Testing Earth-like atmospheric evolution on exo-Earths through oxygen absorption: required sample sizes and the advantage of age-based target selection
Life has had a dramatic impact on the composition of Earth's atmosphere over time, which suggests that statistical studies of other inhabited planets' atmospheres could reveal how they co-evolve with life. While many evolutionary pathways are possible for inhabited worlds, a possible starting hypothesis is that most of them evolve similarly to Earth, which we propose could lead to a positive "age-oxygen correlation" between the ages of inhabited planets and the fraction which have oxygen-rich atmospheres. We demonstrate that next-generation space observatories currently under consideration could test this hypothesis, but only if the stellar age distribution of the target sample is carefully considered. We explore three possible parameterizations of the age-oxygen correlation, finding that they yield similar results. Finally, we examine how abiotic oxygen sources could affect the results, and discuss how measuring the age-dependence of oxygen could shed light on whether it is a reliable biosignature. Future efforts can expand upon this groundwork by incorporating detailed models of the redox balance of terrestrial planets and its dependence on stellar and planetary properties.
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 i…
Oxygen fugacity is a measure of rock oxidation that influences planetary structure and evolution. Most rocky bodies in the Solar System formed at oxygen fugacities approximately five orders of magnitude higher than a hydrogen-rich gas of solar composition. It is unclear whether this oxidation of rocks in the Solar System is typical among other planetary systems. We exploit the elemental abundances observed in six white dwarfs polluted by the accretion of rocky bodies to determine the fraction of oxidized iron in those extrasolar rocky bodies and therefore their oxygen fugacities. The results a…
Exoplanet properties depend on how well the host star is characterized. For instance, the stellar atmospheric parameters (i.e., effective temperature, surface gravity and overall metallicity) are needed to derive the stellar mass and radius by using evolutionary stellar models or empirical calibrations. I studied how the stellar uncertainties impact the determination of planetary characteristics. By using high-resolution spectra and iSpec, I evaluated how discrepancies between two different spectroscopic approaches (the equivalent method and the synthetic spectral fitting technique) can signif…
Here we advocate an observational strategy to help prioritize exoplanet observations. It starts with more easily obtained observational data, and ranks exoplanets for more difficult follow-up observations based on the likelihood of avoiding planets for which oxygen is a false positives or even an inconclusive signature of life. We find that for oxygen to be a robust biosignature, both land and surface water must be present. Landless exoplanets have much slower biogeochemical cycles, so while oxygenic photosynthesizing life could exist on such planets, it could not produce oxygen at a rate comp…
The current explosion in detection and characterization of thousands of extrasolar planets from the Kepler mission, the Hubble Space Telescope, and large ground-based telescopes opens a new era in searches for Earth-analog exoplanets with conditions suitable for sustaining life. As more Earth-sized exoplanets are detected in the near future, we will soon have an opportunity to identify habitable worlds. Which atmospheric biosignature gases from habitable planets can be detected with our current capabilities? The detection of the common biosignatures from nitrogen-oxygen rich terrestrial-type e…
With the aim of connecting the compositions of stars and planets, we present the abundances of carbon and oxygen, as well as iron and nickel, for the transiting exoplanet host star XO-2N and its wide-separation binary companion XO-2S. Stellar parameters are derived from high-resolution, high-signal-to-noise spectra, and the two stars are found to be similar in their Teff, log g, iron ([Fe/H]), nickel ([Ni/H]) abundances. Their carbon ([C/H]) and oxygen ([O/H]) abundances also overlap within errors, although XO-2N may be slightly more C-rich and O-rich than XO-2S. The C/O ratios of both stars (…
In the next decades, the astrobiological community will debate whether the first observations of oxygen in an exoplanet$'$s atmosphere signifies life, so it is critical to establish procedures now for collection and interpretation of such data. We present a step-by-step observational strategy for using oxygen as a robust biosignature, to prioritize exoplanet targets and design future observations. It is premised on avoiding planets lacking subaerial weathering of continents, which would imply geochemical cycles drastically different from Earth$'$s, precluding use of oxygen as a biosignature. T…
