Absorption lines from water, methane and carbon monoxide are detected in the atmosphere of exoplanet HR8799b. A medium-resolution spectrum presented here shows well-resolved and easily identified spectral features from all three molecules across the K band. The majority of the lines are produced by CO and H2O, but several lines clearly belong to CH4. Comparisons between these data and atmosphere models covering a range of temperatures and gravities yield log mole fractions of H2O between -3.09 and -3.91, CO between -3.30 and -3.72 and CH4 between -5.06 and -5.85. More precise mole fractions are obtained for each temperature and gravity studied. A reanalysis of H-band data, previously obtained at similar spectral resolution, results in a nearly identical water abundance as determined from the K-band spectrum. The methane abundance is shown to be sensitive to vertical mixing and indicates an eddy diffusion coefficient in the range of 10^6 to 10^8 cm^2 s^-1, comparable to mixing in the deep troposphere of Jupiter. The model comparisons also indicate a C/O between ~ 0.58 and 0.7, encompassing previous estimates for a second planet in the same system, HR8799c. Super-stellar C/O could indicate planet formation by core-accretion, however, the range of possible C/O for these planets (and the star) is currently too large to comment strongly on planet formation. More precise values of the bulk properties (e.g., effective temperature and surface gravity) are needed for improved abundance estimates.
Context. In 2015, Barman et al. (ApJ, 804, 61) presented detections of absorption from water, carbon monoxide, and methane in the atmosphere of the directly imaged exoplanet HR8799b using integral field spectroscopy (IFS) with OSIRIS on the Keck II telescope. We recently devised a new method to analyse IFU data, called molecule mapping, searching for high-frequency signatures of particular molecules in an IFU data cube. Aims. The aim of this paper is to use the molecule mapping technique to search for the previously detected spectral signatures in HR8799b using the same data, allowing a compar…
Methane has been proposed as an exoplanet biosignature. Imminent observations with the James Webb Space Telescope may enable methane detections on potentially habitable exoplanets, so it is essential to assess in what planetary contexts methane is a compelling biosignature. Methane's short photochemical lifetime in terrestrial planet atmospheres implies that abundant methane requires large replenishment fluxes. While methane can be produced by a variety of abiotic mechanisms such as outgassing, serpentinizing reactions, and impacts, we argue that, in contrast to an Earth-like biosphere, known …
Observations suggest an abundance of water and paucity of methane in the majority of observed exoplanetary atmospheres. We isolate the effect of atmospheric processes to investigate possible causes. Previously, we studied the effect of effective temperature, surface gravity, metallicity, carbon-to-oxygen ratio, and stellar type assuming cloud-free thermochemical equilibrium and disequilibrium chemistry. However, under these assumptions, methane remains a persisting spectral feature in the transmission spectra of exoplanets over a certain parameter space, the Methane Valley. In this work we inv…
We present detections of methane in R of $\sim$1300, L band spectra of VHS 1256 b and PSO 318.5, two low gravity, red, late L dwarfs that share the same colors as the HR 8799 planets. These spectra reveal shallow methane features, which indicate VHS 1256 b and PSO 318.5 have photospheres that are out of chemical equilibrium. Directly imaged exoplanets usually have redder near infrared colors than the field-age population of brown dwarfs on a color magnitude diagram. These objects along the L to T transition show reduced methane absorption and evidence of photospheric clouds. Compared to the H …
Constraints on the Atmospheric Composition of 2002 XV$_{93}$ from JWST Spectroscopy
Space Telescope Science Institute（太空望远镜科学研究所）；Southwest Research Institute（西南研究院）；LIRA, Observatoire de Paris, Université PSL, CNRS, Sorbonne Université, Université Paris Cité（拉格朗日天体物理实验室，巴黎天文台，巴黎文理研究大学，法国国家科学研究中心，索邦大学，巴黎西岱大学）
Triton is the largest moon of the Neptune system and possesses a thin nitrogen atmosphere with trace amounts of carbon monoxide and methane, making it of similar composition to that of the dwarf planet Pluto. Like Pluto and Saturn's moon Titan, Triton has a haze layer thought to be composed of organics formed through photochemistry. Here, we perform atmospheric chamber experiments of 0.5% carbon monoxide and 0.2% methane in molecular nitrogen at 90 K and 1 mbar to generate Triton haze analogues. We then characterize the physical and chemical properties of these particles. We measure their prod…
With no analogues in the Solar System, the discovery of thousands of exoplanets with masses and radii intermediate between Earth and Neptune was one of the big surprises of exoplanet science. These super-Earths and sub-Neptunes likely represent the most common outcome of planet formation. Mass and radius measurements indicate a diversity in bulk composition much wider than for gas giants; however, direct spectroscopic detections of molecular absorption and constraints on the gas mixing ratios have largely remained limited to planets more massive than Neptune. Here, we analyze a combined Hubble…
