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 cloud formation and spectra of K2-18b for H2-dominated atmosphere using a 3D GCM. We analysed the impact of atmospheric composition (with metallicity from 1*solar to 1000*solar), concentration of cloud condensation nuclei and planetary rotation rate. Assuming that K2-18b has a synchronous rotation, we show that the atmospheric circulation in the upper atmosphere essentially corresponds to a symmetric day-to-night circulation. This regime preferentially leads to cloud formation at the substellar point or at the terminator. Clouds form for metallicity >100*solar with relatively large particles. For 100-300*solar metallicity, the cloud fraction at the terminators is small with a limited impact on transit spectra. For 1000*solar metallicity, very thick clouds form at the terminator. The cloud distribution appears very sensitive to the concentration of CCN and to the planetary rotation rate. Fitting HST transit data with our simulated spectra suggests a metallicity of ~100-300*solar. In addition, we found that the cloud fraction at the terminator can be highly variable, leading to a potential variability in transit spectra. This effect could be common on cloudy exoplanets and could be detectable with multiple transit observations.
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）
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）
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…
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$^{…
Hydrocarbon Hazes on Temperate sub-Neptune K2-18b supported by data from the James Webb Space Telescope
University College London（伦敦大学学院）；Universite de Reims Champagne Ardenne（兰斯香槟阿登大学）；King’s College London（伦敦国王学院）；Osservatorio Astronomico di Palermo（巴勒莫天文台）
The Mid-Infrared Transmission Spectrum of the Temperate Sub-Neptune TOI-270 d
Space Telescope Science Institute（太空望远镜科学研究所）；Institute of Astronomy, University of Cambridge（剑桥大学天文研究所）；School of Physics and Astronomy, Cardiff University（卡迪夫大学物理与天文学学院）；Space Science Institute（太空科学研究所）
Studies of the atmospheres of hot Jupiters reveal a diversity of atmospheric composition and haze properties. Similar studies on individual smaller, temperate planets are rare due to the inherent difficulty of the observations and also to the average faintness of their host stars. To investigate their ensemble atmospheric properties, we construct a sample of 28 similar planets, all possess equilibrium temperature within 300-500K, have similar size (1-3 R_e), and orbit early M dwarfs and late K dwarfs with effective temperatures within a few hundred Kelvin of one another. In addition, NASA's Ke…
The atmospheric composition of exoplanets with masses between 2 and 10 M$_\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 $…
Habitable planets are commonly imagined to be temperate planets like Earth, with areas of open ocean and warm land. In contrast, planets in snowball states, where oceans are entirely ice-covered, are believed to be inhospitable. However, we show using a general circulation model that terrestrial planets in the inner habitable zone are able to support large unfrozen areas of land while in a snowball state. Due to their lower albedo, these unfrozen regions reach summer temperatures in excess of 10 $^\circ$Celsius. Such conditions permit CO$_2$ weathering, suggesting that continental weathering c…
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 …
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. Man…
Detection of water vapor in the atmosphere of temperate rocky exoplanets would be a major milestone on the path towards characterization of exoplanet habitability. Past modeling work has shown that cloud formation may prevent the detection of water vapor on Earth-like planets with surface oceans using the James Webb Space Telescope (JWST). Here we analyze the potential for atmospheric detection of H2O on a different class of targets: arid planets. Using transit spectrum simulations, we show that atmospheric H2O may be easier to be detected on arid planets with cold-trapped ice deposits on the …
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…
H$_2$O is a key molecule in characterizing atmospheres of temperate terrestrial planets, and observations of transmission spectra are expected to play a primary role in detecting its signatures in the near future. The detectability of H$_2$O absorption features in transmission spectra depends on the abundance of water vapor in the upper part of the atmosphere. We study the three-dimensional distribution of atmospheric H$_2$O for synchronously rotating Earth-sized aquaplanets using the general circulation model (GCM) ROCKE-3D, and examine the effects of total incident flux and stellar spectral …