Earth's modern atmosphere is highly oxygenated and is a remotely detectable signal of its surface biosphere. However, the lifespan of oxygen-based biosignatures in Earth's atmosphere remains uncertain, particularly for the distant future. Here we use a combined biogeochemistry and climate model to examine the likely timescale of oxygen-rich atmospheric conditions on Earth. Using a stochastic approach, we find that the mean future lifespan of Earth's atmosphere with oxygen levels more than 1% of the present atmospheric level is 1.08+-0.14 billion years. The model projects that a deoxygenation o…
A planetary atmosphere is the outer gas layer of a planet. Besides its scientific significance among the first and most accessible planetary layers observed from space, it is closely connected with planetary formation and evolution, surface and interior processes, and habitability of planets. Current theories of the planetary atmosphere were primarily obtained through the studies of eight large planets, Pluto and three large moons (Io, Titan, and Triton) in the Solar System. Outside the Solar System, more than four thousand extra-solar planets (exoplanets) and two thousand brown dwarfs have be…
The super-Earth exoplanet 55 Cnc e, the smallest member of a five-planet system, has recently been observed to transit its host star. The radius estimates from transit observations, coupled with spectroscopic determinations of mass, provide constraints on its interior composition. The composition of exoplanetary interiors and atmospheres are particularly sensitive to elemental C/O ratio, which to first order can be estimated from the host stars. Results from a recent spectroscopic study analyzing the 6300A [O I] line and two C I lines suggest that 55 Cnc has a carbon-rich composition (C/O=1.12…
Exoplanetary science continues to excite and surprise with its rich diversity. We discuss here some key aspects potentially influencing the range of exoplanetary terrestrial-type atmospheres which could exist in nature. We are motivated by newly emerging observations, refined approaches to address data degeneracies, improved theories for key processes affecting atmospheric evolution and a new generation of atmospheric models which couple physical processes from the deep interior through to the exosphere and consider the planetary-star system as a whole. Using the Solar System as our guide we f…
Stellar impact on exoplanetary atmospheric evolution and habitability
Instituto de Ciencias Nucleares, Universidad Nacional Autónoma de México（墨西哥国立自治大学核科学研究所）；Department of Astrophysics, University of Vienna（维也纳大学天体物理学系）；ASTRON, Netherlands Institute for Radio Astronomy（荷兰射电天文研究所）；Centre for Planetary Habitability (PHAB), Department of Geosciences, University of Oslo（奥斯陆大学地球科学系行星宜居性中心）；Center of Excellence in Space Sciences India, Indian Institute of Science Education and Research Kolkata（印度加尔各答科学教育研究所印度太空科学卓越中心）；Department of Physical Sciences, Indian Institute of Science Education and Research Kolkata（印度加尔各答科学教育研究所物理科学系）；Université Paris Cité, Université Paris-Saclay, CEA, CNRS, AIM（巴黎西岱大学、巴黎萨克雷大学、法国原子能和替代能源委员会、法国国家科学研究中心天体分子生物学实验室）；Jet Propulsion Laboratory, California Institute of Technology（加州理工学院喷气推进实验室）；Goddard Space Flight Center, NASA（美国国家航空航天局戈达德太空飞行中心）
We investigate the abiotic production of oxygen and its photochemical byproduct ozone through water vapor photolysis in moist atmospheres of temperate terrestrial exoplanets. The amount of water vapor available for photolysis in the middle atmosphere of a planet can be limited by an atmospheric cold-trap, the formation of which largely depends on the amount of non-condensable gases. We study this effect using a photochemical model coupled to a 1D radiative-convective equilibrium model in atmospheres with N2, CO2 and H2O as the main constituents. We find that in atmospheres with a low N2 invent…
We present a three-species (H$^+$, O$^+$ and e$^-$) multi-fluid magnetohydrodynamic (MHD) model, endowed with the requisite upper atmospheric chemistry, that is capable of accurately quantifying the magnitude of oxygen ion losses from "Earth-like" exoplanets in habitable zones, whose magnetic and rotational axes are roughly coincidental with one another. We apply this model to investigate the role of planetary obliquity in regulating atmospheric losses from a magnetic perspective. For Earth-like exoplanets orbiting solar-type stars, we demonstrate that the dependence of the total atmospheric i…
