Chemistry of Temperate Super-Earth and Mini-Neptune Atmospheric Hazes from Laboratory Experiments
Sarah E. Moran, Sarah M. Hörst, Véronique Vuitton, Chao He, Nikole K. Lewis, Laurène Flandinet, Julianne I. Moses, Nicole North, François-Régis Orthous-Daunay, … 展开作者
Sarah E. Moran, Sarah M. Hörst, Véronique Vuitton, Chao He, Nikole K. Lewis, Laurène Flandinet, Julianne I. Moses, Nicole North, François-Régis Orthous-Daunay, Joshua Sebree, Cédric Wolters, Eliza M. -R. Kempton, Mark S. Marley, Caroline V. Morley, Jeff A. Valenti
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22 pages, 8 figures, 9 tables, accepted for publication in the Planetary Science Journal
Very little experimental work has been done to explore the properties of photochemical hazes formed in atmospheres with very different compositions or temperatures than that of the outer solar system or of early Earth. With extrasolar planet discoveries now numbering thousands, this untapped phase space merits exploration. This study presents measured chemical properties of haze particles produced in laboratory analogues of exoplanet atmospheres. We used very high resolution mass spectrometry to measure the chemical components of solid particles produced in atmospheric chamber experiments. Many complex molecular species with general chemical formulas C$_w$H$_x$N$_y$O$_z$ were detected. We detect molecular formulas of prebiotic interest in the data, including those for the monosaccharide glyceraldehyde, a variety of amino acids and nucleotide bases, and several sugar derivatives. Additionally, the experimental exoplanetary haze analogues exhibit diverse solubility characteristics, which provide insight into the possibility of further chemical or physical alteration of photochemical hazes in super-Earth and mini-Neptune atmospheres. These exoplanet analogue particles can help us better understand chemical atmospheric processes and suggest a possible source of in situ atmospheric prebiotic chemistry on distant worlds.
Photochemical hazes are important opacity sources in temperate exoplanet atmospheres, hindering current observations from characterizing exoplanet atmospheric compositions. The haziness of an atmosphere is determined by the balance between haze production and removal. However, the material-dependent removal physics of the haze particles is currently unknown under exoplanetary conditions. Here we provide experimentally-measured surface energies for a grid of temperate exoplanet hazes to characterize haze removal in exoplanetary atmospheres. We found large variations of surface energies for haze…
Transmission Spectrum of the Benchmark Temperate Exo-Neptune TOI-1231 b
School of Physics and Astronomy, Cardiff University（卡迪夫大学物理与天文学学院）；Institute of Astronomy, University of Cambridge（剑桥大学天文研究所）；Space Telescope Science Institute（太空望远镜科学研究所）；Space Science Institute（空间科学研究所）
Sub-Neptunes (planets with radii between 2 and 4 R$_{\oplus}$) are abundant around M-dwarf stars, yet the atmospheric dynamics of these planets is relatively unexplored. In this paper, we aim to provide a basic underpinning of the dry dynamics of general low mean molecular weight, temperate sub-Neptune atmospheres. We use the ExoFMS GCM with an idealised grey gas radiation scheme to simulate planetary atmospheres with different levels of instellation and rotation rates, using the atmosphere of K2-18b as our control. We find that the atmospheres of tidally-locked, temperate sub-Neptunes have we…
The upcoming James Webb Space Telescope (JWST) promises a generational shift in the study of temperate mini-Neptune atmospheres using transit spectroscopy. High-altitude clouds however threaten to impede their atmospheric characterisation by muting spectral features. In this study, we systematically investigate JWST instrument configurations for characterising cloudy mini-Neptune atmospheres, assessing the importance of instrument choice and wavelength coverage, focusing on NIRISS and NIRSpec. We consider two temperate mini-Neptunes orbiting nearby M dwarfs, K2-18 b and TOI-732 c, with equilib…
