scattering, while Rayleigh and Mie polarized scattering are considered as most relevant examples. We show that (1) relative contributions of single and multiple scattering depend on the stellar irradiation and opacities in the planetary atmosphere; (2) cloud (particle) physical parameters can be deduced from the wavelength-dependent measurements of the continuum polarization and from a differential analysis of molecular band absorption; (3) polarized scattering in molecular bands increases the reliability of their detections in exoplanets; (4) photosynthetic life can be detected on other planets in visible polarized spectra with high sensitivity. These examples demonstrate the power of spectropolarimetry for exoplanetary research and for searching for life in the universe.

Biological fluorescence as a temporal biosignature for flare star worlds

Biofluorescent Worlds: Biological fluorescence as a temporal biosignature for flare star worlds

Authors:

O'Malley-James et al

Abstract:

Habitability for planets orbiting active stars has been questioned. Especially, planets in the Habitable Zone (HZ) of M-stars, like our closest star Proxima Centauri, experience temporal high-ultraviolet (UV) radiation. The high fraction of M-stars (75%) within the solar neighborhood, the high occurrence rate of rocky planets around M-stars, and the favorable contrast ratio between the star and a potentially habitable rocky planet, makes such planets interesting targets for upcoming observations. During M-star flares, the UV flux on a HZ planet can increase by up to two orders of magnitude. High UV radiation is harmful to life and can cause cell and DNA damage. Common UV protection methods (e.g. living underground, or underwater) would make a biosphere harder to detect. However, photoprotective biofluorescence, "up-shifting" UV to longer, safer wavelengths (a proposed UV protection mechanism for some corals), would increase the detectability of biota and even uncover normally hidden biospheres during a flare. Such biofluorescence could be observable as a "temporal biosignature" for planets around UV-active stars. We model temporal biofluorescence as a biosignature for an exoplanet biosphere exposed to such conditions, based on planets in M-star HZs. We use fluorescing coral proteins to model biofluorescence, comparing observable spectra, and colors, to vegetation and fluorescent minerals. Our planetary models assume a present-day Earth atmosphere and explore the effect of varying cloud coverage and land:ocean fractions. UV flare-induced biofluorescence could be remotely detectable, comparable in strength to vegetation on Earth. On planets in the HZ of M-stars, biofluorescence could be a temporary biosignature, distinguishable from fluorescing minerals and vegetation.

Identifying Planetary Biosignature Impostors: Spectral Features of CO and O4 Resulting from Abiotic O2/O3 Production

Identifying Planetary Biosignature Impostors: Spectral Features of CO and O4 Resulting from Abiotic O2/O3 Production

Authors:

Schwieterman et al

Abstract:

O2 and O3 have been long considered the most robust individual biosignature gases in a planetary atmosphere, yet multiple mechanisms that may produce them in the absence of life have been described. However, these abiotic planetary mechanisms modify the environment in potentially identifiable ways. Here we briefly discuss two of the most detectable spectral discriminants for abiotic O2/O3: CO and O4. We produce the first explicit self-consistent simulations of these spectral discriminants as they may be seen by JWST. If JWST-NIRISS and/or NIRSpec observe CO (2.35, 4.6 um) in conjunction with CO2 (1.6, 2.0, 4.3 um) in the transmission spectrum of a terrestrial planet it could indicate robust CO2 photolysis and suggest that a future detection of O2 or O3 might not be biogenic. Strong O4 bands seen in transmission at 1.06 and 1.27 um could be diagnostic of a post-runaway O2-dominated atmosphere from massive H-escape. We find that for these false positive scenarios, CO at 2.35 um, CO2 at 2.0 and 4.3 um, and O4 at 1.27 um are all stronger features in transmission than O2/O3 and could be detected with SNRs ≳ 3 for an Earth-size planet orbiting a nearby M dwarf star with as few as 10 transits, assuming photon-limited noise. O4 bands could also be sought in UV/VIS/NIR reflected light (at 0.345, 0.36, 0.38, 0.445, 0.475, 0.53, 0.57, 0.63, 1.06, and 1.27 um) by a next generation direct-imaging telescope such as LUVOIR/HDST or HabEx and would indicate an oxygen atmosphere too massive to be biologically produced.

Toward a List of Molecules as Potential Biosignature Gases

Toward a List of Molecules as Potential Biosignature Gases for the Search for Life on Exoplanets and Applications to Terrestrial Biochemistry

Authors:

Seager et al

Abstract:

Thousands of exoplanets are known to orbit nearby stars. Plans for the next generation of space-based and ground-based telescopes are fueling the anticipation that a precious few habitable planets can be identified in the coming decade. Even more highly anticipated is the chance to find signs of life on these habitable planets by way of biosignature gases. But which gases should we search for? Although a few biosignature gases are prominent in Earth's atmospheric spectrum (O2, CH4, N2O), others have been considered as being produced at or able to accumulate to higher levels on exo-Earths (e.g., dimethyl sulfide and CH3Cl). Life on Earth produces thousands of different gases (although most in very small quantities). Some might be produced and/or accumulate in an exo-Earth atmosphere to high levels, depending on the exo-Earth ecology and surface and atmospheric chemistry.

