Life as we know it requires the presence of liquid water and the availability of nutrients, which are mainly based on the elements C, H, N, O, P and S (CHNOPS) and trace metal micronutrients. We aim to understand the presence of these nutrients within atmospheres that show the presence of water cloud condensates, potentially allowing the existence of aerial biospheres. In this paper, we introduce a framework of nutrient availability levels based on the presence of water condensates and the chemical state of the CHNOPS elements. These nutrient availability levels are applied to a set of atmospheric models based on different planetary surface compositions resulting in a range of atmospheric compositions. The atmospheric model is a bottom-to-top equilibrium chemistry atmospheric model which includes the atmosphere–crust interaction and the element depletion due to the formation of clouds. While the reduced forms of CNS are present at the water cloud base for most atmospheric compositions, P and metals are lacking. This indicates the potential bio-availability of CNS, while P and metals are limiting factors for aerial biospheres.

In situ elemental imaging of planetary surface regolith at a spatial resolution of 100s to 1000s of microns can provide evidence of the provenance of rocks or sediments and their habitability, and can identify post-depositional diagenetic alteration affecting preservation. We use high-resolution elemental maps and XRF spectra from MapX, a flight prototype in situ X-ray imaging instrument, to demonstrate this technology in rock types relevant to astrobiology. Examples are given for various petrologies and depositional/diagenetic environments, including ultramafic/mafic rocks, serpentinites, hydrothermal carbonates, evaporites, stromatolitic cherts and diagenetic concretions.

Although astrobiology studies how life functions and evolves, ecology is still largely overlooked in astrobiology research. Here we present an argument for astroecology, a merger of ecology and astrobiology, a self-aware scientific endeavour. Ecology is rarely mentioned in influential documents like the NASA Astrobiology Strategy (2015), and terms such as ‘niche’ can end up being used in a less precise fashion. As ecology deals with sequential levels of organization, we suggest astrobiologically-relevant problems for each of these levels. Organismal ecology provides ecological niche modelling, which can aid in evaluating the probability that Earth-like life would survive in extraterrestrial environments. Population ecology provides a gamut of models on the consequences of dispersal, and if lithopanspermia can be validated as a form of space dispersal for life, then metabiospheres and similar astrobiological models could be developed to understand such complex structure and dynamics. From community ecology, the discussion of habitability should include the concept of true vacant habitats (a misnomer, perhaps better called ‘will-dwells’) and contributions from the blossoming field of microbial ecology. Understanding ecosystems by focusing on abiotic properties is also key to extrapolating from analogue environments on Earth to extraterrestrial ones. Energy sources and their distribution are relevant for ecological gradients, such as the biodiversity latitudinal gradient – would tropics be species-rich in other inhabited planets? Finally, biosphere ecology deals with integration and feedback between living and non-living systems, which can generate stabilized near-optimal planetary conditions (Gaia); but would this work for other inhabited planets? Are there ‘strong’ (like Earth) and ‘weak’ (perhaps like Mars) biospheres? We hope to show ecology can contribute relevant ideas to the interdisciplinary field of astrobiology, helping conceptualize further levels of integration. We encourage new partnerships and for astrobiologists to take ecology into account when studying the origin, evolution and distribution of life in the universe.

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.

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.

This article analyses how the first circumnavigation of the world, from 1519 to 1522, introduced South America as a key space in the formation of the ‘global’, thus producing a historical point of inflection. We examine the commercial and political plans and networks that began to function as a result of this new connectivity, which turned the American continent into a major global axis. The analysis focuses on the way in which this voyage gave new prominence to an unexplored region of the world, namely the southernmost tip of America, thus changing the notion of habitability that had prevailed for centuries in Europe. These changes questioned the authority of ‘ancient’ Greek thinkers and strengthened a European historical narrative that appropriated the discovered territories and distinguished the extreme southern part of America from other southern regions, as symbolized through figures such as the Patagonian giants. I consider these changes based on evidence from Spanish sources.