Three outstanding physical questions for K2-18 b and other temperate sub-Neptunes
Institute of Astronomy & Astrophysics, Academia Sinica（中央研究院天文及天文物理研究所）；HFML-FELIX（高频磁场设施-费利克斯实验室）；JHU Applied Physics Laboratory（约翰斯·霍普金斯大学应用物理实验室）；ETH Zurich, Institute for Particle Physics & Astrophysics（苏黎世联邦理工学院粒子物理与天体物理研究所）；Deutsches Elektronen-Synchrotron DESY（德国电子同步加速器中心）；Department of Earth and Planetary Sciences, University of California, Riverside（加州大学河滨分校地球与行星科学系）；Virtual Planetary Laboratory, University of Washington（华盛顿大学虚拟行星实验室）；Blue Marble Space Institute of Science（蓝大理石空间科学研究所）；Department of Astrophysics, IMAPP, Radboud University（拉德堡德大学IMAPP天体物理学系）；SRON Leiden（莱顿荷兰太空研究所）
Super-Earths and mini-Neptunes are the most abundant types of planets among the ~3500 confirmed exoplanets, and are expected to exhibit a wide variety of atmospheric compositions. Recent transmission spectra of super-Earths and mini-Neptunes have demonstrated the possibility that exoplanets have haze/cloud layers at high altitudes in their atmospheres. However, the compositions, size distributions, and optical properties of these particles in exoplanet atmospheres are poorly understood. Here, we present the results of experimental laboratory investigations of photochemical haze formation withi…
Numerous solar system atmospheres possess aerosols including the characteristic organic hazes of Titan and Pluto. Haze particles substantially impact atmospheric temperatures structures and may provide organic material to the surface of a world, thereby affecting its habitability. Observations of exoplanet atmospheres suggest the presence of aerosols, especially in cooler (<800 K), smaller (<0.3 times Jupiter's mass) exoplanets. It remains unclear if the aerosols muting the spectroscopic features of exoplanet atmospheres are condensate clouds or photochemical hazes, which is difficult to predi…
The search for signs of life on other worlds has largely focused on terrestrial planets. Recent work, however, argues that life could exist in the atmospheres of temperate sub-Neptunes. Here, we evaluate the usefulness of carbon dioxide isotopologues as evidence of aerial life. Carbon isotopes are of particular interest as metabolic processes preferentially use the lighter $^{12}$C over $^{13}$C. In principle, the upcoming James Webb Space Telescope (JWST) will be able to spectrally resolve the $^{12}$C and $^{13}$C isotopologues of CO$_{2}$, but not CO and CH$_{4}$. We simulated observations …
The recent discovery and initial characterization of sub-Neptune-sized exoplanets that receive stellar irradiance of approximately Earth's raised the prospect of finding habitable planets in the coming decade, because some of these temperate planets may support liquid water oceans if they do not have massive H2/He envelopes and are thus not too hot at the bottom of the envelopes. For planets larger than Earth, and especially planets in the 1.7-3.5 R_Earth population, the mass of the H2/He envelope is typically not sufficiently constrained to assess the potential habitability. Here we show that…
The search for signs of life through the detection of exoplanet atmosphere biosignature gases is gaining momentum. Yet, only a handful of rocky exoplanet atmospheres are suitable for observation with planned next-generation telescopes. To broaden prospects, we describe the possibilities for an aerial, liquid water cloud-based biosphere in the atmospheres of sub Neptune-sized temperate exoplanets, those receiving Earth-like irradiation from their host stars. One such planet is known (K2-18b) and other candidates are being followed up. Sub Neptunes are common and easier to study observationally …
Three-dimensional transport-induced chemistry on temperate sub-Neptune K2-18b, Part II: the combined effects of atmospheric dynamics and chemical reactions
Max Planck Institute for Astronomy（马克斯·普朗克天文学研究所）；Department of Atmospheric and Oceanic Sciences, School of Physics, Peking University（北京大学物理学院大气与海洋科学系）；Department of Physics and Astronomy, Faculty of Environment, Science and Economy, University of Exeter（埃克塞特大学环境、科学与经济学院物理与天文系）