To maximize our chances of recognizing biosignature gases, we promote the concept that all stable and potentially volatile molecules should initially be considered as viable biosignature gases. We present a new approach to the subject of biosignature gases by systematically constructing lists of volatile molecules in different categories. An exhaustive list up to six non-H atoms is presented, totaling about 14,000 molecules. About 2500 of these are CNOPSH compounds. An approach for extending the list to larger molecules is described. We further show that about one-fourth of CNOPSH molecules (again, up to N = 6 non-H atoms) are known to be produced by life on Earth. The list can be used to study classes of chemicals that might be potential biosignature gases, considering their accumulation and possible false positives on exoplanets with atmospheres and surface environments different from Earth's. The list can also be used for terrestrial biochemistry applications, some examples of which are provided. We provide an online community usage database to serve as a registry for volatile molecules including biogenic compounds.

Walking on Exoplanets: Is Star Wars Right?

Walking on Exoplanets: Is Star Wars Right?

Authors:

Ballesteros et al

Abstract:

As the number of detected extrasolar planets increases, exoplanet databases become a valuable resource, confirming some details about planetary formation but also challenging our theories with new, unexpected properties.

Archean Era ExoEarths are Hard to Identify, but Phanerozoic Ones are not

Is the Pale Blue Dot unique? Optimized photometric bands for identifying Earth-like exoplanets

Authors:

Krissansen-Totton et al

Abstract:

The next generation of ground and space-based telescopes will image habitable planets around nearby stars. A growing literature describes how to characterize such planets with spectroscopy, but less consideration has been given to the usefulness of planet colors. Here, we investigate whether potentially Earth-like exoplanets could be identified using UV-visible-to-NIR wavelength broadband photometry (350-1000 nm). Specifically, we calculate optimal photometric bins for identifying an exo-Earth and distinguishing it from uninhabitable planets including both Solar System objects and model exoplanets. The color of some hypothetical exoplanets - particularly icy terrestrial worlds with thick atmospheres - is similar to Earth's because of Rayleigh scattering in the blue region of the spectrum. Nevertheless, subtle features in Earth's reflectance spectrum appear to be unique. In particular, Earth's reflectance spectrum has a 'U-shape' unlike all our hypothetical, uninhabitable planets. This shape is partly biogenic because O2-rich, oxidizing air is transparent to sunlight, allowing prominent Rayleigh scattering, while ozone absorbs visible light, creating the bottom of the 'U'. Whether such uniqueness has practical utility depends on observational noise. If observations are photon limited or dominated by astrophysical sources (zodiacal light or imperfect starlight suppression), then the use of broadband visible wavelength photometry to identify Earth twins has little practical advantage over obtaining detailed spectra. However, if observations are dominated by dark current then optimized photometry could greatly assist preliminary characterization. We also calculate the optimal photometric bins for identifying extrasolar Archean Earths, and find that the Archean Earth is more difficult to unambiguously identify than a modern Earth twin.

Is Carbon monoxide Detection in an ExoPlanetary Atmosphere an 'Anti-biosignature?'

On the detection of carbon monoxide as an anti-biosignature in exoplanetary atmospheres

Authors:

Wang et al

Abstract:

Recent works suggest that oxygen can be maintained on lifeless exoplanets in the habitable zones of M dwarfs as the results of photochemical reactions. However, the same photochemical models also predict high concentrations of carbon monoxide (CO) in the corresponding atmospheres. Here we use a line-by-line radiative transfer model to investigate the observation requirements of O2 and CO in such atmospheres. The results show that photochemically produced CO can be readily detected at 1.58, 2.34, and 4.67 μm. We suggest that future missions aiming at characterization of exoplanetary atmospheres consider detections of CO as an anti-biosignature.

Atmospheric Haze may be a Biosignature

An atmospheric haze around a faraway planet -- like the one which probably shrouded and cooled the young Earth -- could show that the world is potentially habitable, or even be a sign of life itself.

Astronomers often use the Earth as a proxy for hypothetical exoplanets in computer modeling to simulate what such worlds might be like and under what circumstances they might be hospitable to life.

In new research from the University of Washington-based Virtual Planetary Laboratory, UW doctoral student Giada Arney and co-authors chose to study Earth in its Archean era, about 2 ½ billion years back, because it is, as Arney said, "the most alien planet we have geochemical data for."

The work builds on geological data from other researchers that suggests the early Earth was intermittently shrouded by an organic pale orange haze that came from light breaking down methane molecules in the atmosphere into more complex hydrocarbons, organic compounds of hydrogen and carbon.