Stellar high-energy radiation (X-ray and extreme ultraviolet, XUV) drives atmospheric escape in close-in exoplanets. Given that stellar irradiation depends on the stellar magnetism and that stars have magnetic cycles, we investigate how cycles affect the evolution of exoplanetary atmospheric escape. Firstly, we consider a hypothetical HD209458b-like planet orbiting the Sun. For that, we implement the observed solar XUV radiation available over one and a half solar cycles in a 1D hydrodynamic escape model of HD209458b. We find that atmospheric escape rates show a cyclic variation (from 7.6 to 1…
In this Letter, we make use of sophisticated 3D numerical simulations to assess the extent of atmospheric ion and photochemical losses from Mars over time. We demonstrate that the atmospheric ion escape rates were significantly higher (by more than two orders of magnitude) in the past at $\sim 4$ Ga compared to the present-day value owing to the stronger solar wind and higher ultraviolet fluxes from the young Sun. We found that the photochemical loss of atomic hot oxygen dominates over the total ion loss at the current epoch whilst the atmospheric ion loss is likely much more important at anci…
We use thermodynamic calculations to model atmospheric chemistry on terrestrial exoplanets that are hot enough for chemical equilibira between the atmosphere and lithosphere, as on Venus. The results of our calculations place constraints on abundances of spectroscopically observable gases, the surface temperature and pressure, and the mineralogy of the surface. These results will be useful in planning future observations of the atmospheres of terrestrial-sized exoplanets by current and proposed space observatories such as the Hubble Space Telescope (HST), Spitzer, James Webb Space Telescope (J…
Exoplanets with substantial Hydrogen/Helium atmospheres have been discovered in abundance, many residing extremely close to their parent stars. The extreme irradiation levels these atmospheres experience causes them to undergo hydrodynamic atmospheric escape. Ongoing atmospheric escape has been observed to be occurring in a few nearby exoplanet systems through transit spectroscopy both for hot Jupiters and lower-mass super-Earths/mini-Neptunes. Detailed hydrodynamic calculations that incorporate radiative transfer and ionization chemistry are now common in one-dimensional models, and multi-dim…
We describe the recent updates to the ExoMol database regarding the molecular spectral line shapes. ExoMol provides comprehensive molecular line lists with a special emphasis on the applications involving characterization of atmospheres of exoplanets and cool stars. Among important requirements of such applications are (i) the broadening parameters for hydrogen and helium dominating atmospheres and (ii) very broad ranges of temperature and pressures. The current status of the available line shape data in the literature, demands from the exoplanetary community and their specific needs are discu…
The oxygen absorption line imprinted in the scattered light from the Earth-like planets has been considered the most promising metabolic biomarker of the exo-life. We examine the feasibility of the detection of the 1.27 micron oxygen band from habitable exoplanets, in particular, around late- type stars observed with a future instrument on a 30 m class ground-based telescope. We analyzed the night airglow around 1.27 micron with IRCS/echelle spectrometer on Subaru and found that the strong telluric emission from atmospheric oxygen molecules declines by an order of magnitude by midnight. By com…
Detecting heavy atoms in the inflated atmospheres of giant exoplanets that orbit close to their parent stars is a key factor for understanding their bulk composition, and the processes that drive their expansion and interaction with the impinging stellar wind. Here, we use archive data obtained with the Cosmic Origins Spectrograph onboard the Hubble Space Telescope to report an absorption of ~6.4+/-1.8% by neutral oxygen during the HD 189733b transit. Scaling published HI results from a simple hydrodynamic model of HD 189733b, a vertical OI column density of ~8x10^15/cm2 produces only a 3.5% a…
We survey the basic principles of atmospheric dynamics relevant to explaining existing and future observations of exoplanets, both gas giant and terrestrial. Given the paucity of data on exoplanet atmospheres, our approach is to emphasize fundamental principles and insights gained from Solar-System studies that are likely to be generalizable to exoplanets. We begin by presenting the hierarchy of basic equations used in atmospheric dynamics, including the Navier-Stokes, primitive, shallow-water, and two-dimensional nondivergent models. We then survey key concepts in atmospheric dynamics, includ…
With over 1800 planets discovered outside of the Solar System in the past two decades, the field of exoplanetology has broadened our perspective on planetary systems. Research priorities are now moving from planet detection to planet characterization. In this context, transiting exoplanets are of special interest due to the wealth of data made available by their orbital configuration. Here, I introduce two methods to gain new insights into the atmospheric and interior properties of exoplanets. The first method aims to map an exoplanet's atmosphere based on the scanning obtained while it is occ…