Hydrocarbon Hazes on Temperate sub-Neptune K2-18b supported by data from the James Webb Space Telescope
University College London（伦敦大学学院）；Universite de Reims Champagne Ardenne（兰斯香槟阿登大学）；King’s College London（伦敦国王学院）；Osservatorio Astronomico di Palermo（巴勒莫天文台）
Atmospheric observations of mini-Neptunes orbiting M-dwarfs are beginning to provide constraints on their chemical and thermal properties, while also providing clues about their interiors and potential surfaces. With their relatively large scale heights and large planet-star contrasts, mini-Neptunes are currently ideal targets towards the goal of characterising temperate low-mass exoplanets. Understanding the thermal structures and spectral appearances of mini-Neptunes is important to understand various aspects of their atmospheres, including radiative/convective energy transport, boundary con…
Wet Removal and Cloud Enhancement: The Microphysics of Cloud-Haze Interactions on Sub-Neptunes
Department of Astronomy & Astrophysics, the University of Chicago（芝加哥大学天文与天体物理系）；Max Planck Institute for Astronomy, 69117 Heidelberg, Germany（马克斯·普朗克天文研究所）；Earth & Planets Laboratory, Carnegie Institution for Science, 5241 Broad Branch Road NW, Washington, DC 20015, USA（卡内基科学研究所地球与行星实验室）
Hubble (HST) spectroscopic transit observations of the temperate sub-Neptune K2-18b were interpreted as the presence of water vapour with potential water clouds. 1D modelling studies also predict the formation of water clouds at some conditions. However, such models cannot predict the cloud cover, driven by atmospheric dynamics and thermal contrasts, and thus their real impact on spectra. The main goal of this study is to understand the formation, distribution and observational consequences of water clouds on K2-18b and other temperate sub-Neptunes. We simulated the atmospheric dynamics, water…
UV radiation can induce photochemical processes in exoplanet atmospheres and produce haze particles. Recent observations suggest that haze and/or cloud layers could be present in the upper atmospheres of exoplanets. Haze particles play an important role in planetary atmospheres and may provide a source of organic material to the surface which may impact the origin or evolution of life. However, very little information is known about photochemical processes in cool, high-metallicity exoplanetary atmospheres. Previously, we investigated haze formation and particle size distribution in laboratory…
Hydrolyzed Hazes on Water-rich Exoplanets: Optical Constants and Detectability
Johns Hopkins University（约翰斯·霍普金斯大学）；NASA Goddard Space Flight Center（美国宇航局戈达德太空飞行中心）；Space Telescope Science Institute（空间望远镜科学研究所）；University of Science and Technology of China（中国科学技术大学）；Univ. Grenoble Alpes（格勒诺布尔阿尔卑斯大学）
We present a reanalysis (using the Minnaert limb-darkening approximation) of visible/near-infrared (0.3 - 2.5 micron) observations of Uranus and Neptune made by several instruments. We find a common model of the vertical aerosol distribution that is consistent with the observed reflectivity spectra of both planets, consisting of: 1) a deep aerosol layer with a base pressure > 5-7 bar, assumed to be composed of a mixture of H2S ice and photochemical haze; 2) a layer of photochemical haze/ice, coincident with a layer of high static stability at the methane condensation level at 1-2 bar; and 3) a…
TOI-270d is a temperate sub-Neptune discovered by the Transiting Exoplanet Survey Satellite (TESS) around a bright (J=9.1mag) M3V host star. With an approximate radius of 2RE and equilibrium temperature of 350K, TOI-270d is one of the most promising small exoplanets for atmospheric characterisation using transit spectroscopy. Here we present a primary transit observation of TOI-270d made with the Hubble Space Telescope (HST) Wide Field Camera 3 (WFC3) spectrograph across the 1.126-1.644 micron wavelength range, and a 95% credible upper limit of $8.2 \times 10^{-14}$ erg s$^{-1}$ cm$^{-2}$ A$^{…
