Earth-like Exoplanets Research Articles

TRAPPIST-1 d: Exo-Venus, Exo-Earth, or Exo-Dead?

Abstract TRAPPIST-1 d is generally assumed to be at the boundary between a Venus-like world and an Earth-like world, although recently published works on TRAPPIST-1 b and c raise concerns that TRAPPIST-1 d may be similarly devoid of a substantial atmosphere. TRAPPIST-1 d is also relatively understudied in comparison with TRAPPIST-1 e. The latter has generally appeared to be within the habitable zone of most atmospheric modeling studies. Assuming that TRAPPIST-1 d still retains a substantial atmosphere, we demonstrate via a series of 3D general circulation model experiments using a dynamic ocean that the planet could reside within the habitable zone in a narrow parameter space. At the same time, it could also be an exo-Venus- or exo-Dead-type world or in transition between between one of these. Studies like this can help distinguish between these types of worlds.

Effects of Planetary Mass Uncertainties on the Interpretation of the Reflectance Spectra of Earth-like Exoplanets

Abstract Atmospheric characterization of Earth-like exoplanets through reflected light spectroscopy is a key goal for upcoming direct imaging missions. A critical challenge in this endeavor is the accurate determination of planetary mass, which may influence the measurement of atmospheric compositions and the identification of potential biosignatures. In this study, we used the Bayesian retrieval framework EXOREL ℜ to investigate the impact of planetary mass uncertainties on the atmospheric characterization of terrestrial exoplanets observed in reflected light. Our results indicate that precise prior knowledge of the planetary mass can be crucial for accurate atmospheric retrievals if clouds are present in the atmosphere. When the planetary mass is known within 10% uncertainty, our retrievals successfully identified the background atmospheric gas and accurately constrained atmospheric parameters together with clouds. However, with less constrained or unknown planetary mass, we observed significant biases, particularly in the misidentification of the dominant atmospheric gas. For instance, the dominant gas was incorrectly identified as oxygen for a modern Earthlike planet or carbon dioxide for an Archean Earth–like planet, potentially leading to erroneous assessments of planetary habitability and biosignatures. These biases arise because the uncertainties in planetary mass affect the determination of surface gravity and atmospheric scale height, leading the retrieval algorithm to compensate by adjusting the atmospheric composition. Our findings emphasize the importance of achieving precise mass measurements—ideally within 10% uncertainty—through methods such as extreme precision radial velocity or astrometry, especially for future missions like the Habitable Worlds Observatory.

Potential Technosignature from Anomalously Low Deuterium/Hydrogen in Planetary Water Depleted by Nuclear Fusion Technology

Abstract Deuterium–deuterium (DD) fusion is viewed as an ideal energy source for humanity in the far future, given a vast seawater supply of D. Here, we consider long-lived, extraterrestrial, technological societies that develop DD fusion. If such a society persisted over geologic timescales, oceanic deuterium would diminish. For an ocean mass and initial deuterium/hydrogen (D/H) ratio that were Earth-like, fusion power use of only ∼10 times that projected for humankind next century would deplete the D/H ratio in ∼(a few) ×108 yr to values below that of the local interstellar medium (ISM). Ocean masses of a few percent of Earth’s would reach an anomalously low D/H in ∼106–107 yr. The timescale shortens with greater energy consumption, smaller oceans, or lower initial D/H. Here, we suggest that anomalous D/H in planetary water below local ISM values of ∼16 × 10−6 (set by Big Bang nucleosynthesis plus deuterium loss onto dust or small admixtures of deuterium-poor stellar material) may be a technosignature. Unlike SETI using radio signals, anomalous D/H would persist for eons, even if civilizations perished or relocated. We discuss the wavelengths of strong absorption features for detecting D/H anomalies in atmospheric water vapor. These are vibrational O–D stretching at 3.7 μm in transmission spectroscopy of Earth-like worlds, ∼1.5 μm (in the wings of the 1.4 μm water band) in the shorter near-infrared for direct imaging by the Habitable Worlds Observatory, and ∼7.5-8 μm (in the wings of the broad 6.3 μm bending vibration of water) for concepts like the Large Interferometer for Exoplanets.

