Viruses are the most numerically abundant biological entities on Earth. As ubiquitous replicators of molecular information and agents of community change, viruses have potent effects on the life on Earth, and may play a critical role in human spaceflight, for life-detection missions to other planetary bodies and planetary protection. However, major knowledge gaps constrain our understanding of the Earth's virosphere: (1) the role viruses play in biogeochemical cycles, (2) the origin(s) of viruses and (3) the involvement of viruses in the evolution, distribution and persistence of life. As viruses are the only replicators that span all known types of nucleic acids, an expanded experimental and theoretical toolbox built for Earth's viruses will be pivotal for detecting and understanding life on Earth and beyond. Only by filling in these knowledge and technical gaps we will obtain an inclusive assessment of how to distinguish and detect life on other planetary surfaces. Meanwhile, space exploration requires life-support systems for the needs of humans, plants and their microbial inhabitants. Viral effects on microbes and plants are essential for Earth's biosphere and human health, but virus–host interactions in spaceflight are poorly understood. Viral relationships with their hosts respond to environmental changes in complex ways which are difficult to predict by extrapolating from Earth-based proxies. These relationships should be studied in space to fully understand how spaceflight will modulate viral impacts on human health and life-support systems, including microbiomes. In this review, we address key questions that must be examined to incorporate viruses into Earth system models, life-support systems and life detection. Tackling these questions will benefit our efforts to develop planetary protection protocols and further our understanding of viruses in astrobiology.

Planetary geodynamics may have an important influence over planetary habitability and the boundaries of the circumstellar habitable zone (CHZ) in space and time. To investigate this we use a minimal parameterized model of the co-evolution of the geosphere and atmosphere of Earth-like planets around F, G, K and M main sequence stars. We found the CHZ for the present Solar System located between 0.92 and 1.09 au for a 1.0 M$_{\oplus }$ Earth-like planet, extendible to 1.36 au for a 4.0 M$_{\oplus }$ planet. In the literature, the CHZ varies considerably in width and border location, but the outer edges tend to be more spread out than the inner edges, showing a higher difficulty in determining the outer edge. Planetary mass has a considerable effect on planetary geodynamics, with low-mass planets cooling down faster and being less capable of maintaining a rich carbon dioxide atmosphere for several billions of years. Age plays a particularly important role in the width of the CHZ as the CHZ contracts in both directions: from the inner edge (as stellar luminosity increases with time), and from the outer edge (as planetary heat flux and seafloor spreading rate decrease with time). This strongly affects long-lived habitability as the 5 Gyr continuous CHZ may be very narrow or even non-existent for low-mass planets (<0.5 M$_{\oplus }$) and fast-evolving high-mass stars (>1.1 M$_{\odot }$). Because of this, the mean age of habitable terrestrial planets in our Galaxy today may be younger than Earth's age. Our results suggest that the best targets for future surveys of biosphere signatures may be planets between 0.5 and 4.0 M$_{\oplus }$, in systems younger than the Solar System. These planets may present the widest and long-lived CHZ.

Free-floating planets (FFPs) can result from dynamical scattering processes happening in the first few million years of a planetary system's life. Several models predict the possibility, for these isolated planetary-mass objects, to retain exomoons after their ejection. The tidal heating mechanism and the presence of an atmosphere with a relatively high optical thickness may support the formation and maintenance of oceans of liquid water on the surface of these satellites. In order to study the timescales over which liquid water can be maintained, we perform dynamical simulations of the ejection process and infer the resulting statistics of the population of surviving exomoons around FFPs. The subsequent tidal evolution of the moons’ orbital parameters is a pivotal step to determine when the orbits will circularize, with a consequential decay of the tidal heating. We find that close-in ($a \lesssim 25$ RJ) Earth-mass moons with carbon dioxide-dominated atmospheres could retain liquid water on their surfaces for long timescales, depending on the mass of the atmospheric envelope and the surface pressure assumed. Massive atmospheres are needed to trap the heat produced by tidal friction that makes these moons habitable. For Earth-like pressure conditions (p0 = 1 bar), satellites could sustain liquid water on their surfaces up to 52 Myr. For higher surface pressures (10 and 100 bar), moons could be habitable up to 276 Myr and 1.6 Gyr, respectively. Close-in satellites experience habitable conditions for long timescales, and during the ejection of the FFP remain bound with the escaping planet, being less affected by the close encounter.

Searching for extrasolar biosignatures is important to understand life on Earth and its origin. Astronomical observations of exoplanets may find such signatures, but it is difficult and may be impossible to claim unambiguous detection of life by remote sensing of exoplanet atmospheres. Here, another approach is considered: collecting grains ejected by asteroid impacts from exoplanets in the Milky Way and then travelling to the Solar System. The optimal grain size for this purpose is around 1 μm, and though uncertainty is large, about 105 such grains are expected to be accreting on Earth every year, which may contain biosignatures of life that existed on their home planets. These grains may be collected by detectors placed in space, or extracted from Antarctic ice or deep-sea sediments, depending on future technological developments.