Direct imaging of widely separated exoplanets from space will obtain their reflected light spectra and measure their atmospheric properties, and small and temperate planets will be the focus of the next generation telescopes. In this work, we used our Bayesian retrieval algorithm ExoReL$^{\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 Mid-Infrared Transmission Spectrum of the Temperate Sub-Neptune TOI-270 d
Space Telescope Science Institute（太空望远镜科学研究所）；Institute of Astronomy, University of Cambridge（剑桥大学天文研究所）；School of Physics and Astronomy, Cardiff University（卡迪夫大学物理与天文学学院）；Space Science Institute（太空科学研究所）
The observed mass-radius relationship of low-mass planets informs our understanding of their composition and evolution. Recent discoveries of low mass, large radii objects ("super-puffs") have challenged theories of planet formation and atmospheric loss, as their high inferred gas masses make them vulnerable to runaway accretion and hydrodynamic escape. Here we propose that high altitude photochemical hazes could enhance the observed radii of low-mass planets and explain the nature of super-puffs. We construct model atmospheres in radiative-convective equilibrium and compute rates of atmospher…
Studies of the atmospheres of hot Jupiters reveal a diversity of atmospheric composition and haze properties. Similar studies on individual smaller, temperate planets are rare due to the inherent difficulty of the observations and also to the average faintness of their host stars. To investigate their ensemble atmospheric properties, we construct a sample of 28 similar planets, all possess equilibrium temperature within 300-500K, have similar size (1-3 R_e), and orbit early M dwarfs and late K dwarfs with effective temperatures within a few hundred Kelvin of one another. In addition, NASA's Ke…
Precise atmospheric observations have been made for a growing sample of warm Neptunes. Here we investigate the correlations between these observations and a large number of system parameters to show that, at 95% confidence, the amplitude of a warm Neptune's spectral features in transmission correlates with either its equilibrium temperature (T_eq) or its bulk H/He mass fraction (f_HHe) --- in addition to the standard kT/mg scaling. These correlations could indicate either more optically-thick, photochemically-produced hazes at lower T_eq and/or higher-metallicity atmospheres for planets with s…
Ultraviolet Radiation Effects on the Optical Properties of Water-Dominated Exoplanet Hazes
University of Arizona（亚利桑那大学）；Space Telescope Science Institute（太空望远镜科学研究所）；Jet Propulsion Laboratory（喷气推进实验室）；University of Science and Technology of China（中国科学技术大学）；Johns Hopkins University（约翰斯·霍普金斯大学）；Planetary Science Institute（行星科学研究所）；Cornell University（康奈尔大学）
Exoplanets with relatively clear atmospheres are prime targets for detailed studies of chemical compositions and abundances in their atmospheres. Alkali metals have long been suggested to exhibit broad wings due to pressure broadening, but most of the alkali detections only show very narrow absorption cores, probably because of the presence of clouds. We report the strong detection of the pressure-broadened spectral profiles of Na, K, and Li absorption in the atmosphere of the super-Neptune WASP-127b, at 4.1$σ$, 5.0$σ$, and 3.4$σ$, respectively. We performed a spectral retrieval modeling on th…
We report the discovery and validation of two TESS exoplanets orbiting faint M dwarfs: TOI-4479b and TOI-2081b. We have jointly analyzed space (TESS mission) and ground based (MuSCAT2, MuSCAT3 and SINISTRO instruments) lightcurves using our multi-color photometry transit analysis pipeline. This allowed us to compute contamination limits for both candidates and validate them as planet-sized companions. We found TOI-4479b to be a sub-Neptune-sized planet ($R_{p}=2.82^{+0.65}_{-0.63}~\rm R_{\oplus}$) and TOI-2081b to be a super-Earth-sized planet ($R_{p}=2.04^{+0.50}_{-0.54}~\rm R_{\oplus}$). Fur…