Recent observations of several planet-hosting M dwarfs show that most have FUV/NUV flux ratios 1000 times greater than that of the Sun. Here we show that the atmospheric oxygen contents (O2 and O3) of potentially habitable planets in this type of UV environment could be 2~3 orders of magnitude greater than those of their counterparts around Sun-like stars as a result of decreased photolysis of O3, H2O2, and HO2. Thus detectable levels of atmospheric oxygen, in combination with the existence of H2O and CO2, may not be the most promising biosignatures on planets around stars with high FUV/NUV ra…
Emission spectroscopy is a promising technique to observe atmospheres of rocky exoplanets, probing both their chemistry and thermal profiles. We present HyDRo, an atmospheric retrieval framework for thermal emission spectra of rocky exoplanets. HyDRo does not make prior assumptions about the background atmospheric composition, and can therefore be used to interpret spectra of secondary atmospheres with unknown compositions. We use HyDRo to assess the chemical constraints which can be placed on rocky exoplanet atmospheres using JWST. Firstly, we identify the best currently-known rocky exoplanet…
Much of the focus of exoplanet atmosphere analysis in the coming decade will be at infrared wavelengths, with the planned launches of the James Webb Space Telescope (JWST) and the Wide-Field Infrared Survey Telescope (WFIRST). However, without being placed in the context of broader wavelength coverage, especially in the optical and ultraviolet, infrared observations produce an incomplete picture of exoplanet atmospheres. Scattering information encoded in blue optical and near-UV (NUV) observations can help determine whether muted spectral features observed in the infrared are due to a hazy/clo…
Atmospheric retrievals on exoplanets usually involve computationally intensive Bayesian sampling methods. Large parameter spaces and increasingly complex atmospheric models create a computational bottleneck forcing a trade-off between statistical sampling accuracy and model complexity. It is especially true for upcoming JWST and ARIEL observations. We introduce ExoGAN, the Exoplanet Generative Adversarial Network, a new deep learning algorithm able to recognise molecular features, atmospheric trace-gas abundances and planetary parameters using unsupervised learning. Once trained, ExoGAN is wid…
The first opportunity to detect indications for life outside the Solar System may be provided already within the next decade with upcoming missions such as the James Webb Space Telescope (JWST), the European Extremely Large Telescope (E-ELT) and/or the Atmospheric Remote-sensing Infrared Exoplanet Large-survey (ARIEL) mission, searching for atmospheric biosignatures on planets in the habitable zone of cool K- and M-stars. Nevertheless, their harsh stellar radiation and particle environment could lead to photochemical loss of atmospheric biosignatures. We aim to study the influence of cosmic ra…
The atmospheric C/O ratio of exoplanets is widely used to constrain their formation. To guarantee that the C/O ratio provides robust information, we need to accurately quantify the amount of C and O in exoplanetary atmospheres. In the case of O, water and carbon monoxide are generally studied as the two key carriers. However, oxygen is a very reactive element and does not bind with carbon; depending on the temperature, it also binds to refractory elements. Estimating the amount of oxygen bound to refractory elements is therefore critical for unbiased estimates of the C/O ratio. In this work, w…
Constraining planet formation based on the atmospheric composition of exoplanets is a fundamental goal of the exoplanet community. Existing studies commonly try to constrain atmospheric abundances, or to analyze what abundance patterns a given description of planet formation predicts. However, there is also a pressing need to develop methodologies that investigate how to transform atmospheric compositions into planetary formation inferences. In this study we summarize the complexities and uncertainties of state-of-the-art planet formation models and how they influence planetary atmospheric com…
Most models used to predict or fit exoplanet transmission spectra do not include all the effects of atmospheric refraction. Namely, the angular size of the star with respect to the planet can limit the lowest altitude, or highest density and pressure, probed during primary eclipses, as no rays passing below this critical altitude can reach the observer. We discuss this geometrical effect of refraction for all exoplanets, and tabulate the critical altitude, density and pressure for an exoplanet identical to Earth with a 1 bar N2/O2 atmosphere, as a function of both the incident stellar flux (Ve…