"Hazy worlds seem common both in our solar system and in the population of exoplanets we've characterized so far," Arney said. "Thinking about Earth with a global haze allows us to put our home planet into the context of these other worlds, and in this case, the haze may even be a sign of life itself."

Arney and co-authors will present their findings Nov. 11 at the American Astronomical Society's Division of Planetary Sciences conference in National Harbor, Maryland.

The researchers used photochemical, climate and radiation simulations to examine the early Earth shrouded by a "fractal" hydrocarbon haze, meaning that the imagined haze particles are not spherical, as used in many such simulations, but agglomerates of spherical particles, bunched together not unlike grapes, but smaller than a raindrop. A fractal haze, they found, would have significantly lowered the planetary surface temperature.

However, they also found the cooling would be partly countered by concentrations of greenhouse gases that tend to warm a planet. They saw that this combination would result in a moderate, possibly habitable average global temperature.

Such a haze, the researchers found, also would have absorbed ultraviolet light so well as to effectively shield the Archean Earth from deadly radiation before the rise of oxygen and the ozone layer, which now provides that protection. The haze was a benefit to just-evolving surface biospheres on Earth, as it could be to similar exoplanets.

The researchers also found that, based on the early Earth data, it's unlikely such a haze would be formed by abiotic, or nonliving means. So for exoplanets with Earthlike amounts of carbon dioxide in their atmospheres, Arney said, "organic haze might be a novel type of biosignature. However, we know these hazes can also form without life on worlds like Saturn's moon Titan, so we are working to come up with more ways to distinguish biological hazes from abiotic ones."

link.

Problems With Detecting ExoEarths With the Proposed High Definition Space Telescope

Issues with the High Definition Space Telescope (HDST) ExoEarth Biosignature Case: A Critique of the 2015 AURA Report "From Cosmic Birth to Living Earths: the future of UVOIR Astronomy"

Author:

Elvis

Abstract:

"From Cosmic Birth to Living Earths" advocates a 12-meter optical/near-IR space telescope for launch ~2035. The goal that sets this large size is the detection of biosignatures from Earth-like planets in their habitable zones around G-stars. The discovery of a single instance of life elsewhere in the universe would be a profound event for humanity. But not at any cost. At 8-9B USD this High Definition Space Telescope (HDST) would take all the NASA astrophysics budget for nearly 20 years, unless new funds are found. For a generation NASA could build no "Greater Observatories" matching JWST in the rest of the spectrum. This opportunity cost prompted me to study the driving exobiosphere detection case for HDST. I find that: (1) the focus on G-stars is not well justified; (2) only G-stars require the use of direct imaging; (3) in the chosen 0.5 - 2.5 micron band, the available biosignatures are ambiguous and a larger sample does not help; (4) the expected number of exobiospheres is 1, with a 5% chance of zero; (5) the accessible sample size is too small to show that exobiospheres are rare; (6) a sufficiently large sample would require a much larger telescope; (7) the great progress in M-star planet spectroscopy - both now and with new techniques, instruments and telescopes already planned - means that a biosignature will likely be found before HDST could complete its search in ~2045. For all these reasons I regretfully conclude that HDST, while commendably ambitious, is not the right choice for NASA Astrophysics at this time. The first exobiosphere discovery is likely to be such a major event that scientific and public pressure will produce new funding across a range of disciplines, not just astrophysics, to study the nature of Life in the Universe. Then will be the time when a broader science community can advocate for a mission that will make definitive exobiosphere measurements.

Titanium(IV) Oxide (Titania) can Produce Oxygen Rich Habitable Zone Atmospheres WITHOUT Life

Titania may produce abiotic oxygen atmospheres on habitable exoplanets

Authors:

Narita et al

Abstract:

The search for habitable exoplanets in the Universe is actively ongoing in the field of astronomy. The biggest future milestone is to determine whether life exists on such habitable exoplanets. In that context, oxygen in the atmosphere has been considered strong evidence for the presence of photosynthetic organisms. In this paper, we show that a previously unconsidered photochemical mechanism by titanium(IV) oxide (titania) can produce abiotic oxygen from liquid water under near ultraviolet (NUV) lights on the surface of exoplanets. Titania works as a photocatalyst to dissociate liquid water in this process. This mechanism offers a different source of a possibility of abiotic oxygen in atmospheres of exoplanets from previously considered photodissociation of water vapor in upper atmospheres by extreme ultraviolet (XUV) light. Our order-of-magnitude estimation shows that possible amounts of oxygen produced by this abiotic mechanism can be comparable with or even more than that in the atmosphere of the current Earth, depending on the amount of active surface area for this mechanism. We conclude that titania may act as a potential source of false signs of life on habitable exoplanets.