The English homelessness scheme has been lauded as being one of the most progressive in the world for offering an individually legally enforceable right to housing to those people who meet the statutory criteria. Its definition of homelessness is also liberal by comparison with many other countries within Europe and beyond, extending significantly beyond the stereotypical rooflessness experienced by rough sleepers. Nevertheless, the scheme is highly selective and targeted, and assesses homelessness through a test of relative need, rather than enshrining a minimally acceptable standard of housing. It thereby creates a category of the marginally housed whose housing needs are assessed as insufficiently poor to be officially categorised as homeless, yet who are living in severely inadequate housing. To reduce the uncertainty and contingency of the current test, the paper proposes the adoption of a new test of habitability.

The appellation ‘habitable zone’ in astrobiology in sooth evinces an overlooked and winding history that can be traced back to the 19th century. This paper sketches how this term from geography was generalized to encompass planetary habitability. The people involved in this narrative are numerous, but the bulk of their musings were rather nebulous. Yet, during this period appear the first true insights, although sadly this saga is not altogether sans blights.

The term habitable is used to describe planets that can harbour life. Debate exists as to specific conditions that allow for habitability but the use of the term as a planetary variable has become ubiquitous. This paper poses a meta-level question: What type of variable is habitability? Is it akin to temperature, in that it is something that characterizes a planet, or is something that flows through a planet, akin to heat? That is, is habitability a state or a process variable? Forth coming observations can be used to discriminate between these end-member hypotheses. Each has different implications for the factors that lead to differences between planets (e.g. the differences between Earth and Venus). Observational tests can proceed independent of any new modelling of planetary habitability. However, the viability of habitability as a process can influence future modelling. We discuss a specific modelling framework based on anticipating observations that can discriminate between different views of habitability.

Earth-Like is an interactive website and twitter bot that allows users to explore changes in the average global surface temperature of an Earth-like planet due to variations in the surface oceans and emerged land coverage, rate of volcanism (degassing) and the level of the received solar radiation. The temperature is calculated using a simple carbon–silicate cycle model to change the level of CO2 in the atmosphere based on the chosen parameters. The model can achieve a temperature range exceeding −100°C to 100°C by varying all three parameters, including freeze-thaw cycles for a planet with our present-day volcanism rate and emerged land fraction situated at the outer edge of the habitable zone. To increase engagement, the planet is visualized by using a neural network to render an animated globe, based on the calculated average surface temperature and chosen values for land fraction and volcanism. The website and bot can be found at earthlike.world and on twitter as @earthlikeworld. Initial feedback via a user survey suggested that Earth-Like is effective at demonstrating that minor changes in planetary properties can strongly impact the surface environment. The goal of the project is to increase understanding of the challenges we face in finding another habitable planet due to the likely diversity of conditions on rocky worlds within our Galaxy.

The search for life in the Universe is a fundamental problem of astrobiology and modern science. The current progress in the detection of terrestrial-type exoplanets has opened a new avenue in the characterization of exoplanetary atmospheres and in the search for biosignatures of life with the upcoming ground-based and space missions. To specify the conditions favourable for the origin, development and sustainment of life as we know it in other worlds, we need to understand the nature of global (astrospheric), and local (atmospheric and surface) environments of exoplanets in the habitable zones (HZs) around G-K-M dwarf stars including our young Sun. Global environment is formed by propagated disturbances from the planet-hosting stars in the form of stellar flares, coronal mass ejections, energetic particles and winds collectively known as astrospheric space weather. Its characterization will help in understanding how an exoplanetary ecosystem interacts with its host star, as well as in the specification of the physical, chemical and biochemical conditions that can create favourable and/or detrimental conditions for planetary climate and habitability along with evolution of planetary internal dynamics over geological timescales. A key linkage of (astro)physical, chemical and geological processes can only be understood in the framework of interdisciplinary studies with the incorporation of progress in heliophysics, astrophysics, planetary and Earth sciences. The assessment of the impacts of host stars on the climate and habitability of terrestrial (exo)planets will significantly expand the current definition of the HZ to the biogenic zone and provide new observational strategies for searching for signatures of life. The major goal of this paper is to describe and discuss the current status and recent progress in this interdisciplinary field in light of presentations and discussions during the NASA Nexus for Exoplanetary System Science funded workshop ‘Exoplanetary Space Weather, Climate and Habitability’ and to provide a new roadmap for the future development of the emerging field of exoplanetary science and astrobiology.