Earth-like Exoplanets in Spin–Orbit Resonances: Climate Dynamics, 3D Atmospheric Chemistry, and Observational Signatures

Abstract Terrestrial exoplanets around M- and K-type stars are important targets for atmospheric characterization. Such planets are likely tidally locked with the order of spin–orbit resonances (SORs) depending on eccentricity. We explore the impact of SORs on 3D atmospheric dynamics and chemistry, employing a 3D coupled climate-chemistry model to simulate Proxima Centauri b in 1:1 and 3:2 SORs. For a 1:1 SOR, Proxima Centauri b is in the Rhines rotator circulation regime with dominant zonal gradients (global mean surface temperature 229 K). An eccentric 3:2 SOR warms Proxima Centauri b to 262 K with gradients in the meridional direction. We show how a complex interplay between stellar radiation, orbit, atmospheric circulation, and (photo)chemistry determines the 3D ozone distribution. Spatial variations in ozone column densities align with the temperature distribution and are driven by stratospheric circulation mechanisms. Proxima Centauri b in a 3:2 SOR demonstrates additional atmospheric variability, including daytime–nighttime cycles in water vapor of +55% to −34% and ozone (±5.2%) column densities and periastron–apastron water vapor cycles of +17% to −10%. Synthetic emission spectra for the spectral range of the Large Interferometer For Exoplanets fluctuate by up to 36 ppm with the orbital phase angle for a 1:1 SOR due to 3D spatial and temporal asymmetries. The homogeneous atmosphere for the 3:2 SOR results in relatively constant emission spectra and provides an observational discriminant from the 1:1 SOR. Our work emphasizes the importance of understanding the 3D nature of exoplanet atmospheres and associated spectral variations to determine habitability and interpret atmospheric spectra.

Revisiting the multi-planetary system of the nearby star HD 20794

Context. Close-by Earth analogs and super-Earths are of primary importance because they will be preferential targets for the next generation of direct imaging instruments. Bright and close-by G-to-M type stars are preferential targets in radial velocity surveys to find Earth analogs. Their brightness allows us to achieve the best precision on RV measurements and search for signals with amplitudes of less than 1 m s−1. Aims. We present an analysis of the RV data of the star HD 20794, a target whose planetary system has been extensively debated in the literature. The broad time span of the observations makes it possible to find planets with signal semi-amplitudes below 1 m s−1 in the habitable zone. Methods. We analyzed RV datasets spanning more than 20 years. We monitored the system with ESPRESSO. We joined ESPRESSO data with the HARPS data, including archival data and new measurements from a recent program. We applied the post-processing pipeline YARARA to HARPS data to correct systematics, improve the quality of RV measurements, and mitigate the impact of stellar activity. Results. We confirm the presence of three planets, with periods of 18.3142 ± 0.0022 d, 89.68 ± 0.10 d, and 647.6−2.7+2.5 d, along with masses of 2.15 ± 0.17 M⊕, 2.98 ± 0.29 M⊕, and 5.82 ± 0.57 M⊕ respectively. For the outer planet, we find an eccentricity of 0.45−0.11+0.10, whereas the inner planets are compatible with circular orbits. The latter is likely to be a rocky planet in the habitable zone of HD 20794. From the analysis of activity indicators, we find evidence of a magnetic cycle with a period of ~3000 d, along with evidence pointing to a rotation period of ~39 d. Conclusions. We have determined the presence of a system of three planets orbiting the solar-type star HD 20794. This star is bright (V=4.34 mag) and close (d = 6.04 pc), and HD 20794 d resides in the stellar habitable zone, making this system a high-priority target for future atmospheric characterization with direct imaging facilities.