The recent discoveries of terrestrial exoplanets and super-Earths extending over a broad range of orbital and physical parameters suggest that these planets will span a wide range of climatic regimes. Characterization of the atmospheres of warm super-Earths has already begun and will be extended to smaller and more distant planets over the coming decade. The habitability of these worlds may be strongly affected by their three-dimensional atmospheric circulation regimes, since the global climate feedbacks that control the inner and outer edges of the habitable zone including transitions to Snow…
Short-period exoplanets potentially lose envelope masses during their evolution because of atmospheric escape caused by the intense XUV radiation from their host stars. We develop a combined model of atmospheric mass loss calculation and thermal evolution calculation of a planet to simulate its evolution and explore the dependences on the formation history of the planet. Thermal atmospheric escape as well as the Roche-lobe overflow contributes to mass loss. The maximum initial planetary model mass depends primarily on the assumed evolution model of the stellar XUV luminosity. We adapt the mode…
The planetary effective surface temperature alone is insufficient to characterize exoplanet atmospheres and their stability or evolution. Considering the star-planet system as a whole is necessary, and a critical component of the system is the photoionizing stellar extreme ultraviolet emission (EUV; 100-912 Å). EUV photons drive atmospheric mass loss through thermal and nonthermal processes, and an accurate accounting of the EUV energy deposition in a planet's energy budget is essential, especially for terrestrial habitable zone planets and close-in gaseous planets. Direct EUV observations of …
The characterization of exoplanetary atmospheres has come of age in the last decade, as astronomical techniques now allow for albedos, chemical abundances, temperature profiles and maps, rotation periods and even wind speeds to be measured. Atmospheric dynamics sets the background state of density, temperature and velocity that determines or influences the spectral and temporal appearance of an exoplanetary atmosphere. Hot exoplanets are most amenable to these characterization techniques; in the present review, we focus on highly-irradiated, large exoplanets (the "hot Jupiters"), as astronomic…
Solar photospheric abundances of refractory elements mirror the Earth's to within ~10 mol% when normalized to the dominant terrestrial planet-forming elements Mg, Si and Fe. This allows for the adoption of Solar composition as an order-of-magnitude proxy for Earth's. It is not known, however, the degree to which this mirroring of stellar and terrestrial planet abundances holds true for other star-planet systems without determination of the composition of initial planetesimals via condensation sequence calculations and post condensation processes. We present the open-source Arbitrary Compositio…
We present results from one-dimensional atmospheric simulations investigating the effect of varying the carbon-to-oxygen (C/O) ratio on the thermal structure, chemical composition and transmission and emission spectra, for irradiated hydrogen-dominated atmospheres. We find that each of these properties of the atmosphere are strongly dependent on the individual abundances (relative to hydrogen) of carbon and oxygen. We confirm previous findings that different chemical equilibrium compositions result from different sets of element abundances but with the same C/O ratio. We investigate the effect…
Near-term studies of Venus-like atmospheres with JWST promise to advance our knowledge of terrestrial planet evolution. However, the remote study of Venus in the Solar System and the ongoing efforts to characterize gaseous exoplanets both suggest that high altitude aerosols could limit observational studies of lower atmospheres, and potentially make it challenging to recognize exoplanets as "Venus-like". To support practical approaches for exo-Venus characterization with JWST, we use Venus-like atmospheric models with self-consistent cloud formation of the seven TRAPPIST-1 exoplanets to invest…
Direct imaging of widely separated exoplanets from space will obtain their reflected light spectra and measure their atmospheric properties, and small and temperate planets will be the focus of the next generation telescopes. In this work, we used our Bayesian retrieval algorithm ExoReL$^{\Re}$ to determine the constraints on the atmospheric properties of sub-Neptune planets from observations taken with a HabEx-like telescope. Small and temperate planets may have a non-H$_2$-dominated atmosphere, and therefore, we introduced the compositional analysis technique in our framework to explore the …
