The Power of Self-Skepticism in Astrobiology

Any claims for evidence of life on other worlds have the potential to be transformative events in human history. Accordingly, any such claims will be met with intense scrutiny from the scientific community. This will be particularly true for claims for evidence of life on exoplanets--planets around other stars--for which we will only have remote-sensing data and no ability to grab a piece of that world and put it under both literal and figurative microscopes. The data upon which these claims will be made will be the integrated product of the entire careers of some of the world's greatest scientists and engineers, paid for by considerable taxpayer expense. This presents astrobiologists with a paradox: How can such investments be justified if the end goal is destined to be a highly scrutinized discovery

On the Frequency of Potential Venus Analogs from Kepler Data

The field of exoplanetary science has seen a dramatic improvement in sensitivity to terrestrial planets over recent years. Such discoveries have been a key feature of results from the {\it Kepler} mission which utilizes the transit method to determine the size of the planet. These discoveries have resulted in a corresponding interest in the topic of the Habitable Zone (HZ) and the search for potential Earth analogs. Within the Solar System, there is a clear dichotomy between Venus and Earth in terms of atmospheric evolution, likely the result of the large difference ($\sim$ factor of two) in incident flux from the Sun. Since Venus is 95\% of the Earth's radius in size, it is impossible to distinguish between these two planets based only on size. In this paper we discuss planetary insolation in the context of atmospheric erosion and runaway greenhouse limits for planets similar to Venus. We define a ``Venus Zone'' (VZ) in which the planet is more likely to be a Venus analog rather than an Earth analog. We identify 43 potential Venus analogs with an occurrence rate (\eta_{\venus}) of $0.32^{+0.05}_{-0.07}$ and $0.45^{+0.06}_{-0.09}$ for M dwarfs and GK dwarfs respectively.Comment: 6 pages, 3 figures, 2 tables. Accepted for publication in the Astrophysical Journal Letters. More information and graphics can be found at the Habitable Zone Gallery (http://hzgallery.org

Organic Haze as a Biosignature in Anoxic Earth-like Atmospheres

Early Earth may have hosted a biologically-mediated global organic haze during the Archean eon (3.8-2.5 billion years ago). This haze would have significantly impacted multiple aspects of our planet, including its potential for habitability and its spectral appearance. Here, we model worlds with Archean-like levels of carbon dioxide orbiting the ancient sun and an M4V dwarf (GJ 876) and show that organic haze formation requires methane fluxes consistent with estimated Earth-like biological production rates. On planets with high fluxes of biogenic organic sulfur gases (CS2, OCS, CH3SH, and CH3SCH3), photochemistry involving these gases can drive haze formation at lower CH4/CO2 ratios than methane photochemistry alone. For a planet orbiting the sun, at 30x the modern organic sulfur gas flux, haze forms at a CH4/CO2 ratio 20% lower than at 1x the modern organic sulfur flux. For a planet orbiting the M4V star, the impact of organic sulfur gases is more pronounced: at 1x the modern Earth organic sulfur flux, a substantial haze forms at CH4/CO2 ~ 0.2, but at 30x the organic sulfur flux, the CH4/CO2 ratio needed to form haze decreases by a full order of magnitude. Detection of haze at an anomalously low CH4/CO2 ratio could suggest the influence of these biogenic sulfur gases, and therefore imply biological activity on an exoplanet. When these organic sulfur gases are not readily detectable in the spectrum of an Earth-like exoplanet, the thick organic haze they can help produce creates a very strong absorption feature at UV-blue wavelengths detectable in reflected light at a spectral resolution as low as 10. In direct imaging, constraining CH4 and CO2 concentrations will require higher spectral resolution, and R > 170 is needed to accurately resolve the structure of the CO2 feature at 1.57 {\mu}m, likely, the most accessible CO2 feature on an Archean-like exoplanet.Comment: accepted for publication in Astrobiolog

Astrobiology as a NASA Grand Challenge

"Are we alone" is a question whose ambition can only be met with a NASA-led global collaboration. In this white paper, we describe how this makes "The Search for Life Beyond Earth" a new Grand Challenge for NASA. As described in the White House Office of Science and Technology Policy and the White House National Economic Council, Grand Challenges are "ambitious but achievable goals that harness science, technology, and innovation to solve important national or global problems and that have the potential to capture the public's imagination." NASA had identified an "Asteroid Grand Challenge" centered on the Asteroid Retrieval Mission, which was closed out in June, 2017. Here, we explain how NASA's next Grand Challenge could be focused on "The Search for Life Beyond Earth," with a flagship-scale mission in Astrophysics as its centerpiece

Lower Limits on Aperture Size for an ExoEarth-Detecting Coronagraphic Mission

The yield of Earth-like planets will likely be a primary science metric for future space-based missions that will drive telescope aperture size. Maximizing the exoEarth candidate yield is therefore critical to minimizing the required aperture. Here we describe a method for exoEarth candidate yield maximization that simultaneously optimizes, for the first time, the targets chosen for observation, the number of visits to each target, the delay time between visits, and the exposure time of every observation. This code calculates both the detection time and multi-wavelength spectral characterization time required for planets. We also refine the astrophysical assumptions used as inputs to these calculations, relying on published estimates of planetary occurrence rates as well as theoretical and observational constraints on terrestrial planet sizes and classical habitable zones. Given these astrophysical assumptions, optimistic telescope and instrument assumptions, and our new completeness code that produces the highest yields to date, we suggest lower limits on the aperture size required to detect and characterize a statistically-motivated sample of exoEarths.Comment: Accepted for publication in ApJ; 38 pages, 16 Figures, 3 Table