Polycyclic Aromatic Hydrocarbons as an Extraterrestrial Atmospheric Technosignature

Abstract Polycyclic aromatic hydrocarbons (PAHs) are prevalent in the Universe and interstellar medium but are primarily attributed to anthropogenic sources on Earth, such as fossil fuel combustion and firewood burning. Drawing upon the idea of PAHs being suitable candidates for technosignatures, we investigate the detectability of those PAHs that have available absorption cross sections in the atmospheres of Earth-like exoplanets (orbiting G-type stars at a distance of 10 pc) with an 8 m mirror of the Habitable Worlds Observatory (HWO). Specifically, we focus on Naphthalene, Anthracene, Phenanthrene, and Pyrene. Our simulations indicate that under current-Earth-like conditions, detecting PAH signatures between 0.2 and 0.515 μm is infeasible. To account for the historical decline in PAH production post the industrial revolution, we explore varying PAH concentrations to assess instrumental capabilities of detecting civilizations resembling modern Earth. We also evaluate telescope architectures (6 m, 8 m, and 10 m mirror diameters) to put our results into the context of the future HWO mission. With these four molecules, PAH detection remains infeasible, even at concentrations 10 times higher than current levels. While larger mirrors provide some advantages, they fail to resolve the spectral signatures of these molecules with significant signal-to-noise ratios. The UV absorption features of PAHs—caused by π-orbital → π*-orbital electronic transitions—serve as valuable markers, due to their distinct and detectable nature, preserved by the aromatic stability of PAHs. Additional lab measurements are necessary to gather absorption cross-sectional data beyond UV for more abundant PAHs. This may help further in improving the detectability of these molecules.

Amplicon Sequencing Reveals Diversity in Spatially Separated Microbial Communities in the Icelandic Mars Analog Environment Mælifellssandur.

Exploration missions to Mars rely on landers or rovers to perform multiple analyses over geographically small sampling regions, while landing site selection is done using large-scale but low-resolution remote-sensing data. Utilizing Earth analog environments to estimate small-scale spatial and temporal variation in key geochemical signatures and biosignatures will help mission designers ensure future sampling strategies meet mission science goals. Icelandic lava fields can serve as Mars analog sites due to conditions that include low nutrient availability, temperature extremes, desiccation, and isolation from anthropogenic contamination. This work reports analysis of samples collected using methods analogous to those of planetary missions to characterize microbial communities at different spatial scales in Mælifellssandur, Iceland, an environment with homogeneity at "remote imaging" resolution (overall temperature, apparent moisture content, and regolith grain size). Although microbial richness did not vary significantly among samples, the phylogenetic composition of the sediment microbiome differed significantly among sites separated by 100 m, which suggests distinct microbial signatures despite apparent homogeneity from remote observations. This work highlights the importance of considering microenvironments in future life-detection missions to extraterrestrial planetary bodies.

Volatile-rich Sub-Neptunes as Hydrothermal Worlds: The Case of K2-18 b

Abstract Temperate exoplanets between the sizes of Earth and Neptune, known as “sub-Neptunes,” have emerged as intriguing targets for astrobiology. It is unknown whether these planets resemble Earth-like terrestrial worlds with a habitable surface, Neptune-like giant planets with deep atmospheres and no habitable surface, or something exotic in between. Recent JWST transmission spectroscopy observations of the canonical sub-Neptune, K2-18 b, revealed ~1% CH4, ~1% CO2, and a nondetection of CO in the atmosphere. While previous studies proposed that the observed atmospheric composition could help constrain the lower atmosphere's conditions and determine the interior structure of sub-Neptunes like K2-18 b, the possible interactions between the atmosphere and a hot, supercritical water ocean at its base remain unexplored. In this work, we investigate whether a global supercritical water ocean, resembling a planetary-scale hydrothermal system, can explain these observations on K2-18 b–like sub-Neptunes through equilibrium aqueous geochemical calculations. We find that the observed atmospheric CH4/CO2 ratio implies a minimum ocean temperature of ~710 K, whereas the corresponding CO/CO2 ratio allows ocean temperatures up to ~1070 K. These results indicate that a global supercritical water ocean on K2-18 b is plausible. While life cannot survive in such an ocean, this work represents the first step toward understanding how a global supercritical water ocean may influence observable atmospheric characteristics on volatile-rich sub-Neptunes. Future observations with better-constrained CO and NH3 mixing ratios could further help distinguish between possible interior compositions of K2-18 b.

