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In the past, measures of the "Earth-likeness" of exoplanets have been qualitative, considering an abiotic Earth, or requiring discretionary choices of what parameters make a planet Earth-like. With the advent of high-resolution exoplanet spectroscopy, there is a growing need for a method of quantifying the Earth-likeness of a planet that addresses these issues while making use of the data available from modern telescope missions. In this work, we introduce an informational-entropic metric that makes use of the spectrum of an exoplanet to directly quantify how Earth-like the planet is. To illustrate our method, we generate simulated transmission spectra of a series of Earth-like and super-Earth exoplanets, as well as an exoJupiter and several gas giant exoplanets. As a proof of concept, we demonstrate the ability of the information metric to evaluate how similar a planet is to Earth, making it a powerful tool in the search for a candidate Earth 2.0.

Water activity (aw) quantifies the free water available for microbial growth. At the cellular level, liquid water is paramount for replication and proliferation. Research on Earth-like life suggests that microbial replication is limited by an aw threshold of ≥ 0.585, below which replication ceases. On Mars, liquid water is typically unstable, but gaseous water exchanges between the atmosphere and the upper regolith are substantial. A variety of salts widespread across the Martian surface are capable of hydration and deliquescence, including sulfates that can undergo hydration–dehydration changes when exposed to different levels of aw. This study investigates microbial growth at different aw levels in a commercially available Mojave Mars Simulant 2 (MMS-2), a fine-grade basaltic soil modified with 2-4 wt% calcium sulfate and oxides (Fe₂O₃, SiO₂, MgO, CaO) to mimic Martian regolith composition. Growth was monitored by quantifying extracted deoxyribonucleic acid (DNA) from samples incubated at aw 1, 0.75, 0.65, 0.34 and 0.12 under Earth-like conditions (30 °C, ≈1 bar). At aw = 1, DNA mass from MMS-2 and Bacillus subtilis-spiked MMS-2 peaked on day 15 and day 3, respectively. At lower aw levels (0.75±0.02 and 0.65±0.02), DNA mass reached its peak after 20 and 30 days, respectively. Samples incubated at aw = 0.34±0.02 exhibited reduced DNA yields, with a maximum on day 30, whereas no detectable increase in DNA occurred at aw = 0.12 ± 0.02 over 60 days. Statistical comparisons with the aw = 0.12 control were significant (e.g., aw = 0.34 ± 0.02, cleanroom vs. aw = 0.12 ± 0.02 at day 30: p = 0.0098; Benjamini–Hochberg (BH) False Discovery Rate across day 20, 30, 45: q = 0.0153). These findings suggest that atmospheric water can be adsorbed into regolith grains and salts, supporting microbial persistence and DNA accumulation consistent with possible replication at reduced water activity under Earth ambient conditions.

Environmental conditions fundamentally shape host-pathogen interactions; however, how multiple extreme abiotic stressors combine to influence infection outcomes remains poorly understood. All living beings have evolved under specific gravitational and radiation regimes; deviations from these conditions-whether in extreme terrestrial environments or beyond Earth-may alter physiological homeostasis, including immune function and pathogen replication. In this study, we investigated the effects of reduced gravity and lowered muon flux on Orsay virus infection in the nematode Caenorhabditis elegans. We employed a fully factorial experimental design, examining how each factor, alone and in combination, influences fecundity and developmental traits and viral load. While below-background radiation radically affected viral accumulation dynamics, reduced gravity had a minor effect. Both factors significantly impacted reproduction and morphology, with some effects magnified by viral infection. These results reveal how even partial modifications of Earth-like gravity and radiation levels can alter host-pathogen interactions. By integrating experimental observations with mathematical modeling, we suggest that these environmental stressors may primarily affect prezygotic reproductive processes and modulate viral replication through distinct and sometimes antagonistic mechanisms. Although this work does not encompass the full complexity of space environments, where cosmic radiation includes high-energy protons and heavy ions, it provides insight into how adjustable models of reduced gravity and radiation can advance our understanding of biological adaptation beyond standard terrestrial conditions.IMPORTANCEUnderstanding how extreme environmental conditions affect host-pathogen interactions is critical for exploring fundamental principles of stress biology. This study demonstrates that reduced gravity and diminished muon radiation flux can significantly alter viral infection dynamics and host physiology in Caenorhabditis elegans. By integrating experimental data with mathematical modeling, we propose that these abiotic stresses impact prezygotic reproductive processes and modulate viral replication in distinct and sometimes antagonistic ways. Our findings suggest that even partial deviations from Earth-like conditions can reshape infection outcomes and developmental trajectories, highlighting the need for deeper mechanistic insights into biological adaptation beyond terrestrial norms. These results have implications for space biosciences, evolutionary virology, radiation hormesis, and the design of countermeasures to preserve organismal health in extreme or non-terrestrial habitats.