The elemental compositions of hot Jupiters are informative relics of planet formation that can help us answer long-standing questions regarding the origin and formation of giant planets. Here, I present the main conclusions from a comprehensive atmospheric retrieval survey of eight hot Jupiters with detectable molecular absorption in their near-infrared transmission spectra. I analyze the eight transmission spectra using the newly-developed, self-consistent atmospheric retrieval framework, SCARLET. Unlike previous methods, SCARLET combines the physical and chemical consistency of complex atmos…
The investigation of planets around other stars began with the study of gas giants, but is now extending to the discovery and characterization of super-Earths and terrestrial planets. Motivated by this observational tide, we survey the basic dynamical principles governing the atmospheric circulation of terrestrial exoplanets, and discuss the interaction of their circulation with the hydrological cycle and global-scale climate feedbacks. Terrestrial exoplanets occupy a wide range of physical and dynamical conditions, only a small fraction of which have yet been explored in detail. Our approach …
Two of TESS's major science goals are to measure masses for 50 planets smaller than 4 Earth radii and to discover high-quality targets for atmospheric characterization efforts. It is important that these two goals are linked by quantifying what precision of mass constraint is required to yield robust atmospheric properties of planets. Here, we address this by conducting retrievals on simulated JWST transmission spectra under various assumptions for the degree of uncertainty in the planet's mass for a representative population of seven planets ranging from terrestrials to warm Neptunes to hot J…
Planets with masses between 0.1 - 10 M_earth are believed to host dense atmospheres. These atmospheres can play an important role on the planet's spin evolution, since thermal atmospheric tides, driven by the host star, may counterbalance gravitational tides. In this work we study the long-term spin evolution of Earth-sized exoplanets. We generalize previous works by including the effect of eccentric orbits and obliquity. We show that under the effect of tides and core-mantle friction, the obliquity of the planets evolve either to 0 or 180 degrees. The rotation of these planets is also expecte…
The emergence of oxygenic photosynthesis created a new niche with dramatic potential to transform energy flow through Earth's biosphere. However, more primitive forms of photosynthesis that fix CO2 into biomass using electrons from reduced species like Fe(II) and H2 instead of water would have competed with Earth's early oxygenic biosphere for essential nutrients. Here, we combine experimental microbiology, genomic analyses, and Earth system modeling to demonstrate that competition for light and nutrients in the surface ocean between oxygenic phototrophs and Fe(II)-oxidizing, anoxygenic photos…
The oxidation of rocky planet surfaces and atmospheres, which arises from the twin forces of stellar nucleosynthesis and gravitational differentiation, is a universal process of key importance to habitability and exoplanet biosignature detection. Here we take a generalized approach to this phenomenon. Using a single parameter to describe redox state, we model the evolution of terrestrial planets around nearby M-stars and the Sun. Our model includes atmospheric photochemistry, diffusion and escape, line-by-line climate calculations and interior thermodynamics and chemistry. In most cases we fin…
We review the field of exoplanetary biosignatures with a main focus upon atmospheric gas-phase species. Due to the paucity of data in Earth-like planetary atmospheres a common approach is to extrapolate knowledge from the Solar System and Early Earth to Earth-like exoplanets. We therefore review the main processes (e.g. atmospheric photochemistry and transport) affecting the most commonly-considered species (e.g. O2, O3, N2O, CH4 etc.) in the context of the modern Earth, Early Earth, the Solar System and Earth-like exoplanets. We consider thereby known abiotic sources for these species in the …
Atmospheric chemistry models have shown molecular oxygen can build up in CO2-dominated atmospheres on potentially habitable exoplanets without an input of life. Existing models typically assume a surface pressure of 1 bar. Here we present model scenarios of CO2-dominated atmospheres with the surface pressure ranging from 0.1 to 10 bars, while keeping the surface temperature at 288 K. We use a one-dimensional photochemistry model to calculate the abundance of O2 and other key species, for outgassing rates ranging from a Venus-like volcanic activity up to 20x Earth-like activity. The model maint…