The Detectability of CH4/CO2/CO and N2O Biosignatures Through Reflection Spectroscopy of Terrestrial Exoplanets

The chemical makeup of Earth’s atmosphere during the Archean (4–2.5 Ga) and Proterozoic eon (2.5–0.5 Ga) contrast considerably with the present-day: the Archean was rich in carbon dioxide and methane, and the Proterozoic had potentially higher amounts of nitrous oxide. CO2 and CH4 in an Archean Earth analog may be a compelling biosignature because their coexistence implies methane replenishment at rates unlikely to be abiotic. However, CH4 can also be produced through geological processes, and setting constraints on volcanic molecules such as CO may help address this ambiguity. N2O in a Proterozoic Earth analog may be evidence of life because N2O production on Earth is mostly biological. Motivated by these ideas, we use the code EXORELR to generate forward models and simulate spectral retrievals of an Archean and Proterozoic Earth-like planet to determine the detectability of CH4, CO2, CO, and N2O in their reflected light spectrum for wavelength range 0.25–1.8 μm. We show that it is challenging to detect CO in an Archean atmosphere for volume mixing ratio (VMR) ≤ 10%, but CH4 is readily detectable for both the full wavelength span and truncated ranges cut at 1.7 μm and 1.6 μm, although for the latter two cases the dominant gas of the atmosphere is misidentified. Meanwhile, N2O in a Proterozoic atmosphere is detectable for VMR = 10−3 and long wavelength cutoff ≥1.4 μm, but undetectable for VMR ≤ 10−4 . The results presented here will be useful for the strategic design of the future Habitable Worlds Observatory and the components needed to potentially distinguish between inhabited and lifeless planets.

Impact of solar-wind turbulence on a planetary bow shock

Context. The interaction of the solar-wind plasma with a magnetized planet generates a bow-shaped shock ahead of the wind. Over recent decades, near-Earth spacecraft observations have provided insights into the physics of the bow shock, and the findings suggest that solar-wind intrinsic turbulence influences the bow shock dynamics. On the other hand, theoretical studies, primarily based on global numerical simulations, have not yet investigated the global three-dimensional (3D) interaction between a turbulent solar wind and a planetary magnetosphere. This paper addresses this gap for the first time by presenting an investigation of the global dynamics of this interaction that provides new perspectives on the underlying physical processes. Aims. We use the newly developed numerical code MENURA to examine how the turbulent nature of the solar wind influences the 3D structure and dynamics of magnetized planetary environments, such as those of Mercury, Earth, and magnetized Earth-like exoplanets. Methods. We used the hybrid particle-in-cell code MENURA to conduct 3D simulations of the turbulent solar wind and its interaction with an Earth-like magnetized planet through global numerical simulations of the magnetosphere and its surroundings. MENURA runs in parallel on graphics processing units, enabling efficient and self-consistent modeling of turbulence. Results. By comparison with a case in which the solar wind is laminar, we show that solar-wind turbulence globally influences the shape and dynamics of the bow shock, the magnetosheath structures, and the ion foreshock dynamics. Also, a turbulent solar wind disrupts the coherence of foreshock fluctuations, induces large fluctuations on the quasi-perpendicular surface of the bow shock, facilitates the formation of bubble-like structures near the nose of the bow shock, and modifies the properties of the magnetosheath region. Conclusions. The turbulent nature of the solar wind impacts the 3D shape and dynamics of the bow shock, magnetosheath, and ion foreshock region. This influence should be taken into account when studying solar-wind-planet interactions in both observations and simulations. We discuss the relevance of our findings for current and future missions launched into the heliosphere.