The Great Oxidation Event (GOE), which marked the transition from an anoxic to an oxygenated atmosphere, occurred 2.4 billion years ago on Earth, several hundreds of millions of years after the emergence of oxygenic photosynthesis. This long delay implies that specific conditions in terms of biomass productivity and burial were necessary to trigger the GOE. It could be a limiting factor for the development of oxygenated atmospheres on inhabited exoplanets. In this study, we explore the specificities of a terrestrial planet in the habitable zone of an M dwarf for a GOE. Using a 1D coupled photochemical-climate model, we simulate the atmospheric evolution of TRAPPIST-1e, an Earth-like exoplanet, exploring the effect of oxygen sources (biotic or abiotic). Our results show that the stellar energy distribution promotes O[Formula: see text] production at lower O[Formula: see text] concentrations compared to Earth, and the ozone layer on TRAPPIST-1e forms more efficiently. This lowers the threshold for atmospheric oxidation, suggesting that the GOE on TRAPPIST-1e would occur quickly after the rise of oxygenic photosynthesis, up to 1Gyrs earlier than on Earth, and would reach O[Formula: see text] enabling oxygenic respiration and thus the development of animals. We may question whether this is a general behavior around several M-stars. Furthermore, we discuss how the overproduction of ozone could make O[Formula: see text] detection possible using the James Webb Space Telescope, providing a potential method to observe oxygenation signatures on exoplanets in the near future. Previous studies predicted that for an Earth-like atmosphere O[Formula: see text] would require over 150 transits for detection, but our results show that significantly fewer transits could be needed.

Marine habitability for complex life on Earth and Earth-like planets requires bioavailable nutrients and dissolved oxygen. The cycling of nutrients and oxygen is controlled by physical ocean circulation. However, our understanding of how circulation has varied through time and space is incomplete for Earth and unconstrained for Earth-like exoplanets. Earth's rotation has slowed over time, affecting ocean circulation by modifying the Coriolis effect. We use a three-dimensional Earth system model to explore how slowing planetary rotation influences ocean circulation and biogeochemistry. We show that slower rotation enhances wind-driven upwelling and global circulation. Nutrient recycling is consequently more efficient, increasing photosynthetic productivity. Additionally, enhanced ocean oxygenation improves habitability for aerobic life under a well-oxygenated atmosphere. However, under a poorly oxygenated atmosphere, slowing rotation increases oxygen fluxes from the ocean to the atmosphere. Therefore, Earth's rotational history may have been a long-term background control on surface oxygenation and the evolution of animals.

Tropical cyclones (TCs) are among the most energetic phenomena in the climate system. Given their energetic nature, TCs induce upscale effects on the climate system, although these effects have not been extensively assessed. Understanding the influence of TCs on climate is important given the expected changes in TC activity with a warming climate. In this study, we provide evidence that TCs induce large-scale radiative cooling. We compare the climates of a coupled global climate model in two configurations: a control configuration with Earth-like TC activity and a perturbed configuration where TC activity is reduced by suppressing wind-induced surface heat exchange in areas with TC-like conditions. By comparing climate states, we find a reduction of incoming top-of-atmosphere radiation with more TC activity, largely in the subtropics. Globally, TC activity drives increases to longwave emission that outweigh increases to absorbed solar radiation. We show that decreases in sea surface temperature occur as an adjustment to TC-induced perturbations, coincident with increases in longwave emission caused by a drying of the free troposphere and a reduction in high cloud fraction. The results demonstrate the role of TCs in organizing convection, suggesting that TCs play a role in modulating climate sensitivity and large-scale energy transport.