Abiotic Ozone and Oxygen in Atmospheres Similar to Prebiotic Earth

The search for life on planets outside our solar system will use spectroscopic identification of atmospheric biosignatures. The most robust remotely-detectable potential biosignature is considered to be the detection of oxygen (O_2) or ozone (O_3) simultaneous to methane (CH_4) at levels indicating fluxes from the planetary surface in excess of those that could be produced abiotically. Here, we use an altitude-dependent photochemical model with the enhanced lower boundary conditions necessary to carefully explore abiotic O_2 and O_3 production on lifeless planets with a wide variety of volcanic gas fluxes and stellar energy distributions. On some of these worlds, we predict limited O_2 and O_3 build up, caused by fast chemical production of these gases. This results in detectable abiotic O_3 and CH_4 features in the UV-visible, but no detectable abiotic O_2 features. Thus, simultaneous detection of O_3 and CH_4 by a UV-visible mission is not a strong biosignature without proper contextual information. Discrimination between biological and abiotic sources of O_2 and O_3 is possible through analysis of the stellar and atmospheric context - particularly redox state and O atom inventory - of the planet in question. Specifically, understanding the spectral characteristics of the star and obtaining a broad wavelength range for planetary spectra should allow more robust identification of false positives for life. This highlights the importance of wide spectral coverage for future exoplanet characterization missions. Specifically, discrimination between true- and false-positives may require spectral observations that extend into infrared wavelengths, and provide contextual information on the planet's atmospheric chemistry.Comment: Accepted for publication in The Astrophysical Journal. 43 pages, 6 figure

Detecting and Constraining N2_2 Abundances in Planetary Atmospheres Using Collisional Pairs

Characterizing the bulk atmosphere of a terrestrial planet is important for determining surface pressure and potential habitability. Molecular nitrogen (N$_2$) constitutes the largest fraction of Earth$'$s atmosphere and is likely to be a major constituent of many terrestrial exoplanet atmospheres. Due to its lack of significant absorption features, N$_2$ is extremely difficult to remotely detect. However, N$_2$ produces an N$_2$-N$_2$ collisional pair, (N$_2$)$_2$, which is spectrally active. Here we report the detection of (N$_2$)$_2$ in Earth$'$s disk-integrated spectrum. By comparing spectra from NASA$'$s EPOXI mission to synthetic spectra from the NASA Astrobiology Institute$'$s Virtual Planetary Laboratory three-dimensional spectral Earth model, we find that (N$_2$)$_2$ absorption produces a ~35$\%$ decrease in flux at 4.15 $\mu$m. Quantifying N$_2$ could provide a means of determining bulk atmospheric composition for terrestrial exoplanets and could rule out abiotic O$_2$ generation, which is possible in rarefied atmospheres. To explore the potential effects of (N$_2$)$_2$ in exoplanet spectra, we used radiative transfer models to generate synthetic emission and transit transmission spectra of self-consistent N$_2$-CO$_2$-H$_2$O atmospheres, and analytic N$_2$-H$_2$ and N$_2$-H$_2$-CO$_2$ atmospheres. We show that (N$_2$)$_2$ absorption in the wings of the 4.3 $\mu$m CO$_2$ band is strongly dependent on N$_2$ partial pressures above 0.5 bar and can significantly widen this band in thick N$_2$ atmospheres. The (N$_2$)$_2$ transit transmission signal is up to 10 ppm for an Earth-size planet with an N$_2$-dominated atmosphere orbiting within the HZ of an M5V star and could be substantially larger for planets with significant H$_2$ mixing ratios.Comment: Accepted for publication in The Astrophysical Journal. 46 pages, 12 figures, 3 table

An Ensemble of Bayesian Neural Networks for Exoplanetary Atmospheric Retrieval

Machine learning is now used in many areas of astrophysics, from detecting exoplanets in Kepler transit signals to removing telescope systematics. Recent work demonstrated the potential of using machine learning algorithms for atmospheric retrieval by implementing a random forest to perform retrievals in seconds that are consistent with the traditional, computationally-expensive nested-sampling retrieval method. We expand upon their approach by presenting a new machine learning model, \texttt{plan-net}, based on an ensemble of Bayesian neural networks that yields more accurate inferences than the random forest for the same data set of synthetic transmission spectra. We demonstrate that an ensemble provides greater accuracy and more robust uncertainties than a single model. In addition to being the first to use Bayesian neural networks for atmospheric retrieval, we also introduce a new loss function for Bayesian neural networks that learns correlations between the model outputs. Importantly, we show that designing machine learning models to explicitly incorporate domain-specific knowledge both improves performance and provides additional insight by inferring the covariance of the retrieved atmospheric parameters. We apply \texttt{plan-net} to the Hubble Space Telescope Wide Field Camera 3 transmission spectrum for WASP-12b and retrieve an isothermal temperature and water abundance consistent with the literature. We highlight that our method is flexible and can be expanded to higher-resolution spectra and a larger number of atmospheric parameters

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

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 $\gtrsim$ 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.Comment: 7 pages, 4 figures, accepted to the Astrophysical Journal Letter