We present new 3.6 and 4.5 micron secondary eclipse measurements for five cool (less than approximately 1000 K) transiting gas giant planets: HAT-P-15b, HAT-P-17b, HAT-P-18b, HAT-P-26b, and WASP-69b. We detect eclipses in at least one bandpass for all planets except HAT-P-15b. We confirm and refine the orbital eccentricity of HAT-P-17b, which is also the only planet in our sample with a known outer companion. We compare our measured eclipse depths in these two bands, which are sensitive to the relative abundances of methane versus carbon monoxide and carbon dioxide, respectively, to prediction…
The disequilibrium combination of abundant methane and carbon dioxide has been proposed as a promising exoplanet biosignature that is readily detectable with upcoming telescopes such as the James Webb Space Telescope. However, few studies have explored the possibility of non-biological CH4 and CO2 and related contextual clues. Here, we investigate whether magmatic volcanic outgassing on terrestrial planets can produce atmospheric CH4 and CO2 with a thermodynamic model. Our model suggests that volcanoes are unlikely to produce CH4 fluxes comparable to biological fluxes. Improbable cases where v…
Chemical disequilibrium in planetary atmospheres has been proposed as a generalized method for detecting life on exoplanets through remote spectroscopy. Among solar system planets with substantial atmospheres, the modern Earth has the largest thermodynamic chemical disequilibrium due to the presence of life. However, how this disequilibrium changed over time and, in particular, the biogenic disequilibria maintained in the anoxic Archean or less oxic Proterozoic eons are unknown. We calculate the atmosphere-ocean disequilibrium in the Precambrian using conservative proxy- and model-based estima…
Giant exoplanets located >1 AU away from their parent stars have atmospheric environments cold enough for water and/or ammonia clouds. We have developed a new equilibrium cloud and reflected light spectrum model, ExoREL, for widely separated giant exoplanets. The model includes the dissolution of ammonia in liquid water cloud droplets, an effect studied for the first time for exoplanets. While preserving the causal relationship between temperature and cloud condensation, ExoREL is simple and fast to enable efficient exploration of parameter space. Using the model, we find that the mixing ratio…
JWST reveals anomalously enhanced methane outgassing from below Chiron's water ice and carbon dioxide bearing surface
Space Telescope Science Institute（太空望远镜科学研究所）；Southwest Research Institute（西南研究院）；Université Lyon 1（里昂第一大学）；NASA Goddard Space Flight Center（美国国家航空航天局戈达德太空飞行中心）；Université Paris-Saclay（巴黎萨克雷大学）；Northern Arizona University（北亚利桑那大学）；University of Oviedo（奥维耶多大学）；University of Central Florida（中佛罗里达大学）
Oxygen and methane are considered to be the canonical biosignatures of modern Earth, and the simultaneous detection of these gases in a planetary atmosphere is an especially strong biosignature. However, these gases may be challenging to detect together in the planetary atmospheres because photochemical oxygen radicals destroy methane. Previous work has shown that the photochemical lifetime of methane in oxygenated atmospheres is longer around M dwarfs, but M dwarf planet habitability may be hindered by extreme stellar activity and evolution. Here, we use a 1-D photochemical-climate model to s…
Hydrogen cyanide (HCN) is a key feedstock molecule for the production of life's building blocks. The formation of HCN in an N$_2$-rich atmospheres requires first that the triple bond between N$\equiv$N be severed, and then that the atomic nitrogen find a carbon atom. These two tasks can be accomplished via photochemistry, lightning, impacts, or volcanism. The key requirements for producing appreciable amounts of HCN are the free availability of N$_2$ and a local carbon to oxygen ratio of C/O $\geq 1$. We discuss the chemical mechanisms by which HCN can be formed and destroyed on rocky exoplane…
The super-Neptune exoplanet WASP-107b is an exciting target for atmosphere characterization. It has an unusually large atmospheric scale height and a small, bright host star, raising the possibility of precise constraints on its current nature and formation history. We report the first atmospheric study of WASP-107b, a Hubble Space Telescope measurement of its near-infrared transmission spectrum. We determined the planet's composition with two techniques: atmospheric retrieval based on the transmission spectrum and interior structure modeling based on the observed mass and radius. The interior…
The search for life on exoplanets is motivated by the universal ways in which life could modify its planetary environment. Atmospheric gases such as oxygen and methane are promising candidates for such environmental modification due to the evolutionary benefits their production would confer. However, confirming that these gases are produced by life, rather than by geochemical or astrophysical processes, will require a thorough understanding of planetary context, including the expected counterfactual atmospheric evolution for lifeless planets. Here, we evaluate current understanding of planetar…
In their Letter, Tsiaras et al. reported the detection of water vapour in the atmosphere of K2-18 b, an exoplanet of 7 to 10 Earth masses located in the habitable zone of an M-dwarf star. The detection is based on an absorption feature seen at 1.4 $μ$m in observations of the transiting exoplanet with the Hubble Space Telescope/Wide Field Camera 3. We have simulated the mean temperature structure and composition of K2-18b using a radiative-convective equilibrium model and we present here the corresponding transit spectroscopy calculations. We argue that the reported absorption is most likely du…