Liquid is a fundamental requirement for life as we understand it, but whether that liquid has to be water is not known. We propose the hypothesis that ionic liquids (ILs) and deep eutectic solvents (DES) constitute a class of non-aqueous planetary liquids capable of persisting on a wide range of bodies where stable liquid water cannot exist. This hypothesis is motivated by key physical properties of ILs and DES. Many exhibit vapor pressures orders of magnitude lower than that of water and remain liquid across exceptionally wide temperature ranges, from cryogenic to well above terrestrial temperatures. These properties permit stable liquids to exist where liquid water would rapidly evaporate or freeze and outside of bulk phases as persistent microscale reservoirs-such as thin films and pore-filling droplets. In other words, ILs and DES can persist in environments without requiring oceans, thick atmospheres, or narrowly regulated climate conditions. We further hypothesize that ILs and DES could act as solvents for non-Earth-like life, based on their polar nature and the demonstrated stability and functionality of proteins and other biomolecules in ionic liquids. More speculatively, our hypothesis extends to the idea that ILs and DES could enable prebiotic chemistry by providing long-lived, protective liquid environments for complex organic molecules on bodies such as comets and asteroids, where liquid water is absent. Additionally, based on the occurrence of DES-like mixtures as protective intracellular liquids in desiccation-tolerant plants, we propose that ILs and DES might be solvents that life elsewhere purposefully evolves. We review protein and other biomolecule studies in ILs and DES and outline planetary environments in which ILs and DES might occur by discussing available anions and cations. We present strategies to advance the IL/DES solvent hypothesis using laboratory studies, computational chemistry, planetary missions, analysis of existing spectroscopic datasets, and modeling of liquid microniches and chemical survival on small bodies.

Many petrological, geodynamical, and geochemical perspectives have offered circumstantial evidence for either an early onset of plate tectonics in the first 10% of Earth's history or a late onset after the great oxidation event (2.5 Ga ago). This calls into question over what timescales plate tectonics have influenced terrestrial geological and geochemical processes. We present geochemical data from the products of ancient crustal subduction, which were recycled into the deep mantle and then tapped by the modern Marquesas volcanic hotspot. We demonstrate that these materials have distinct short-lived radiogenic (146Sm-142Nd, t1/2 = 103 Ma) isotopic compositions (average μ142Nd = +2.3 ±1.3, n = 3) compared to other Marquesas lavas (average μ142Nd = -0.8 ±1.2, n = 7). These results require that the history of these subduction products diverged from those of other Marquesas magmas more than four billion years ago. Quantitative modeling suggests that the most geochemically enriched Marquesas samples represent up to ~0.6% recycled crustal sediments, which may have been subducted at any time in Earth's history but were most likely subducted in the late Hadean Eon to early Eoarchean Era. The inferred felsic composition of such materials further requires that both crustal melting and sedimentation processes were active in some form on the early Earth. Further, the preservation of evidence for foundational planetary events in geologically young rocks reveals that Earth's volcanic hotspots could provide a defining perspective on the early planetary-scale processes that build Earth-like planets.