Dry permafrost - ground with temperature always below 0°C and containing negligible ice - overlying ice-cemented ground has been reported in the Dry Valleys of Antarctica and on Mars. Here we report on a new site (79°49.213'S, 83°18.860'W, 718 m elevation) located on the side of Mount Dolence in Ellsworth Land, Antarctica. Year-round temperature and humidity measurements indicate that dry permafrost is present between depths of 13.5 and 49.0 cm - the location of ice-cemented ground. The mean annual frost point of the ice-cemented ground is -17.0 ± 0.2°C and the mean annual frost point of the atmosphere is -22.7 ± 1°C. The corresponding mean annual temperatures are -19.2°C and -20.3°C. Neither the temperature of the ice-cemented ground nor the air rise above freezing. Both the dry permafrost and the ice table may be habitable. In the dry soil at 3 cm depth there are 80 hours in the summer when temperature exceeds -5°C and water activity exceeds 0.8. At the ice table, temperature exceeds -10°C and water activity exceeds 0.8 for 35 hours in the year. The ice table and the dry permafrost above it would be considered a ‘Special Region’ on Mars. Further microbial investigation of this site is indicated.

The concept of exoplanetary habitability is evolving. The driving force is a desire to define the biological potential of planets and identify which can host complex and possibly intelligent life. To assess this in a meaningful manner, climate models need to be applied to realistic surfaces. However, the vast majority of climate models are developed using generic aquaplanet, or swamp planet, scenarios that provide uniform, surface frictional coefficients. However, aside from planets with largely uniform oceans, these models are not obviously useful when it comes to understanding the impact of climate on biodiversity. Here, we show that contrary to expectation, the aquaplanet models can be directly applied to planets with a variety of land areas, with little need for modification. Using this premise, this paper provides a simple mathematical framework that may be applied to more complex planetary surfaces and identifies the majority of the climate-model components that are needed to accurately determine the biological potential of habitable exoplanets. As a proof-of-concept, an available climate model for Proxima b is used to determine its biological potential, given a suitable atmosphere.

Environmental conditions have changed in the past of our planet but were not hostile enough to extinguish life. In the future, an aged Earth and a more luminous Sun may lead to harsh or even uninhabitable conditions for life. In order to estimate the life span of the biosphere we built a minimal model of the co-evolution of the geosphere, atmosphere and biosphere of our planet, taking into account temperature boundaries, CO2 partial pressure lower limits for C3 and C4 plants, and the presence of enough surface water. Our results indicate that the end of the biosphere will happen long before the Sun becomes a red giant, as the biosphere faces increasingly more difficult conditions in the future until its collapse due to high temperatures. The lower limit for CO2 partial pressure for C3 plants will be reached in 170(+ 320, − 110) Myr, followed by the C4 plants limit in 840(+ 270, − 100) Myr. The mean surface temperature will reach 373 K in 1.63(+ 0.14, − 0.05) Gyr, a point that would mark the extinction of the biosphere. Water loss due to internal geophysical processes will not be dramatic, implying almost no variation in the surface ocean mass and ocean depth for the next 1.5 billion years. Our predictions show qualitative convergence and some quantitative agreement with results found in the literature, but there is considerable scattering in the scale of hundreds of millions of years for all the criteria devised. Even considering these uncertainties, the end of the biosphere will hardly happen sooner than 1.5 Gyr.