The atmospheric composition of exoplanets with masses between 2 and 10 M$_\oplus$ is poorly understood. In that regard, the sub-Neptune K2-18b, which is subject to Earth-like stellar irradiation, offers a valuable opportunity for the characterisation of such atmospheres. Previous analyses of its transmission spectrum from the Kepler, Hubble (HST), and Spitzer space telescopes data using both retrieval algorithms and forward-modelling suggest the presence of H$_2$O and an H$_2$--He atmosphere, but have not detected other gases, such as CH$_4$. We present simulations of the atmosphere of K2-18 b…
We present the discovery of three modestly-irradiated, roughly Neptune-mass planets orbiting three nearby Solar-type stars. HD 42618 b has a minimum mass of $15.4 \pm 2.4$ M$_{\oplus}$, a semi-major axis of 0.55 AU, an equilibrium temperature of 337 K, and is the first planet discovered to orbit the solar analogue host star, HD 42618. We also discover new planets orbiting the known exoplanet host stars HD 164922 and HD 143761 ($ρ$ CrB). The new planet orbiting HD 164922 has a minimum mass of $12.9 \pm 1.6$ M$_{\oplus}$ and orbits interior to the previously known Jovian mass planet orbiting at …
We report the discovery and characterization of TOI-1759~b, a temperate (400 K) sub-Neptune-sized exoplanet orbiting the M~dwarf TOI-1759 (TIC 408636441). TOI-1759 b was observed by TESS to transit on sectors 16, 17 and 24, with only one transit observed per sector, creating an ambiguity on the orbital period of the planet candidate. Ground-based photometric observations, combined with radial-velocity measurements obtained with the CARMENES spectrograph, confirm an actual period of $18.85019 \pm 0.00014$ d. A joint analysis of all available photometry and radial velocities reveal a radius of $…
Quantified Estimation of Molecular Detections across Different Classes of Neptunian Atmospheres Using Cross-Correlation Spectroscopy: Prospects for Future Extremely Large Telescopes with High-Resolution Spectrographs
Triton is the largest moon of the Neptune system and possesses a thin nitrogen atmosphere with trace amounts of carbon monoxide and methane, making it of similar composition to that of the dwarf planet Pluto. Like Pluto and Saturn's moon Titan, Triton has a haze layer thought to be composed of organics formed through photochemistry. Here, we perform atmospheric chamber experiments of 0.5% carbon monoxide and 0.2% methane in molecular nitrogen at 90 K and 1 mbar to generate Triton haze analogues. We then characterize the physical and chemical properties of these particles. We measure their prod…
One of the most profound questions about the newly discovered class of low-density super-Earths is whether these exoplanets are predominately H2-dominated mini-Neptunes or volatile-rich worlds with gas envelopes dominated by H2O, CO2, CO, CH4, or N2. Transit observations of the super-Earth GJ 1214b rule out a cloud-free H2-dominated atmosphere, but are not able to determine whether the lack of deep spectral features is due to high-altitude clouds or the presence of a high mean molecular mass atmosphere. Here, we demonstrate that one can unambiguously distinguish between cloudy mini-Neptunes an…
The atmosphere of K2-18 b: The role of hazes, clouds and photoelectrons
Université Reims Champagne Ardenne（兰斯香槟阿登大学）；Institut d’Astrophysique de Paris, UMR CNRS 7095（巴黎天体物理研究所）；University College London（伦敦大学学院）；King’s College London（伦敦国王学院）；Observatoire de Paris, Université PSL, Sorbonne Université, Université Paris Cité, CY Cergy Paris Université, CNRS（巴黎天文台）
We report the discovery of a transiting, temperate, Neptune-sized exoplanet orbiting the nearby ($d$ = 27.5 pc), M3V star TOI-1231 (NLTT 24399, L 248-27, 2MASS J10265947-5228099). The planet was detected using photometric data from the Transiting Exoplanet Survey Satellite and followed up with observations from the Las Cumbres Observatory and the Antarctica Search for Transiting ExoPlanets program. Combining the photometric data sets, we find that the newly discovered planet has a radius of 3.65$^{+0.16}_{-0.15}$ R$_{\oplus}$, and an orbital period of 24.246 days. Radial velocity measurements …
Characterizing planetary systems with SPIRou: a temperate sub-Neptune exoplanet orbiting the nearby fully-convective star GJ 1289 and a candidate around GJ 3378