Cascade adaptive optics with a second stage based on a Zernike wavefront sensor for exoplanet observations

Context. Over the past decade, the high-contrast observation of disks and gas giant planets around nearby stars has been made possible with ground-based instruments using extreme adaptive optics (XAO). These facilities produce images with a Strehl ratio higher than 90% in the H band, in median observing conditions and high-flux regime. However, the correction leaves behind adaptive optics (AO) residuals, which impede studies of fainter or less massive exoplanets. Aims. Cascade AO systems with a fast second stage based on a Pyramid wavefront sensor (PWFS) have recently emerged as an appealing solution to reduce the atmospheric wavefront errors. Since these phase aberrations are expected to be small, they can also be accurately measured by a Zernike wavefront sensor (ZWFS), a well-known concept for its high sensitivity and moderate linear capture range. We propose an alternative second stage that relies on the ZWFS to correct for the AO residuals. Methods. We implemented the cascade AO with a ZWFS-based control loop on the ESO’s GPU-based High-order adaptive OpticS Testbench (GHOST) to validate the scheme in monochromatic light. We emulated the XAO first stage in different observing conditions (wind speed, seeing) and determined the corresponding operation parameters (e.g., number of controlled modes, integrator gain, loop calibration) that lead to stable loop operation and good correction performance. Our strategy was assessed in terms of corrected wavefront errors and contrast gain in the images with a Lyot coronagraph to probe its efficiency. Results. In median wind speed and seeing, our second-stage AO with a ZWFS and a basic integrator was able to reduce the atmospheric residuals by a factor of 6 and increase the wavefront error stability with a gain of 2 between open and closed loop. In the presence of non-common path aberrations, we also achieved a contrast gain of a factor of 2 in the coronagraphic images at short separations from the source, proving the ability of our scheme to work in cascade with an XAO loop. In addition, it may prove useful for imaging fainter or lighter close-in companions. In more challenging conditions, contrast improvements are also achieved by adjusting the control loop features. Conclusions. Our study validates the ZWFS-based second-stage AO loop as an effective solution to address small residuals left over from a single-stage XAO system for the coronagraphic observations of circumstellar environments. Our first in-lab demonstration paves the way for more advanced versions of our approach with different temporal control laws, non-linear reconstructors, and spectral widths. This would allow our approach to operate in high-contrast facilities on the current 8–10 m class telescopes and Extremely Large Telescopes to observe exoplanets, all the way down to Earth analogs around M dwarfs.

EXOLIFE: Detecting Exoplanets and Predicting Habitability Using Machine Learning

Exoplanets are among the most studied and remarkable topics in astronomy. Over the years, various methods have emerged for exoplanet detection, allowing for the identification of numerous exoplanet types. In this context, remote sensing and machine learning, which are central to our research, have significantly accelerated the detection process by leveraging algorithms. Our study involved training several machine learning models, including XGBoost, Random Forest, Multilayer Perceptron, K-Nearest Neighbor, Logistic Regression, and Support Vector Classifier, to compare their performance in both habitability assessment and exoplanet detection. The research utilized machine learning models trained on space observation data obtained from NASA, with the Python programming language serving as the foundation for the system's infrastructure. Our hypothesis was that "The detection of exoplanets and their evaluation within the scope of the habitability criterion can be increased to high accuracy rates with machine learning." Unlike merely detecting exoplanets, this study specifically aimed to identify Earth-like exoplanets. The XGBoost algorithm emerged as the most successful model in determining habitability, achieving an accuracy rate of 97.46% and demonstrating high precision and sensitivity. For exoplanet detection, all models achieved a main test accuracy rate of 96%; however, when considering sensitivity and precision, XGBoost was again the most effective. This research, following the synthesis and analysis of these two parameters, achieved a very high success rate compared to previous studies and made a significant contribution to the astronomy/astrophysics literature. Additionally, a Graphical User Interface (GUI) was developed, making the tested models functional through an application. The study successfully reached its goal of contributing important findings to the field.