Interior models of Uranus and Neptune often assume discrete layers, but sharp interfaces are expected only if major constituents are immiscible. Diffuse interfaces could arise if accretion favored a central concentration of the least volatile constituents (also incidentally the most dense); compositional gradients arising in such a structure would likely inhibit convection. Currently, two lines of evidence suggest possible hydrogen-water immiscibility in ice giant interiors. The first arises from crude extrapolation of the experimental hydrogen-water critical curve to $\sim 3$ GPa (Bali et al.…
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…
Characterizing planetary systems with SPIRou: a temperate sub-Neptune exoplanet orbiting the nearby fully-convective star GJ 1289 and a candidate around GJ 3378
Understanding the internal structures of planets with a large H$_2$O component is important for the characterisation of sub-Neptune planets. The finding that the mini-Neptune K2-18b could host a liquid water ocean beneath a mostly hydrogen envelope motivates a detailed examination of the phase structures of water-rich planets. To this end, we present new internal structure models for super-Earths and mini-Neptunes that enable detailed characterisation of a planet's water component. We use our models to explore the possible phase structures of water worlds and find that a diverse range of inter…
With high-resolution spectroscopy we can study exoplanet atmospheres and learn about their chemical composition, temperature profiles, and presence of clouds and winds, mainly in hot, giant planets. State-of-the-art instrumentation is pushing these studies towards smaller exoplanets. Of special interest are the few planets in the 'Neptune desert', a lack of Neptune-size planets in close orbits around their hosts. Here, we assess the presence of water in one such planet, the bloated super-Neptune WASP-166 b, which orbits an F9-type star in a short orbit of 5.4 days. Despite its close-in orbit, …
A temperate sub-Neptune transiting the M4 Dwarf TOI-210 identified by NIRPS and TESS -- Uncovering hidden M-dwarf planetary systems in the near-infrared
Observatoire de Genève, Département d’Astronomie, Université de Genève（日内瓦天文台，天文学系，日内瓦大学）；Institut Trottier de recherche sur les exoplanètes, Département de Physique, Université de Montréal（特罗蒂埃系外行星研究所，物理系，蒙特利尔大学）；Department of Physics & Astronomy, McMaster University（麦克马斯特大学物理与天文学系）；Origins Institute, McMaster University（麦克马斯特大学起源研究所）；Observatoire du Mont-Mégantic（蒙梅甘蒂克天文台）；Departamento de Física Teórica e Experimental, Universidade Federal do Rio Grande do Norte（北里奥格兰德联邦大学理论与实验物理学系）；Univ. Grenoble Alpes, CNRS, IPAG（格勒诺布尔阿尔卑斯大学，法国国家科学研究中心，IPAG）
We investigate the deep water abundance of Neptune using a simple 2-component (core + envelope) toy model. The free parameters of the model are the total mass of heavy elements in the planet (Z), the mass fraction of Z in the envelope (f_env), and the D/H ratio of the accreted building blocks (D/H_build ). We systematically search the allowed parameter space on a grid and constrain it using Neptune's bulk carbon abundance, D/H ratio, and interior structure models. Assuming solar C/O ratio and cometary D/H for the accreted building blocks forming the planet, we can fit all of median ~ 7%), and …
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 …
Water clouds are expected to form on Y dwarfs and giant planets with equilibrium temperatures near or below that of Earth, drastically altering their atmospheric compositions and their albedos and thermal emission spectra. Here we use the 1D Community Aerosol and Radiation Model for Atmospheres (CARMA) to investigate the microphysics of water clouds on cool substellar worlds to constrain their typical particle sizes and vertical extent, taking into consideration nucleation and condensation, which have not been considered in detail for water clouds in H/He atmospheres. We compute a small grid o…
The internal heat flows of both Uranus and Neptune remain major outstanding problems in planetary science. Uranus' surprisingly cold effective temperature is inconsistent with adiabatic thermal evolution models, while Neptune's substantial internal heat flow is twice its received insolation. In this work we constrain the magnitude of influence condensation, including latent heat and inhibition of convection, can have on the thermal evolution of these bodies. We find that while the effect can be significant, it is insufficient to solve the Uranus faintness problem on its own. Self-consistently …
We investigate the potential for the James Webb Space Telescope (JWST) to detect and characterize the atmospheres of the sub-Neptunian exoplanets in the TOI-270 system. Sub-Neptunes are considered more likely to be water worlds than gas dwarfs. We model their atmospheres using three atmospheric compositions - two examples of hydrogen-dominated atmospheres and a water-dominated atmosphere. We then simulate the infrared transmission spectra of these atmospheres for JWST instrument modes optimized for transit observation of exoplanet atmospheres : NIRISS, NIRSpec and MIRI. We then predict the obs…