Mapping the Cloud-Driven Atmospheric Dynamics & Chemistry of an Isolated Exoplanet Analog with Harmonic Signatures
United States Air Force Academy（美国空军学院）；Boston University（波士顿大学）；School of Physics, Trinity College Dublin, The University of Dublin（都柏林三一学院物理学院）；Départment de Physique, Université de Montréal, IREX（蒙特利尔大学物理系）；Observatoire du Mont-Mégantic, Université de Montréal（蒙特利尔大学蒙梅格尼天文台）；McGill University（麦吉尔大学）
The James Webb Space Telescope (JWST) may be capable of finding biogenic gases in the atmospheres of habitable exoplanets around low mass stars. Considerable attention has been given to the detectability of biogenic oxygen, which could be found using an ozone proxy, but ozone detection with JWST will be extremely challenging, even for the most favorable targets. Here, we investigate the detectability of biosignatures in anoxic atmospheres analogous to those that likely existed on the early Earth. Arguably, such anoxic biosignatures could be more prevalent than oxygen biosignatures if life exis…
Study of exoplanets is one of the main goals of present research in planetary sciences and astrobiology. Analysis of huge planetary data from space missions such as CoRoT and Kepler is directed ultimately at finding a planet similar to Earth -- the Earth's twin, and answering the question of potential exo-habitability. The Earth Similarity Index (ESI) is a first step in this quest, ranging from 1 (Earth) to 0 (totally dissimilar to Earth). It was defined for the four physical parameters of a planet: radius, density, escape velocity and surface temperature. The ESI is further sub-divided into i…
A carbon-to-oxygen ratio (C/O) of around unity is believed to act as a natural separator of water- and methane-dominated spectra when characterizing exoplanet atmospheres. In this paper we quantify the C/O ratios at which this separation occurs by calculating a large self-consistent grid of cloud-free atmospheric models in chemical equilibrium, using the latest version of petitCODE. Our study covers a broad range of parameter space: 400 K$<$T$_{eff}<$2600 K, 2.0$<$log(g)$<$5.0, -1.0$<$[Fe/H]$<$2.0, 0.25$<$C/O$<$1.25, and stellar types from M to F. We make the synthetic transmission and emissio…
High Resolution Optical Methane Linelist from observations of Titan for Cross-Correlation studies
University College London（伦敦大学学院）；Instituto de Astrofísica e Ciências do Espaço, Universidade de Lisboa（里斯本大学空间科学与天体物理研究所）；Departamento de Física, Faculdade de Ciências, Universidade de Lisboa（里斯本大学理学院物理系）；Bard College（巴德学院）；NMES Faculty, King’s College London（伦敦国王学院非医学教育系）
Metal Oxide Clusters in Gas Giant Exoplanet Atmospheres
Space Research Institute, Austrian Academy of Sciences（奥地利科学院空间研究所）；Institute of Theoretical and Computational Physics, TU Graz（格拉茨工业大学理论与计算物理研究所）；Swiss Federal Institute of Intellectual Property (IGE)（瑞士联邦知识产权局）；Space Research and Planetary Sciences, Physics Institute, University of Bern（伯尔尼大学物理学院空间研究与行星科学系）
We have investigated the information content in reflected-starlight spectra of exoplanets. We specify our analysis to Barnard's Star b candidate super-Earth, for which we assume a radius 0.6 times that of Neptune, an atmosphere dominated by H$_2$-He, and a CH$_4$ volume mixing ratio of 5$\cdot$10$^{-3}$. The main conclusions of our study are however planet-independent. We set up a model of the exoplanet described by seven parameters including its radius, atmospheric methane abundance and basic properties of a cloud layer. We generate synthetic spectra at zero phase (full disk illumination) fro…
Aerosols are common in the atmospheres of exoplanets across a wide swath of temperatures, masses, and ages. These aerosols strongly impact observations of transmitted, reflected, and emitted light from exoplanets, obfuscating our understanding of exoplanet thermal structure and composition. Knowing the dominant aerosol composition would facilitate interpretations of exoplanet observations and theoretical understanding of their atmospheres. A variety of compositions have been proposed, including metal oxides and sulphides, iron, chromium, sulphur, and hydrocarbons. However, the relative contrib…
WFIRST-CGI is a NASA technology demonstration mission that is charged with demonstrating key technologies for future exo-Earth imaging missions in space. In the process, it will obtain images and low-resolution spectra of a handful to a dozen extrasolar planets and possibly protoplanetary disks. Its unprecedented contrast levels in the optical will provide astronomers with their first direct look at mature, Jupiter sized planets at moderate separations. This paper addresses the question: what science can be done with such data? An analytic noise model, which is informed by the ongoing engineer…
The study of exoplanetary atmospheres is key to understand the differences between their physical, chemical and dynamical processes. Up to now, the bulk of atmospheric characterization analysis has been conducted on transiting planets. On some sufficiently bright targets, high-resolution spectroscopy (HRS) has also been successfully tested for non-transiting planets. We study the dayside of the non-transiting planet HD 102195b using the GIANO spectrograph mounted at TNG, demonstrating the feasibility of atmospheric characterization measurements and molecular detection for non-transiting planet…