Detecting and sizing the Earth with PLATO: A feasibility study based on solar data

Context. The PLAnetary Transits and Oscillations of stars (PLATO) mission will observe the same area of the sky continuously for at least two years in an effort to detect transit signals of an Earth-like planet orbiting a solar-like star. Aims. We aim to study how short-term solar-like variability caused by oscillations and granulation would affect PLATO’s ability to detect and size Earth if PLATO were to observe the Solar System itself. We also compare different approaches to mitigate noise caused by short-term solar-like variability and perform realistic transit fitting of transit signals in PLATO-like light curves. Methods. We injected Earth-like transit signals onto real solar data taken by the Helioseismic and Magnetic Imager (HMI) instrument on board the Solar Dynamics Observatory (SDO). We isolated short-term stellar variability in the HMI observations by removing any variability with characteristic timescales longer than five hours using a smooth Savitzky-Golay filter. We then added a noise model for a variety of different stellar magnitudes computed by PlatoSim assuming an observation by all 24 normal cameras. We first compared four different commonly used treatments of correlated noise in the time domain by employing them in a transit fitting scheme. We then tried to recover pairs of transit signals using an algorithm similar to the transit least squares algorithm. Finally, we performed transit fits using realistic priors on planetary and stellar parameters and assessed how accurately the pair of two injected transits was recovered. Results. We find that short-term solar-like variability affects the correct retrieval of Earth-like transit signals in PLATO data. Variability models accounting for variations with typical timescales at the order of one hour are sufficient to mitigate these effects. We find that when the limb-darkening coefficients of the host star are properly constrained, the impact parameter does not negatively affect the detectability of a transit signal or the retrieved transit parameters, except for high values (b > 0.8). For bright targets (8.5–10.5 mag), the transit signal of an Earth analogue can reliably be detected in PLATO data. For faint targets a detection is still likely, though the results of transit search algorithms have to be verified by transit-fitting algorithms to avoid false positive detections being flagged. For bright targets (V-mag ≤ 9.5), the radius of an Earth-like planet orbiting a solar-like star can be correctly determined at a precision of 3% or less, assuming that at least two transit events are observed and the characteristics of the host star are well understood.

Tunable 30 GHz laser frequency comb for astronomical spectrograph characterization and calibration.

The search for Earth-like exoplanets with the Doppler radial velocity (RV) technique is an extremely challenging and multifaceted precision spectroscopy problem. Currently, one of the limiting instrumental factors in reaching the required long-term 10-10 level of radial velocity precision is the defect-driven subpixel quantum efficiency (QE) variations in the large-format detector arrays used by precision echelle spectrographs. Tunable frequency comb calibration sources that can fully map the point spread function (PSF) across a spectrograph's entire bandwidth are necessary for quantifying and correcting these detector artifacts. In this work, we demonstrate a combination of laser frequency and mode spacing control that allows full and deterministic tunability of a 30 GHz electro-optic comb together with its filter cavity. After supercontinuum generation, this gives access to any optical frequency across 700-1300 nm. Our specific implementation is intended for the comb deployed at the Habitable-Zone Planet Finder (HPF) spectrograph and its near-infrared Hawaii-2RG array, but the techniques apply to all laser frequency combs (LFCs) used for precision astronomical spectrograph calibration and other applications that require broadband tuning.

Structural comparison of [Ce(OtBu)Cl(THF)5](BPh4) to smaller rare earth analogues.