Astrocombs are promising calibration sources for high-resolution astronomical spectrographs, offering stable, uniformly spaced calibration lines whose positions are traceable to an atomic frequency reference. Most spectrograph pixels fall in the gaps between astrocomb lines, remaining uninterrogated by the calibration light; however, it is becoming clear that detector inhomogeneities and intra-order variations in the spectrograph performance make it important to characterize the entire detector area to obtain the best radial-velocity precision. Here, this requirement is fulfilled by the demonstration of an astrocomb architecture in which lines can be swept in any chosen increment in frequency. Based on Fabry-Pérot filtering of a primary comb with a 1 GHz spacing, the astrocomb employs feed-forward locking to overlap a single-frequency laser onto a chosen primary comb line, a process that precisely selects which subset of primary comb modes will be transmitted by the Fabry-Pérot cavity. Operating from 650-1030 nm, and exhibiting continuous performance for >12 hours, the system demonstrates the long-term stability required by a practical astrocomb, while providing the fine sampling of the spectrograph instrument function likely to be necessary for achieving radial-velocity precisions supporting Earth-like exoplanet detection.

Prolonged exposure to reduced gravity in space leads to bone demineralization and muscle atrophy, which current countermeasures of simple body loading can partially address. To address this aetiology, a resistive hypogravity exosuit (R-HEXsuit) is proposed for dynamic body loading in low gravity. R-HEXsuit is a lightweight (1.4 kg) soft wearable exosuit that uses pneumatic artificial muscles to provide programmed bilateral resistance during walking, stimulating primary leg muscles. Tests on healthy subjects in Earth gravity and simulated Moon gravity reveal that the suit increases metabolic cost by 29.3% in Moon gravity, aligning it with Earth-like metabolic cost. Muscle activation in key knee joint muscles also increases, matching or exceeding Earth levels, without altering natural gait patterns. These results highlight the R-HEXsuit as a promising tool for replicating Earth-like physical demands during low-gravity missions, offering a potential solution for mitigating musculoskeletal degradation in space.

A key question in the planetary sciences centers on the divergence between the sibling planets, Venus and Earth. Venus currently does not operate with plate tectonics, and its thick atmosphere has led to extreme greenhouse conditions. It is unknown if this state was set primordially or if Venus was once more Earth-like. Here, we explore Venus as an example of a planet that recently transitioned between tectonic regimes. Our results show that transitions naturally lead to substantial resurfacing and melt-generated outgassing from lithosphere-breaking events and overturns, with 3 to 10 bars of atmosphere generated per overturn over ~60-million year timescales and ~10 to 100 bars outgassed over billion-year time frames. We find that the observation of Venus with a thick greenhouse atmosphere and the inferences of currently low volcanic rates and previous prodigious volcanic rates are consistent with a planet that has undergone a transition in tectonics, suggesting that Venus once hosted clement surface conditions and was more Earth-like.

We report the pure rotational spectra of three alkaline earth(-like) metal-bearing molecules: calcium peroxide (CaO2), strontium dicarbide (SrC2), and ytterbium dicarbide (YbC2), produced in a laser ablation-electric discharge supersonic expansion source and detected by cavity Fourier transform microwave spectroscopy. The semiexperimental equilibrium structure of each molecule has been derived to ≲1 mÅ uncertainty by combining comprehensive isotopic measurements with highly accurate ab initio rovibrational corrections. These precise structures provide direct physical probes of not only the highly ionic metal-ligand bonding but also the electronic structure of the dianionic ligands themselves. Our detection of CaO2, in particular, appears to represent the only high-resolution gas-phase spectroscopy of a metal peroxide (M2+O22-) molecule, providing a unique intramolecular proxy of the unstable O22- dianion. In addition to these fundamental insights, we discuss our observation of highly nonequilibrated vibrational population distributions in the expansion source and the relevance of our results to the chemistry of circumstellar envelopes studied by radio astronomy.

The early start to life naively suggests that abiogenesis is a rapid process on Earth-like planets. However, if evolution typically takes ∼4 Gyr to produce intelligent life-forms like us, then the limited lifespan of Earth's biosphere (∼5-6 Gyr) necessitates an early (and possibly highly atypical) start to our emergence-an example of the weak anthropic principle. Our previously proposed objective Bayesian analysis of Earth's chronology culminated in a formula for the minimum odds ratio between the fast and slow abiogenesis scenarios (relative to Earth's lifespan). Timing from microfossils (3.7 Gya) yields 3:1 odds in favor of rapid abiogenesis, whereas evidence from carbon isotopes (4.1 Gya) gives 9:1, both below the canonical threshold of "strong evidence" (10:1). However, the recent result of a 4.2 Gya LUCA pushes the odds over the threshold for the first time (nominally 13:1). In fact, the odds ratio is >10:1 for all possible values of the biosphere's ultimate lifespan and speculative hypotheses of ancient civilizations. For the first time, we formally have strong evidence that favors the hypothesis that life rapidly emerges in Earth-like conditions (although such environments may themselves be rare).