Typicality arguments attempt to use the Copernican Principle to draw conclusions about the cosmos and presently unknown conscious beings within it, including extraterrestrial intelligences (ETI). The most notorious is the Doomsday Argument, which purports to constrain humanity's future from its current lifespan alone. These arguments rest on a likelihood calculation that penalizes models in proportion to the number of distinguishable observers. I argue that such reasoning leads to solipsism, the belief that one is the only being in the world, and is therefore unacceptable. Using variants of the ‘Sleeping Beauty’ thought experiment as a guide, I present a framework for evaluating observations in a large cosmos: Weighted Fine Graining (WFG). WFG requires the construction of specific models of physical outcomes and observations. Valid typicality arguments then emerge from the combinatorial properties of third-person physical microhypotheses. Indexical (observer-relative) facts do not directly constrain physical theories, but instead weight different provisional evaluations of credence. As indexical knowledge changes, the weights shift. I show that the self-applied Doomsday Argument fails in WFG, even though it can work for an external observer. I argue that the Copernican Principle does not let us apply self-observations to constrain ETIs.

The origin of carbon fixation is a fundamental question in astrobiology. While the Calvin cycle is the most active on the modern Earth, the reductive tricarboxylic acid (TCA) cycle (rTCA) pathway for carbon fixation has been proposed to have played an important role in early evolution. In this study, we examined the evolution of key enzymes in the rTCA, which are rare in extant organisms, occurring in a few groups of Bacteria and Archaea. We investigated one of the least common reactions of this pathway, cleavage of citrate into oxaloacetate and acetyl-CoA, which can be performed by either a two-enzyme system (CCS/CCL) or a single enzyme (ACL) that is assumed to be the result of fusion of the two active sites into a single polypeptide. For broader context, we also studied functionally diverged homologues of these enzymes, succinyl-CoA synthetase (SCS) and citrate synthase. Our phylogenetic analysis of these enzymes in Bacteria and Archaea shows that SCS, a homologue of CCS from distant bacterial taxa capable of citrate cleavage, are monophyletic, suggesting linked horizontal gene transfers of SCS and citrate cleavage enzymes. We also found evidence of the horizontal transfer of SCS from a clade of anaerobic Archaea (Archaeoglobi, Methanomicrobia or Crenarchaeota) to an ancestor of Cyanobacteria/Melainabacteria clade – both of which share a succinate semialdehyde shunt in their oxidative TCA cycles. We identified new bacterial and archaeal taxa for which complete rTCA cycles are theoretically possible, including Syntrophobacter, Desulfofundulus, Beggiatoa, Caldithrix, Ca. Acidulodesulfobacterales and Ca. Micrarchaeota. Finally, we propose a mechanism for syntrophically-regulated fluxes through oxidative and rTCA reactions in microbial communities particularly Haloarchaea-Nanohaloarchaea symbiosis and its implications for carbon fixation during retinal-based phototrophy and the Purple Earth hypothesis. We discuss how the inclusion of an ecological perspective in the studies of evolution of ancient metabolic pathways may be beneficial to understanding the origin of life.

Recent astrophysical findings suggest that the era during which the Universe is habitable has just begun. This raises the question whether the entire Universe may at some point in the future be filled with intelligent life. Hanson et al. (2021, The Astrophysical Journal 922, 182) argued that we can be confident that the Universe will, by cosmic standards, soon be dominated by imperialist civilizations which expand rapidly, persist long and make drastic changes to the volumes they control. The main motivation for this ‘grabby civilizations’ hypothesis is that it supposedly provides a good explanation of why we are so early in cosmic history. In this paper, we criticize this motivation and suggest that it fails, for reasons analogous to why the notorious Doomsday argument fails. In the last part of the paper we broaden our discussion and argue that it may be rational to assign a rather low prior probability to the grabby civilizations hypothesis. For instance, if there are any civilizations that expand rapidly and indefinitely, they may well not make any drastic changes to the volumes they inhabit, potentially for strategic reasons. Hence, we call for epistemic caution and humility regarding the question of the long-term evolution of intelligence in the Universe.

The authors use the mathematical tool of Maccone's lognormal distribution to further factor the Drake equation, which calculates the number of advanced civilizations in the galaxy, from the seven original levels of the Drake equation to 49 levels of overall analysis. The Maccone approach, in fact, supported by the central limit theorem, becomes more reliable the more levels are introduced. The resulting study necessarily draws upon an array of disciplines ranging from astronomy, chemistry and geology to biology, palaeontology and futurology. The final result calculates the number of planetary systems suitable for life in its various stages of development: those which have probably hosted life in the past and those which still host it at its various evolutionary levels. The final evolutionary level is the so-called galactic civilization (often called ETC, or extraterrestrial civilizations). The number of resulting galactic civilizations is divided between static civilizations, which do not move around the galaxy and whose Kardašëv rating is still low (<1.4), of which we find three examples (we ourselves plus, perhaps, two others), and potentially dynamic civilizations, which move around the galaxy and have a sufficiently high Kardašëv rating (≥1.4), of which we find 2000.