The introduction of the cerium(III) analogue (1-Ce, Ln = Ce) of (tert-butoxido)chloridopentakis(tetrahydrofuran)lanthanide(III) tetraphenylborate tetrahydrofuran disolvate, [Ln{OC(CH3)3}Cl(C4H8O)5][B(C6H5)4]·2C4H8O or [Ln(OtBu)Cl(THF)5](BPh4)·2THF (1-Ln) has been achieved with a structural comparison between the existing solid-state structures of other rare earth analogues and the title compound at 100 and 180 K. The cation in 1-Ce is targeted as the cerium(III) ion possesses the criteria to exhibit slow magnetic relaxation in axial point-charge crystal fields, akin to the dysprosium(III) ion in 1-Dy. AC magnetic susceptibility experiments reveal no such behaviour for 1-Ce, putting the viability of cerium-based SMMs into question.

Multi-aperture telescopes at the quantum limit of superresolution imaging : Detecting subRayleigh object near a star

Multi-aperture telescopes at the quantum limit of superresolution imaging : Detecting subRayleigh object near a star

A sub-Earth-mass planet orbiting Barnard’s star

Context. ESPRESSO guaranteed time observations (GTOs) at the 8.2m VLT telescope were performed to look for Earth-like exoplanets in the habitable zone of nearby stars. Barnard’s star is a primary target within the ESPRESSO GTO as it is the second closest neighbour to our Sun after the α Centauri stellar system. Aims. We present here a large set of 156 ESPRESSO observations of Barnard’s star carried out over four years with the goal of exploring periods of shorter than 50 days, thus including the habitable zone (HZ). Methods. Our analysis of ESPRESSO data using Gaussian process (GP) to model stellar activity suggests a long-term activity cycle at 3200 d and confirms stellar activity due to rotation at 140 d as the dominant source of radial velocity (RV) variations. These results are in agreement with findings based on publicly available HARPS, HARPS-N, and CARMENES data. ESPRESSO RVs do not support the existence of the previously reported candidate planet at 233 d. Results. After subtracting the GP model, ESPRESSO RVs reveal several short-period candidate planet signals at periods of 3.15 d, 4.12 d, 2.34 d, and 6.74 d. We confirm the 3.15 d signal as a sub-Earth mass planet, with a semi-amplitude of 55 ± 7 cm s−1, leading to a planet minimum mass mp sin i of 0.37 ± 0.05 M⊕, which is about three times the mass of Mars. ESPRESSO RVs suggest the possible existence of a candidate system with four sub-Earth mass planets in circular orbits with semi-amplitudes from 20 to 47 cm s−1, thus corresponding to minimum masses in the range of 0.17–0.32 M⊕. Conclusions. The sub-Earth mass planet at 3.1533 ± 0.0006 d is in a close-to circular orbit with a semi-major axis of 0.0229 ± 0.0003 AU, thus located inwards from the HZ of Barnard’s star, with an equilibrium temperature of 400 K. Additional ESPRESSO observations would be required to confirm that the other three candidate signals originate from a compact short-period planet system orbiting Barnard’s star inwards from its HZ.

Revisiting the dynamical masses of the transiting planets in the young AU Mic system: Potential AU Mic b inflation at ~20 Myr