The discovery of many low-mass exoplanets, including several planets within the habitable zone of their host stars, has led to the question of which kind of atmosphere surrounds them. Recent exoplanet detections have revealed the existence of a large population of low-mass planets (<3&#x2009;M &#x2295;) with H2-dominated atmospheres that must have been accreted from the protoplanetary disk. As the gas disk usually has an ~10% fraction of helium, we model the possible enrichment of the primordial He fraction in the atmosphere of planets with mass between 0.75&#x2009;M &#x2295; and 3.0&#x2009;M &#x2295; that orbit in the classical habitable zone of Sun-like stars. Depending on the mass accreted by the planet during the gas disk phase and the stellar high-energy flux between ~10 and 120&#x2009;nm, we find that Earth-like planets with masses between ~0.95&#x2009;M &#x2295; and 1.25&#x2009;M &#x2295; inside the habitable zone of Sun-like stars can end up with He-dominated primordial atmospheres. This finding has important implications for the evolution of Earth-like habitats, as these thick helium-enriched primordial atmospheres can inhibit the habitability of these planets. The upcoming generation of giant telescopes, such as the Extremely Large Telescope, may enable us to observe and explore these atmospheres.

The hard steps model is a 'toy' mathematical representation of evolution towards complex life on Earth or Earth-like planets. It assumes that, at the longest time scale, the rate of evolution towards increased complexity is governed by unlikely transitions that happen randomly and rarely. Applied to Earth, the model suggests a small number of such transitions in the pathway to 'intelligent observers'-humans. The transitions are usually envisaged as occurring instantaneously, but this ignores the reality that on Earth, the evolution of life and the planetary environment have been inextricably linked. The critical steps should be seen as initiating whole Earth-system transitions that take hundreds of millions of years to complete, as in the events that caused, and followed after, the Paleoproterozoic and Neoproterozoic glacial episodes. These were both caused by, and drivers of, evolutionary advances that were necessary for complex life to arise. I adapt the model to include such delays and show that it then suggests just two or three hard steps to humans. The model predicts that the search for biosignatures in exoplanet atmospheres may find planets with Archean-like atmospheres, but probably will not find the signature of a planet with a Proterozoic or modern Earth-like atmosphere.This article is part of the discussion meeting issue 'Chance and purpose in the evolution of biospheres'.

The exploration of our solar system for microbial extraterrestrial life is the primary goal of several space agencies. Mars has attracted substantial attention owing to its Earth-like geological history and potential niches for microbial life. This study evaluated the suitability of the polyextremophilic fungal strain Rhinocladiella similis LaBioMMi 1217 as a model eukaryote for astrobiology. Comprehensive genomic analysis, including taxonomic and functional characterization, revealed several R. similis genes conferring resistance to Martian-like stressors, such as osmotic pressure and ultraviolet radiation. When cultured in a synthetic Martian regolith (MGS-1), R. similis exhibited altered morphology and produced unique metabolites, including oxylipins, indolic acid derivatives, and siderophores, which might be potential biosignatures. Notably, oxylipins were detected using laser desorption ionization mass spectrometry, a technique slated for its use in the upcoming European Space Agency ExoMars mission. Our findings enhance the understanding of extremophilic fungal metabolism under Martian-like conditions, supporting the potential of black yeasts as viable eukaryotic models in astrobiological studies. Further research is necessary to validate these biosignatures and explore the broader applicability of R. similis in other extraterrestrial environments.