Context. Understanding planet formation is important in the context of the origin of planetary systems in general and of the Solar System in particular, as well as to predict the likelihood of finding Jupiter, Neptune, and Earth analogues around other stars. Aims. We aim to precisely determine the radii and dynamical masses of transiting planets orbiting the young M star AU Mic using public photometric and spectroscopic datasets. Methods. We performed a joint fit analysis of the TESS and CHEOPS light curves and more than 400 high-resolution spectra collected with several telescopes and instruments. We characterise the stellar activity and physical properties (radius, mass, density) of the transiting planets in the young AU Mic system through joint transit and radial velocity fits with Gaussian processes. Results. We determine a radius of Rpb = 4.79 ± 0.29 R⊕, a mass of Mpb = 9.0 ± 2.7 M⊕, and a bulk density of ρpb = 0.49 ± 0.16 g cm−3 for the innermost transiting planet AU Mic b. For the second known transiting planet, AU Mic c, we infer a radius of Rpc = 2.79 ± 0.18 R⊕, a mass of Mpc = 14.5 ± 3.4 M⊕, and a bulk density of ρpc = 3.90 ± 1.17 g cm−3. According to theoretical models, AU Mic b may harbour an H2 envelope larger than 5% by mass, with a fraction of rock and a fraction of water. AU Mic c could be made of rock and/or water and may have an H2 atmosphere comprising at most 5% of its mass. AU Mic b has retained most of its atmosphere but might lose it over tens of millions of years due to the strong stellar radiation, while AU Mic c likely suffers much less photo-evaporation because it lies at a larger separation from its host. Using all the datasets in hand, we determine a 3σ upper mass limit of Mp[d] sin i = 8.6 M⊕ for the AU Mic’d’ TTV-candidate. In addition, we do not confirm the recently proposed existence of the planet candidate AU Mic ’e’ with an orbital period of 33.4 days. We investigated the level of the radial velocity variations and show that it is lower at longer wavelength with smaller changes from one observational campaign to another.

Covariation of hot spring geochemistry with microbial genomic diversity, function, and evolution

The geosphere and the microbial biosphere have co-evolved for ~3.8 Ga, with many lines of evidence suggesting a hydrothermal habitat for life’s origin. However, the extent that contemporary thermophiles and their hydrothermal habitats reflect those that likely existed on early Earth remains unknown. To address this knowledge gap, 64 geochemical analytes were measured and 1022 metagenome-assembled-genomes (MAGs) were generated from 34 chemosynthetic high-temperature springs in Yellowstone National Park and analysed alongside 444 MAGs from 35 published metagenomes. We used these data to evaluate co-variation in MAG taxonomy, metabolism, and phylogeny as a function of hot spring geochemistry. We found that cohorts of MAGs and their functions are discretely distributed across pH gradients that reflect different geochemical provinces. Acidic or circumneutral/alkaline springs harbor MAGs that branched later and are enriched in sulfur- and arsenic-based O2-dependent metabolic pathways that are inconsistent with early Earth conditions. In contrast, moderately acidic springs sourced by volcanic gas harbor earlier-branching MAGs that are enriched in anaerobic, gas-dependent metabolisms (e.g. H2, CO2, CH4 metabolism) that have been hypothesized to support early microbial life. Our results provide insight into the influence of redox state in the eco-evolutionary feedbacks between thermophiles and their habitats and suggest moderately acidic springs as early Earth analogs.

Impact of vegetation albedo on the habitability of Earth-like exoplanets

ABSTRACT Vegetation can modify the planetary surface albedo via the Charney mechanism, as plants are usually darker than the bare surface of the continents. We updated ESTM (Earth-like surface temperature model) to incorporate the presence, distribution and evolution of two dynamically competing vegetation types that resemble grasslands and trees (the latter in the double stages of life: adults and seedlings). The newly developed model was applied to estimate how the climate-vegetation system reaches equilibrium across different rocky planetary configurations, and to assess its impact on temperature and habitability. With respect to a world with bare granite continents, the effect of vegetation-albedo feedback is to increase the average surface temperature. Since grasses and trees exhibit different albedos, they affect temperature to different degrees. The ultimate impact on climate depends on the outcome of the competition between these vegetation types. The change in albedo due to vegetation extends the habitable zone and enhances the overall planetary habitability beyond its traditional outer edge. This effect is especially relevant for planets that have a larger extension of continents than Earth. For Earth, the semimajor axis d = 1.04 au represents the turning point where vegetation enhances habitability from h = 0.0 to 0.485 (in the grass-dominance case), to h = 0.584 (in the case of coexistence between grasses and trees), and to h = 0.612 (in the tree-dominance case). This illustrates the transition from a snowball state to a planet with intermediate habitability at the outer edge of the circumstellar habitability zone.